Guidance document 2 : characterisation of the storage complex, CO2 stream composition, monitoring and corrective measures.
Date: 2025-08-13
Institutions: Directorate-General for Climate Action
Subjects: EU Member State, EU environmental policy, EU financial instrument, energy storage, user guide

Guidance document 2 

Characterisation of the storage complex, CO2 stream composition, 
monitoring and corrective measures 

 
 
 
 
 
 
 
EUROPEAN COMMISSION 

Directorate-General for Climate Action 

This document has been prepared with the assistance of DNV Netherlands B.V. under contract to the 
European Commission (Service contract No 090203/2022/881796/SER/CLIMA.C.2) 

It has also greatly benefited from discussions with, and information supplied by, experts from Member 
States within the framework of the Expert Group on the geological storage of carbon dioxide and key 
stakeholders within the framework of a targeted stakeholder consultation. DG CLIMA gratefully 
acknowledges contributions from other Commission departments (COMP, ENER, ENV, MARE, GROW, 
REGIO, SANTE, RTD, JRC, SG, and SJ) in finalising the guidance documents. 

E-mail:    CLIMA-CCS-DIRECTIVE@ec.europa.eu 

European Commission 
B-1049 Brussels 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
EUROPEAN COMMISSION 

Guidance document 2 
Characterisation of the storage complex, CO2 stream composition, 
monitoring and corrective measures 

2025 

Directorate-General for Climate Action 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Guidance document 2 

The aim of this guidance document is to contribute to a better understanding of the requirements of Directive 
2009/31/EC on the geological storage of carbon dioxide. It has been prepared by the European Commission 
on  the  basis  of  the  views  and  knowledge  provided  by  stakeholders.  The  purpose  of  this  guidance  is 
explanatory and illustrative. It does not create any rights or obligations. 

Manuscript completed in July 2024 

This document has been prepared for the European Commission however it reflects the views only of the 
authors, and the European Commission is not liable for any consequence stemming from the reuse of this 
publication. 

ISBN: 978-92-68-31056-4 DOI: 10.2834/7032239 ML-01-25-044-EN-N 

© European Union, 2025 

The reuse policy of European Commission documents is implemented by Commission Decision 
2011/833/EU of 12 December 2011 on the reuse of Commission documents (OJ L 330, 14.12.2011, 
p. 39). Unless otherwise noted, the reuse of this document is authorised under a Creative Commons 
Attribution 4.0 International (CC BY 4.0) licence (https://creativecommons.org/licenses/by/4.0). This means 
that reuse is allowed provided appropriate credit is given and any changes are indicated. 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Guidance document 2 

Contents 

1.  Revision history ........................................................................................ 7 

2.  Purpose and scope of guidance documents .......................................... 8 

3.  Characterising the storage complex and surrounding area .................. 9 

3.1.  Legislative context ................................................................................... 9 

3.2.  Evaluating storage capacity in the Member States ............................... 10 

3.3.  Data collection....................................................................................... 13 

3.3.1.  Geology and geophysics ......................................................................... 13 
3.3.2.  Hydrogeology .......................................................................................... 16 
3.3.3.  Reservoir engineering and petrophysics .................................................. 17 
3.3.4.  Geochemistry .......................................................................................... 18 
3.3.5.  Geomechanics ........................................................................................ 18 
3.3.6.  Seismicity ................................................................................................ 20 
3.3.7.  Presence and condition of natural and man-made pathways ................... 22 
3.3.8.  Domains surrounding the storage complex that may be affected ............. 25 
3.3.9.  Population distribution ............................................................................. 26 
3.3.10.  Proximity to valuable natural resources ................................................... 26 
3.3.11.  Activities around the storage complex and possible interactions .............. 27 
3.3.12.  Proximity to the potential CO2 source(s) .................................................. 29 

3.4.  Building the three-dimensional static geological Earth model ............... 30 

3.5.  Dynamic modelling ................................................................................ 31 

3.5.1.  Characterisation of the storage dynamic behaviour ................................. 31 
3.5.2.  Sensitivity characterisation ...................................................................... 33 
3.5.3.  Risk assessment ..................................................................................... 33 

4.  CO2 composition ..................................................................................... 35 

4.1.  Legislative context ................................................................................. 35 

4.2.  CO2 stream consists overwhelmingly of carbon dioxide ........................ 36 

4.3.  No waste or other matter may be added for the purpose of disposal .... 36 

4.4.  Acceptable concentrations of incidental and added substances............ 37 

5.  Monitoring ................................................................................................ 39 

5.1.  Legislative context ................................................................................. 39 

5.2.  Establishing the monitoring plan ........................................................... 40 

5 

 
 
 
Guidance document 2 

5.2.1.  Approach ................................................................................................ 40 
5.2.2.  Scope and format of monitoring plans ..................................................... 45 
5.2.3.  Plan implementation, reporting and performance management ............... 52 

5.3.  Updating the plan .................................................................................. 55 

5.4.  Post-closure monitoring ........................................................................ 55 

6.  Corrective measures ............................................................................... 57 

6.1.  Legislative context ................................................................................. 57 

6.2.  Relationship to monitoring and monitoring plan updates ....................... 58 

6.3.  Responsibilities during project phases .................................................. 59 

6.4.  Scope and format of corrective measures plan ..................................... 59 

6.5.  Documentation and reporting ................................................................ 60 

6.6. 

Interpreting the results and performance of corrective measures .......... 61 

6.7. 

Inspections ............................................................................................ 61 

6.8.  Updates to the corrective measures plan .............................................. 61 

7.  References ............................................................................................... 62 

6 

 
1.  Revision history 

Key changes since the previous version 

•  Broadened description of a more balanced site-specific and risk-based approach to ensure 

containment through validation of effective trapping mechanisms. 

•  More detailed section on geomechanical characterisation. 

•  More emphasis on legacy wells as part of the process to assess site suitability. 

•  New subsection on evaluating legacy wells as part of site characterisation. 

•  Sensitivity case dynamic modelling included to feed into the range of uncertainty. 

•  Additional guidance on interpretation of Article 12 on CO2 stream composition, including statement 

that water co-injected with the CO2  should not be considered part of the CO2  stream. 

•  More balanced discussion of monitoring technologies and risk-based monitoring plan. 

•  Additional guidance specific to depleted field storage. 

•  Additional guidance related to considerations around induced seismicity. 

•  New section on evaluating storage capacity in Member States in line with Article 4(2) of the CCS 

Directive. 

7 

 
 
2.  Purpose and scope of guidance documents 

This guidance document (GD) forms part of a set of guidance documents as follows: 

●  Guidance document 1: CO2 storage life cycle and risk management framework; 

●  Guidance  document  2:  Characterisation  of  the  storage  complex,  CO2  stream 

composition, monitoring and corrective measures; 

●  Guidance  document  3:  Criteria  for  transfer  of  responsibility  to  the  competent 

authority; 

●  Guidance document 4: Financial security and financial contribution. 

The  aim  of  these  GDs  is  to  improve  understanding  of  the  requirements  of  Directive 
2009/31/EC on the geological storage of carbon dioxide (the ‘CCS Directive’) and give 
indications on how it can be implemented. They should therefore facilitate a correct and 
uniform application of the CCS Directive across the EU. The guidance does not represent 
an  official  position  of  the  Commission  and  is  not  legally  binding.  The  binding 
interpretation  of  EU  legislation  is  the  exclusive competence  of  the  European  Court  of 
Justice that can make final judgments concerning the interpretation of the CCS Directive. 

GD 2 provides guidance on: 

● 

● 

site selection; 

the composition of the CO2 stream; 

●  monitoring; and 

● 

corrective measures. 

Note:  See GD  1,  Section  2.4,  for  interpretations  of the main defined  and non-defined 
terms used in the CCS Directive. 

8 

 
 
3.  Characterising the storage complex and surrounding 

area 

The  goal  of  the  process  to  characterise  the  storage  site,  storage  complex  and 
surrounding area is to assess the containment, capacity1, injectivity and monitorability, 
and also to demonstrate that storage activities pose no significant risk to human health 
or the environment. The risk assessment process (see GD 1) provides the foundation for 
data  collection,  site  characterisation  and  site  selection.  Site-specific  risks  will  be 
identified and evaluated through several steps, such as data analysis, 3D static geologic 
modelling, dynamic modelling and sensitivity characterisation. 

The  CCS  Directive  allows  for  many  CO2  storage  settings  and  trapping  mechanisms, 
provided  they  meet  the  requirements  of  the  CCS  Directive.  Saline  aquifers,  depleted 
hydrocarbon fields, CO2 mineralisation, coal seams and other settings, such as still active 
hydrocarbon  fields,  in  case  of  EHR,  or  geothermal  systems,  in  the  case  where  the 
objective  is  to  reduce  GHG  emissions,  may  all  apply  under  the  CCS  Directive.  The 
process to characterise the storage site, the storage complex and the surrounding area 
will  vary  depending  on  the  storage  type  of  the  site,  data  availability  from  previous 
activities, and site-specific conditions. 

3.1.  Legislative context 

This  section  provides  guidance  to  operators  and  competent  authorities  on  how  to 
interpret and meet the obligations stipulated by Article 4(2) and Article 4(3) in the CCS 
Directive. Article 4(2) states that Member States that intend to allow geological storage 
of CO2 in their territory must assess the storage capacity available in parts or in the whole 
of their territory. Article 4(3) states that ‘the suitability of a geological formation for use 
as a storage site must be determined through a characterisation and assessment of the 
potential  storage  complex  and  surrounding  area  pursuant  to  the  criteria  specified  in 
Annex I’. 

Annex  I  to  the  CCS  Directive  sets  out  the  specific  criteria  to  be  met  in  order  to 
characterise and assess the potential storage complex and surrounding area. It requires 
operators  to  complete  the  assessment  according  to  best  practice  at  the  time  of  the 
assessment.  Best  practice  may  vary  depending  on  site-specific  considerations.  The 
steps to carry out during the site characterisation process in Annex I include: 

1 Capacity of prospective areas for geological storage of CO2  within storage sites relying on 
buoyancy/structural trapping can be assessed using the CO2  Storage Resources 
Management System (SRMS) methodology developed by SPE. In mineralisation projects, 
the capacity is determined by the reactivity of the rock (over time) and the ability of the 
operation to avoid CO2  exsolution prior to mineralisation, without causing a negative 
environmental impact. From a project perspective, the emphasis is generally on verifying 
sufficient capacity for project volumes, rather than estimating the overall available capacity 
of the selected storage site. 

9 

 
 
 
 
 
 
1. 

Data collection. 

o  Sufficient  data  must  be  collected  to  construct  a  volumetric  and  three- 
dimensional  static  (3D)-earth  model  for  the  storage  site  and  storage 
complex,  including  the  caprock,  the  surrounding  area  and  hydraulically 
connected areas. 

2. 

Three-dimensional (3D) static geological modelling. 

o  Using the data collected in Step 1, a three-dimensional static geological 
earth model, or a set of such models, of the candidate storage complex, 
including  the  caprock  and  the  hydraulically  connected  area  and  fluids 
must be built using computer reservoir simulators. 

3.  Characterising  storage dynamic behaviour, sensitivity and risk assessment. 

o  The process to characterise storage dynamic behaviour must be based 
on dynamic modelling. It must comprise multiple time-step simulations of 
CO2  injection  into  the  storage  site  using  the  three-dimensional  static 
geological earth model(s) in the computerised storage complex simulator 
constructed. 

o  Multiple simulations must be carried out to identify the sensitivity of the 
assessment  to  assumptions  made  about  particular  parameters.  The 
simulations must be based on altering parameters in the static geological 
earth  model(s)  and  changing  rate  functions  and  assumptions  in  the 
dynamic  modelling  exercise.  The  risk  assessment  must  factor  in  any 
significant sensitivity. 

o  The risk assessment must include the following steps: 

■  hazard characterisation 

■  exposure assessment 

■  effects assessment 

■ 

risk characterisation. 

3.2.  Evaluating storage capacity in the Member States 

To  meet  the  Member  States'  obligations  under  Article  4(2),  the  competent  authority 
should  carry  out  or  commission  work  to  develop  a  storage  atlas  for  the  areas  in their 
national territory considered to be potentially suitable for the geological storage of CO2. 
The storage atlas should be made available as a digital, searchable database. Examples 
of  storage  atlas’  developed  for  this  purpose  include  the  CO2  storage  atlas  for  the 
Norwegian  Continental  Shelf2,  the  storage  atlas  developed  for  the  UK3,  the  North 

2 https://www.npd.no/en/whats-new/publications/co2-atlases/co2-atlas-for-the-norwegian- 

continental-shelf/. 

3 https://www.co2stored.co.uk/home/index. 

10 

 
 
 
 
 
 
American Storage Atlas4 and the OGCI storage resource catalogue5. A European CO2 
storage database was also developed by the CO2StoP project (CO2  Storage Potential 
in Europe)6. The work to develop the storage atlas should factor in considerations that 
may  exclude  certain  areas  for  geological  storage,  such  as  protected  nature  areas,  or 
other socioeconomic aspects, such as human settlements, even though the subsurface 
in the area may be technically suitable for geological storage. 

To carry out an initial high-level evaluation of the capacity available in those areas of the 
Member State's territory, a dual approach is often taken. 

Depleted fields 

Depleted  hydrocarbon  fields  are  those  fields  for  which  it  has  been  demonstrated  that 
hydrocarbon  reserves  have  been  depleted  (either  fully  or  partially)  or  the  remaining 
resources may no longer be recovered/developed economically. For depleted fields, the 
capacity  is  often  estimated  based  on  the  volume  of  hydrocarbons  –  at  reservoir 
conditions – that have been produced, and the equivalent volume of CO2 that will occupy 
the same reservoir volume. This requires access to production data and an evaluation 
of CO2  density at reservoir conditions. 

A discount factor (i.e. storage efficiency factor) is typically applied since the injected CO2 
may  not  be  able  to  access  the  full  reservoir  volume,  and  geomechanical  effects  of 
depletion  and  reinflation  may  not  permit  re-pressurisation  to  initial  reservoir  pressure. 
This may also be necessary if reinflation could lead to higher reservoir pressures than 
virgin pressure if the full equivalent volume is injected. A storage efficiency factor of 50% 
may be considered conservative and used to compare different projects, but the value 
for a given reservoir should be scrutinised on a case-by-case basis. For instance, the 
storage  efficiency  factor  will  vary  depending  on  the  nature  of  the  reservoir  and 
corresponding connected aquifer (i.e. the hydraulic unit), where a field with strong aquifer 
pressure support (i.e. production from water drive resulting in pressure recharge) would 
have a lower storage efficiency factor and lower estimated CO2  capacity compared to a 
field  lacking  aquifer  pressure  support  (i.e.  little  to  no  production  from  water  drive  or 
pressure recharge). 

Aquifer sites 

Aquifer  sites  are  often  not  depleted  and  CO2  injection  operations  generally  increase 
pressure  significantly  above  virgin  pressure  (unless  active  pressure  management  is 
carried out using brine production). The storage capacity for such sites is constrained by 
allowable pressure build-up without fracturing the rock, or causing activation of faults, as 
well  as  by  constraints  on  CO2  migration  to  achieve  long-term  containment.  High-level 
(e.g. basin-wide) capacity estimates are, however, generally derived without considering 

4  https://www.netl.doe.gov/sites/default/files/2018-10/ATLAS-V-2015.pdf. 

5  https://www.ogci.com/ccus/co2-storage-catalogue. 

6 https://setis.ec.europa.eu/european-co2-storage-database_en hosted by the Energy and 
Industry Geography Lab (https://energy-industry-geolab.jrc.ec.europa.eu), platform 
developed by the Joint Research Centre. 

11 

 
 
 
 
 
 
 
migration. Instead, estimates are typically derived by using rough estimates of mass that 
can be injected without over-pressurising the storage system. 

The typical method to  derive  initial  high-level  capacity  estimates for  aquifer  sites  links 
capacity  to  the  volume  that  can  be  made  available  by  water  displacement  and 
compressibility using the following formula: 

MCO2 = Vb x 𝜙 x n/g x ρCO2 x E Where: 

●  MCO2 = mass of CO2 (kg); 

●  Vb = bulk volume (m3); 

● 

● 

ϕ = average effective porosity (% or ratio); 

n/g = net to gross ratio (or %); 

●  ρCO2 = density of CO2 at reservoir conditions (kg/m3); and 

●  E = storage efficiency factor. 

As it is conventional to represent CO2  capacity in tonnes (t) or million tonnes (Mt), MCO2 
in  kilograms  (kg)  must  be  multiplied  by  1000  or  1.0x109,  respectively.  Here  the  bulk 
volume (Vb) is sometimes expressed as Vb  = A x h, where A is the area of the basin or 
hydraulic unit being considered, and h is the net thickness of the hydraulic unit. Hydraulic 
units may have been mapped by national geological surveys, and the net thickness can 
be derived from well logs of wells that penetrate the hydraulic unit. Similarly, porosity and 
n/g can be derived from analysing cores from the hydraulic unit. 

For entire hydraulic units, it is reasonable to assume that the unit is a closed system. The 
storage  efficiency  factor  depends  on  the  window  to  maximum  allowable  pressure 
constraints,  generally  estimated  using  evaluations  of  hydraulic  systems  around  the 
world.  For  instance,  the  storage  efficiency  factor  has  been  estimated  for  four  closed 
systems (Mount Simon (USA), Basal Cambrian (Canada), and Bunter and Rotliegend in 
the North Sea) in Sylvain Thibeau et al. (2014). These estimates range from 0.62% for 
the  Basal  Cambrian  to  0.92%  for  Mount  Simon.  Without  specific  knowledge  about 
allowable  pressure  constraints  and  specific  modelling  for  the  hydraulic  unit,  it  is 
recommended that the competent authorities use a conservative value for E, e.g. E = 
0.5%. 

Note that this volumetric approach can also be used to derive initial high-level capacity 
estimates  for  individual  storage  sites.  However, at site  level,  capacity  depends  on the 
extent of the hydraulic unit containing the aquifer used for geological storage (allowing 
pressure dissipation) relative to the volume of the aquifer in the storage complex. If a 
storage site (the aquifer proposed for geological storage) is part of a large hydraulic unit, 
then the storage efficiency factor may be much higher. Such assessments at the scale 
of individual sites are not expected to be carried out by the Member State. 

12 

 
3.3.  Data collection 

Data collection is an important component of site characterisation and modelling. Data 
collection plans should be developed that are specific to the type of storage site, trapping 
mechanisms, environmental and human health risks, and subsurface complexities. 

In  some  cases,  access  to  existing  data  from  oil  and  gas  exploration,  geothermal 
operations,  or  other  subsurface  activities  will  facilitate  a  reliable  characterisation  and 
assessment of the potential storage complex and surrounding area. If access to these 
data is available in a region nearby to a proposed storage site with a similar geology, 
then  a  storage  operator  may  be  able  to  make  considerable  progress  in  proving  the 
existence of a viable storage site. 

3.3.1.  Geology and geophysics 

The primary objectives of the geology and geophysical data collection are to describe 
and characterise the storage site, storage complex, surrounding area, and hydraulically 
connected areas with enough detail to build the 3D geological static model and underpin 
the  monitoring  plan.  The  data  must  cover  at  least  the  intrinsic  characteristics  of  the 
storage complex discussed in the following subsections. The data must be taken from 
multiple sources including well logs, well cuttings analyses, core samples and analyses, 
fluid  samples,  injection  and  production  tests,  outcrop  studies,  seismic/geophysical 
surveys and/or remote sensing surveys. The level of detail required for each component 
will depend on the trapping mechanism(s) and the key risks identified for the CCS project. 

Additional  subsurface  characterisation  data  may  be  required  to  provide  a  complete 
understanding  of  site  conditions  over  time  as  part  of  monitoring  (see  Section  5).  For 
example,  accurate  4D  seismic  monitoring  requires  calibration  to  core  measurements, 
petrophysical data, rock physics information and/or saturation models. These data may 
be required to accurately predict the seismic response to injected CO2  in hydrocarbon 
fields, and to monitor the movement of CO2  in the subsurface over time. 

In  both  saline  aquifers  and  depleted  hydrocarbon  fields,  the  volume  and  area  of  the 
storage site may be defined by the maximum extent of the injected CO2  stream in the 
reservoir throughout the project lifecycle. In depleted hydrocarbon fields, the vertical and 
lateral boundaries will be defined based on the natural limits and characteristics of the 
field, such as trap geometry, spill points, lithology or facies changes, or bounding faults. 
Both saline aquifers and depleted hydrocarbon field sites are likely to involve a porous 
and  permeable  injection  zone  comprised  of  sandstone  or  carbonate.  In  sites  which 
leverage predominantly CO2 dissolution or mineralisation trapping, the volume-area may 
be defined by the maximum extent of the injected CO2  stream, factoring in natural limits 
such as lithology, facies changes or non-transmissible faults. The injection zone is likely 
to consist of mafic or ultramafic rocks, such as basalt or peridotite with permeable facies 
and distributed fractures or faults. 

Competent authorities and operators are encouraged to be involved at an early stage to 
achieve consensus on the vertical and lateral extent of the defined storage complex. 

13 

 
 
 
 
The description of the storage complex will include any caprocks or seals, faults, facies 
changes  or  other  geologic  features  that  would  impact  the  containment  of  the  injected 
stream (see Figure 1). A storage complex may consist of multiple injection zones, each 
with applicable trapping mechanisms and natural limits. 

For mineralisation projects, a storage site will include the geological stratum (or strata) 
into which the CO2  stream is injected (referred to as the storage reservoir) as well as 
surface and injection facilities. The vertical extent of the storage site will be determined 
by the vertical extent of the storage reservoir(s). The lateral extent should include the 
domain where CO2  may exist in free-phase and should be contained within the storage 
complex. 

Additional guidance on the infrastructure to be included in the storage site definition is 
provided in GD 1, Table 1: Clarification of the key defined terms used in CCS Directive. 
The definition of the storage complex should include the subsurface component of the 
storage  site.  It  is  recommended  that  protected  groundwater  is  clearly  identified  and 
excluded from the definition of the storage complex. Caprocks and seals are not required 
components  of  the  mineralisation  storage complex  definition  if  it  is  demonstrated  that 
they are not critical to ensure the containment of injected CO2 within the storage complex 
(see Section 3.4 Box 1). 

The description of the surrounding area should include the area surrounding the storage 
complex where negative effects to the environment or human health are possible (see 
Figure  1).  Member  States  may  also  include  specific  requirements  or  descriptions  to 
consider  in  the  surrounding  area.  The  surrounding  area  will  be  determined  on  a  site- 
specific  basis,  and  will  be  based  on  a  risk  assessment.  The  recommended  geologic 
considerations for the surrounding area include areas of increased pressure, horizons 
containing groundwater consumable to humans, or areas with a risk of natural or induced 
seismicity. Additional considerations are included in Section 3.3.8. 

Geologic data should be collected to characterise the following areas, as appropriate: 

1. 

Storage site: 

o  a  diagram  indicating  the  vertical and  lateral extent  of the  storage  site, 
indicating the vertical and lateral extent of key horizons within the site; 

o  properties  of  the  injection  zone  rock  such  as  lithology,  thickness, 

mineralogy, porosity, permeability, facies types and distributions; 

o  properties of the caprock, where applicable, such as lithology, thickness, 

mineralogy, porosity, permeability, facies types and distributions; 

o 

faults  and  fractures:  location,  orientation,  fault  throw,  transmissibility, 
fracture distribution (also see Section 3.3.5); 

o  capacity estimates. 

14 

 
Figure 1. Illustrative example of the storage site, storage complex, surrounding area and 
leakage pathway in a saline aquifer or depleted oil and gas field. 

2. 

Storage complex: 

o  a diagram indicating the vertical and lateral extent of the storage complex, 
including  the  vertical  and  lateral  extent  of  all  horizons  in  the  storage 
complex; 

o 

o 

o 

rock  properties  for  key  horizons:  lithology,  porosity,  permeability, 
thickness, facies types and distributions; 

faults  and  fractures:  location,  orientation,  fault  throw,  transmissibility, 
fracture distribution (also see Section 3.3.5); 

leakage pathways: faults, fractures, permeable zones in caprock, capillary 
pressure estimates (for more details, see Section 3.3.8); 

o  paleogeography,  depositional  history,  structural  evolution  and  nearby 

petroleum systems (if present). 

15 

 
 
3.3.2.  Hydrogeology 

The  primary  goals  of  the  hydrogeology  characterisation  process  are  to  identify  the 
groundwater7 intended for human consumption8, or necessary to avoid impacts on health 
or the environment within the storage complex and surrounding area, and to prevent it 
from  becoming  contaminated  by  CO2  leakage.  The  additional  goals  are  to  provide 
information on the movement and interaction of fluids within the groundwater as they link 
to modelling of the pressure and temperature gradients and groundwater salinity, which 
in turn link to the evaluation of containment, capacity and hydrodynamics. 

Groundwater may contain CO2  in solution due to the natural generation of CO2  from the 
subsurface  during  volcanic  activity,  mantle  degassing,  diagenesis,  and  geochemical 
alterations of the rock formations over time. Baseline monitoring (see Chapter 4) can be 
used to characterise the chemical composition of groundwater prior to injection. 

injection 

Some  storage  sites  may  propose  CO2 
into  aquifers.  which  require 
demonstration  that  the  site  would  pose  no  significant  risk  to  human  health  or  the 
environment, and that groundwater intended for human consumption is protected, with 
no CO2  injection being performed in such aquifers (also see Section 3.3.10). In these 
cases, it is recommended that groundwater intended for human consumption is clearly 
differentiated from groundwater that is not intended for human consumption and that a 
geologic barrier or system is in place preventing mixing of these components. 

It  may  also  be  crucial  to  carry  out  detailed  monitoring  of  both  these  components  to 
ensure  that  early  corrective  action  can  be  taken  if  groundwater  intended  for  human 
consumption  is  threatened  (see  Section  5).  Injection  into  aquifers,  however,  may  be 
subject  to  other  local  regulation.  It  is  recommended  that  operators  refer  to  local 

7 ‘Groundwater’ is defined as all water which is below the surface of the ground in the saturation 
zone and in direct contact with the ground or subsoil (ref. Article 2, Directive 2000/60/EC 
Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 
establishing a framework for Community action in the field of water policy, retrieved from: 
https://eur-lex.europa.eu/eli/dir/2000/60/oj). 

8 Water intended for human consumption is defined as (ref. Article 2, Directive 98/83/EC 
of 3 November 1998 on the quality of water intended for human consumption, retrieved 
from: https://eur-lex.europa.eu/legal- 
content/EN/TXT/?uri=celex%3A31998L0083): 

(a) all water either in its original state or after treatment, intended for drinking, 
cooking, food preparation or other domestic purposes, regardless of its origin and 
whether it is supplied from a distribution network, from a tanker, or in bottles or 
containers; 

(b) all water used in any food-production undertaking for the manufacture, 

processing, preservation or marketing of products or substances intended for human 
consumption unless the competent national authorities are satisfied that the quality of 
the water cannot affect the wholesomeness of the foodstuff in its finished form. 

16 

 
 
 
 
 
 
 
 
groundwater protection regulation for other applicable definitions as these may vary by 
jurisdiction. 

3.3.3.  Reservoir engineering and petrophysics 

Reservoir engineering is the study of the characteristics and movement of fluids within a 
subsurface porous medium (reservoir or injection zone). Reservoir engineering is closely 
linked to the dynamic modelling step of the site characterisation process. Information on 
reservoir engineering is used to evaluate the capacity, containment and injectivity of a 
storage site. It can also be used to monitor the movement of injected fluids (see Section 
5). Key data types will include chemical compositions of reservoir fluids, pressure and 
temperature profiles, and the relative permeability of reservoir fluid to the injected fluid. 

A description of the hydraulic unit9 will also be a component of the reservoir engineering 
characterisation. For shared hydraulic units, it is the responsibility of the operator, as part 
of  the  storage  permit,  to  identify  and  characterise  activities  around  the  storage  site, 
including characterisation of the hydraulic unit (Article 9, Annex I to the CCS Directive). 
The competent authority must ensure that ‘potential pressure interactions are such that 
both sites can simultaneously meet the requirements of the directive’ (Article 8). If the 
storage complex spans across national boundaries, multiple competent authorities will 
need to collaborate to ensure the requirements are met. 

Petrophysics  is  the  study  of rock  and  pore-fluid properties from  physical  samples (i.e. 
core and cuttings) and digital measurements (well logs). It includes studying the physical 
and  chemical  properties  of  the  rock  and  its  petrological  (mineral  composition) 
characteristics and how these elements impact upon the development of and interaction 
of fluids in a subsurface formation. Petrophysical data and evaluation are critical inputs 
into the 3D static geological model (see Section 3.3.3) and will feed into the evaluation 
of  capacity,  containment,  injectivity  and  likely  monitorability  of  storage  sites.  Key 
petrophysical data types include porosity and permeability evaluations, fluid properties, 
wettability of the rock and capillary pressure measurements. 

In  sites  with  a  history  of  operations,  such  as  depleted  hydrocarbon  field  sites  and 
geothermal fields, there may  be  plentiful reservoir  engineering  and  petrophysical  data 
due to exploration and production activity. However, it is recommended to ensure that 
data  available  are  representative  of  the  ‘current’  site  conditions,  and  not  of  the  initial 
condition or production condition. Production can impact porosity, permeability and fluid 

9  ‘Hydraulic unit’ is defined as a hydraulically connected pore space where pressure 

communication can be measured by technical means and which is bordered by flow 
barriers, such as faults, salt domes, lithological boundaries, or by the wedging out or 
outcropping of the formation. 

The hydraulic unit containing the subsurface volume for the storage site is important for 

determining the expected pressure build-up from the geological storage project, which is 
also a key determinant for storage capacity and sustained injectivity. The hydraulic unit 
should therefore be mapped and described. This mapping should also describe other 
known activities within the hydraulic unit that may impact pressure within the storage site. 

17 

 
 
 
 
 
 
 
characteristics  leading  to  misrepresentation  of  geologic  conditions  in  3D  static  and 
dynamic models if older data are used. 

3.3.4.  Geochemistry 

Geochemistry  is  the  study  of  the  chemical  constituents  of  rocks  and  the  fluids  they 
contain at  the  elemental  and  mineralogical  level and identifies  the  conditions  in which 
minerals  are  likely  to  precipitate  or  dissolve.  These  processes  have  implications  for 
assessing capacity, containment, injectivity and for monitoring the injected CO2  stream. 

A  geochemical  analysis  may  identify  the  likelihood  for  minerals  in  a  storage  zone  or 
caprock to dissolve, which can potentially result in leakage. In some cases, geochemical 
analysis can identify the propensity for mineralisation reactions to take place, leading to 
mineral storage of CO2. It may also identify the propensity of a geochemical system to 
result in precipitation of solids, which may be another indicator of mineral trapping, or 
lead to  a reduction  in  injectivity.  A  geochemical analysis  may  identify the likelihood  of 
mobilisation  of  other  elements  that  could  have  site-specific  environmental  or  health 
implications in the event of leakage. 

Geochemical  investigations  may  also  be  used  to  assess  interactions  between  the 
contents in the CO2  stream and wellbore cement to identify potential leakage pathways. 
Geochemical analysis and modelling will also help to identify capacity in systems that 
rely on geochemical interactions, such as mineralisation projects. 

The extent and nature of any geochemical changes that could occur will be site-specific, 
and therefore appropriate sampling and assessment will need to be completed for each 
storage  complex.  Geochemical  sampling  from  appropriate  and  key  intervals  (e.g.  the 
water  leg  of  a  petroleum  field  or  the  injection  zone  of  a  mineralisation  site)  may  be 
important in producing representative results for storage complex characterisation. 

A geochemical analysis may include reactive transport modelling, reservoir simulation of 
pressure and temperature conditions, as well as chemical and mineralogical sampling of 
rock, reservoir fluids and injected fluids. 

3.3.5.  Geomechanics 

Geomechanics is the study of forces (stresses) and deformation in the subsurface and 
the evaluation of potential failure (breaking) of rock fabric, which is one of the root causes 
of the creation of permeable pathways that can result in leakage. Knowledge of the stress 
condition in the subsurface (which is the force per unit of area) is essential because rock 
failure is described by stress-based criteria. 

Changing the pore pressure during oil and gas production and again during geological 
storage of CO2  changes the stress condition in the subsurface both within and outside 
the  reservoir.  A  changing  temperature  also  impacts  the  stress  condition  in  the 
subsurface. Geomechanical analyses assess how the stress conditions change due to 
prior oil and gas production in depleted field sites and planned geological storage of CO2; 
identifying where in the subsurface critical stress condition and rock failure may occur. 
Geomechanical analyses also provide information about expected deformations such as 

18 

 
 
 
 
 
subsidence and heave at surface or seabed, and compaction and extension along new 
and/or legacy well trajectories. 

Geomechanical  analyses  impact  evaluations  of  containment  (e.g.  through  induced 
fracturing of the top-seal and reactivation of fractures and faults), capacity (as prescribed 
pressure  gradients  and/or  and thresholds  for  rock  fracture gradients  or minimum total 
principal  stress  may  be  required  by  the  competent  authority),  and  injectivity  (as 
geomechanical considerations constrain injection rates, pressure and temperature). It is 
closely linked with thermal modelling (temperature changes can result in fractures) and 
geochemical analysis (for example, dissolution can weaken rock and precipitation can 
strengthen rock). Operators are recommended to consider taking an integrated approach 
to these subjects. 

Geomechanics analyses require characterisation of the geological structure (geometry 
of  the  storage  and  sealing  formations,  faults),  the  mechanical  (elastic  and  failure) 
properties and the initial conditions in terms of stress, pore pressure and temperature. 
The  geomechanical  characterisation  is  dependent  on  data  from  petrophysical  logging 
(such as density, porosity and sonic velocity logs), drilling experience (observations of 
bore hole instability and losses), well testing (e.g. extended leak-off test and step-rate 
test), core analyses and various rock mechanical testing, and seismic interpretation of 
formations, faults, salt bodies etc. 

It is important to collect data to characterise the current stress regime within the injection 
zone, caprock, and/or surrounding geologic environment, as well as data to characterise 
changes in the stress as injection proceeds, because previous production and planned 
storage operation impact an area much larger than the reservoir formations. 

Geomechanical impacts and considerations will differ depending on the geologic setting, 
past  and  present  stress  regime,  and  the  risks  or  issues  associated  with  the  type  of 
geological storage. All sites are recommended to evaluate the store and overburden to 
identify faults (location, geometry, extent, orientation, throw) extending through the store 
or  caprock,  establish  the  initial  conditions  (stress,  temperature)  and  induced  changes 
(compaction, pressure, temperature). They should also evaluate the natural or induced 
seismicity  risk  (see  Section  3.3.6),  and  consider  the  impact  of  injection  operations  on 
changes in the geomechanical setting. 

Additional  considerations  may  be  relevant  to  different  sites  and  settings.  For 
mineralisation  projects,  for  example,  the  geomechanical  characterisation  may  include 
identification of fracture sets to assess storage capacity (mineralisation reactions may 
be more likely to occur in permeable fractured zones) and the reactivation of critically 
induced  seismicity  potential  and  other 
stressed  pre-existing 
considerations. 

identify 

faults 

to 

For depleted oil and gas reservoirs, geomechanical analyses are critical to identify stress 
changes  and  compaction  impacts  to  the  current  stress  state  to  ensure  that  leakage 
pathways are identified during the site screening and that no new pathways are created 
through  injection.  The  stress  condition  will  change  from  the  production  phase  to  the 
injection  phase  in  these  projects,  but  a  return  to  the  pre-production  stress  state  is 
unlikely. New data will likely need to be collected to properly evaluate these settings. 

19 

 
The  operator  should  assess  the  injection  pressure  limits  based  on  minimum  total 
principal stress (S3), the fracture gradient (FG), and geomechanical constraints. The term 
FG stems from drilling engineering to generally indicate the mud weight at which the bore 
hole begins to lose drilling fluid into the formation due to the nucleation and/or growth of 
a fracture. The FG requires a reference depth to obtain the absolute stress value, which 
can  be  lost  or  misinterpreted  from  reports.  The  S3  is  a  more  accurate  value  to  use  to 
assess injection pressure limits as it is not biased by near-wellbore stress variations and 
wellbore orientation and is a better measure of in-situ stress in the reservoir. 

Lower  injection  pressure  limits  will  result  in  a  lower  capacity  and  lower  sustainable 
injection  rates  for  individual  wells.  The  operator  should  communicate  clearly  to  the 
competent  authority  the  basis  used  to  assess  the  injection  pressure  limits,  including 
reference  depths  as  applicable  and  expected  safety  margins.  Site-specific 
geomechanical  assessments,  testing  and  modelling  should  be  used  to  assess  the 
appropriate thresholds for the site. 

There  are  a  range  of  thresholds  and  approaches  proposed  (for  example,  in  US  EPA 
Class  VI  regulations,  Offshore  Norge  ARMA  22-559  and  ARMA  2016-887,  California 
LCFS and others). Any injection pressure limits established must ensure there are no 
significant risks to human health and the environment. 

3.3.6.  Seismicity 

Seismicity is an important consideration for CO2 storage sites due to associated leakage 
risk, the potential impact on health, safety, the environment and damage to infrastructure. 
Earthquakes are caused when two structural blocks abruptly slip past one another along 
faults, generating seismic waves that propagate through the earth. Blocks may also slip 
slowly past one another, without generating seismic waves. This is referred to as an a- 
seismic slip. It is assumed that an a-seismic slip precedes an abrupt seismic slip. Seismic 
and  a-seismic  slips  may  both  create  leakage  pathways  along  fault  planes  and  occur 
when the fault plane is critically stressed. 

Seismicity  can  be  broken  down  into  three  categories:  natural,  induced  and  triggered 
seismicity. 

●  Natural  seismicity  refers  to  earthquake  activity  that  occurs  due  to  natural 
geologic  (tectonic)  processes.  It  means  that  the  critical  stress  condition  along 
fault planes is dominated by natural processes and that storage site operations 
have a minor or negligible contribution. 

● 

Induced  seismicity  refers  to  earthquakes  that  are  directly  caused  by  human 
activities, typically associated with industrial operations such as mining, hydraulic 
fracturing  (fracking),  geothermal  energy  extraction  and  reservoir-induced 
seismicity from large water impoundments. These earthquakes occur as a result 
of changing the stress conditions within the Earth’s crust. The seismic events are 
directly linked to human interventions and would not have occurred naturally in 
the absence of such activities. 

●  Triggered  seismicity  refers  to  earthquakes  that  are  indirectly  influenced  by 
human  activities  but  are  primarily  caused  by  natural  stress  conditions  in  the 
Earth’s crust. These earthquakes occur when pre-existing faults or stresses in 

20 

 
 
 
the  Earth’s crust  are affected  by  external  factors,  such as  seismic  waves  from 
large  distant  earthquakes, volcanic  activity  or  changes  in fluid  pressure due  to 
activities like reservoir filling or fluid extraction. In this case, human activities act 
as a trigger that initiates the earthquake, but the underlying cause is the release 
of stress accumulated naturally over time. 

Fault  movement  and  seismic  waves  can  be  initiated  due  to  increased  subsurface 
pressure  or  sometimes  due  to  geochemical  processes  or  subsurface  temperature 
changes. Triggered seismicity may be more likely to occur in areas of natural seismicity 
and where existing faults are further stressed. 

Evaluations  of  natural,  induced  or  triggered  seismicity  risk  are  important  components 
and operators are recommended to include this work in the site risk assessment (see 
GD 1) and in the site characterisation process. This evaluation will require information 
about i) the presence, extent and orientation of pre-existing faults systems, which may 
be  inferred  by  studying  2D  and  3D  seismic  reflection  data  and  assessed  using 
earthquake  catalogues  available  for  the  area.  This  will  include  i)  historical  data  ii)  the 
stress condition in the subsurface, iii) the fault strength (before it starts to slip), and iv) 
the change in pore pressure and temperature induced by previous oil & gas production 
and  planned  CO2  storage  operations.  The  seismicity  evaluation  may  use  data  from 
geomechanical  modelling  (see  Section  3.3.6),  and  it  may  have  an  influence  on  site 
suitability decisions, well placement and injection operation plans (i.e. allowable injection 
and reservoir pressure and temperature). 

Some  faults  may  not  be  able  to  be  identified  through  site  characterisation  tools  and 
techniques. Seismicity may occur also on invisible faults (e.g. faults with little throw that 
are  not  visible  on  seismic  data).  This  scenario  may  be  tackled  through  uncertainty 
management and included in monitoring plans (see Section 5). 

Seismicity can be generated at multiple scales: at grain level causing micro-seismicity 
with a very small moment magnitude (Mw<<0) not felt by humans, or at a very large level 
resulting  in  a  safety  and  environmental  hazard.  Moment  magnitude  is,  by  definition, 
related to the seismic slip area on the fault plane. 

The larger the moment magnitude, the larger the size of the seismic slip patch. So, larger 
earthquakes  may  increase  permeability  over  a  larger  area  and  create  permeable 
pathways across larger/thicker sealing formations. Seismic activity accumulating along 
a fault line and migrating upwards, even at relatively low levels (magnitude less than 3) 
may result in the generation of leakage pathways (faults or fractures) out of the storage 
complex (Willacy et al 2019). Larger events may impact nearby infrastructure and pose 
a  safety  hazard  that  increases  with  increasing  moment  magnitude.  Operators  are 
recommended to review materials standards to identify the relevant thresholds. 

Not every seismic event will result in a leakage pathway; in some cases seismic activity 
may  result  in the  sealing  of  existing  leakage  pathways  or  faults.  The risks  associated 
with  the  seismic  hazard  depend  on  the  potential  damage,  which  is  expected  to  be 
different  for  onshore  and  for  offshore  geological  storage  sites  for  CO2.  Site 
characterisation is recommended to include an assessment of seismic risks and related 
impacts. 

Seismicity can also be measured and monitored at multiple scales, providing different 
forms of insight to feed into the site characterisation process. Permanent networks of 

21 

 
seismic  monitoring  stations  supported  by  geologic  surveys  can  identify  large-moment 
magnitude earthquakes that impact broad areas and pose a hazard to communities and 
infrastructure. Additional measurement and monitoring systems are often required to be 
installed at CO2 storage sites to identify smaller-scale seismic activity that can jeopardise 
containment.  More  precise  measurement,  for  example  using  wellbore  geophysical 
monitoring  and/or  site-specific  measurement  arrays,  is  also often required  to uniquely 
identify  responsible  faults  in  the  storage  complex  to  assess  potential  leakage.  Higher 
resolution measurement may enable injection operations to be modified when low-level 
seismicity thresholds are reached, potentially avoiding inducing a larger earthquake. 

Under GD 1, the thresholds for acceptable level of seismicity should be based on a site- 
specific, risk-based approach (see Zhang et al 2016; Porter et al 2019). These thresholds 
should factor in the level of completeness10 of the seismicity monitoring network in the 
monitoring  area  and  should  be  set  in  concert  with  the  competent  authority  and  the 
operator. Seismic monitoring is recommended to begin as early as feasible before CO2 
injection starts. Some approaches to date have included a traffic light system indicating 
increasingly stringent operational constraints as seismic activity increases. This can be 
based  on  multiple  factors  including  ground  motion,  duration  or  frequency  of  seismic 
events, local building codes or offshore platform structural limitations (Grigoli et al 2017; 
Zoback 2012; Thorsteinsson and Gunnarsson 2014). 

3.3.7.  Presence and condition of natural and man-made 

pathways 

Predicting the presence of leakage pathways for the potential migration of CO2 out of the 
targeted  storage formations  is  a  vital  component  of  the  characterisation  process.  It  is 
included  explicitly  in  the  containment  evaluation  and  it  links  to  risk  assessment,  site 
characterisation, CO2  composition, monitoring and corrective action. 

The project’s description of the storage complex will form the basis and context for this 
evaluation, and will need to factor in the trapping mechanism(s). All leakage scenarios 
that have the potential to pose a significant risk to human health and the environment 
should be considered. 

Natural pathways 

Natural  pathways  include  geological  features  that  can  provide  either  a  conduit  to 
overlying and adjacent geological formations outside the targeted storage formation, or 
a conduit to the surface. The risk associated with some natural pathways will differ by 
trapping mechanism and geologic setting. It may include, for example, faults, variation in 
the caprock quality or a lack of adequate side seals. 

For sedimentary basin storage sites, additional considerations may include delineation 
of the structural trap (e.g. height of a spill point or characteristics of a migration pathway), 
and may extend to geochemical risks associated with the injection of acidic fluids (i.e. 
dissolution of caprock or seals). 

10  The level of completeness refers to the minimum magnitude above which all pertinent 

earthquakes in a certain area are reliably identified. This value may vary in time and space. 

22 

 
 
 
 
 
For mineralisation sites, pressure changes in the subsurface may lead to CO2  exsolving 
from  solution  before  the  minerals  are  formed.  Or  there  may  be  geochemical 
considerations  related  to  acidic  fluids  mobilising  previously  formed  carbon-bearing 
minerals.  Depleted  field  storage  sites  may  have  additional  caprock  risks  related  to 
compaction due to previous production activities (see Section 3.3.5). 

Human-made pathways 

Human-made pathways include wellbores and boreholes. As part of the characterisation 
of the storage complex, data should be collected to inform the status and condition of all 
existing wells within the storage complex, as well as all wells outside the storage complex 
that may represent conduits for formation fluids from the hydraulic unit. This data should 
be  evaluated  in  a  risk  assessment  context  to  assess the risk  of  leakage  along legacy 
wells  and  the  risk  of  movement  of  formation  fluids  and  CO2  charged  fluids  to 
environmental or economic receptors (risk related to principal effects 1-3 in GD 1). 

The  risk  assessment  of  relevant  legacy  wells  within  and  outside  the  storage  complex 
should be based on available data on the status and condition of all wells. It should follow 
the  guidance  provided  in  applicable  regulations,  guidelines  or  standards.  Relevant 
guidance is provided in the following sources: 

● 

ISO 27914:2017 Carbon dioxide capture, transportation and geological storage 
– Geological storage Clause 7.6 – Evaluation of wells, and Clause 7.8 – 
Abandonment of wells; 

●  DNV-RP-J203 Geological storage of carbon dioxide, Section 7 – Well 

qualification, 2021; and 

●  OEUK Guidelines: Well Decommissioning for CO2 storage, 202211. 

Guidance  issued  by  the  Petroleum  Safety  Authority  Norway  recommends  using  ISO 
27914:2017 Clause 7.6 and DNV-RP-J203 (2021) Section 7 to evaluate well barriers in 
existing  (legacy)  wells12.  All  the  above  guidance  documents  describe  a  risk-based 
approach to assess the need for risk mitigation, including monitoring, intervention and 
the  materials  or  well 
remediation.  Neither  document 
abandonment design, for example. 

is  prescriptive  regarding 

Allowing a risk-based, non-prescriptive approach is particularly important to enable cost- 
effective storage in depleted hydrocarbon fields. Depleted hydrocarbon fields may have 
many legacy wells within the storage complex and surrounding area. While all wells will 
need  to  be  thoroughly  assessed  on  a  case-by-case  basis,  the  need  to  perform 
monitoring,  intervention  or  remediation  should  be  determined  by  the  risk  assessment 
process and the evaluated performance of the well barriers in terms of zonal isolation. 

11 Additional guidance can be found in standards developed for hydrocarbon recovery activities, 
such as NORSOK D-010:2021, Well integrity in drilling and well operations, ISO 16530- 
1:2017, Petroleum and natural gas industries - Well integrity - Part 1: Life cycle 
governance; and NOGEPA ST045, Decommissioning of Wells. 

12 https://www.ptil.no/contentassets/272a208eb9a94b1f833737d378dd58d4/veiledning-til- 
forskrift-om-sikkerhet-og-arbeidsmiljo-ved-transport-og-injeksjon-av-co2-04.12.19.pdf. 

23 

 
 
 
 
 
The evaluation of well barriers should also factor in the exposure of the wells to CO2, 
other buoyant fluids or elevated pressure13. Taking a risk-based approach also enables 
operators to factor in different geological contexts and the reliance on physical trapping 
by well barriers. For instance, if a well can be demonstrated to not be exposed to free- 
phase CO2 or other buoyant fluids, e.g. if all injected CO2 is fully dissolved in the storage 
site, then the number of potential mechanisms which could lead to fluid migration along 
the well are limited (e.g. if pressure is elevated) and the related risks may be reduced. 

The key considerations for evaluating well barrier performance are requirements to the 
following elements: 

● 

● 

● 

● 

barrier materials 

number of barriers 

length of barriers 

position and placement of barriers. 

Guidance  on  each  of  these  elements  is  provided  in  the  OEUK  Guidelines:  Well 
Decommissioning  for  CO2  storage,  2022,  Section  3.  Regulations  may  also  stipulate 
requirements for each of these elements. 

For CO2  storage projects, specific considerations should be given to the chemical and 
mechanical impact that CO2 and CO2 charged fluids and elevated pressure may have on 
the  long-term  performance  of  the  well  barrier.  For  instance,  the  guidance  in  ISO 
27914:2017 states that geochemical modelling should be carried out to: 

● 

● 

‘evaluate  the  response  of  wells  to  geochemical  reactions,  including  cement 
and/or  casing  degradation,  which  may  lead  to  potential  flow  of  CO2  or  CO2- 
saturated formation fluid’; and 

‘evaluate the predicted pH and chemical composition of the fluids in contact with 
the cement sheath in order to select suitable cements and tubular metallurgy for 
new wells, or remedial materials for legacy wells, to resist chemical degradation'. 

Similarly, the guidance provided in ISO 27914:2017 states that geomechanical modelling 
should be carried out to evaluate the mechanical aspects of well integrity. 

A particular consideration related to the potential chemical or mechanical degradation of 
casing and cement is the potential for debonding of casing and cement, or corrosion of 
casing  by  CO2-charged  fluids  contained  in  the  annulus.  To  mitigate  this  risk  and  limit 
possible leak-paths in the wellbore zone, it is sometimes recommended that wells in CO2 
storage projects are decommissioned which may utilise equipment, such as ‘bridge’ or 
‘pancake’ plugs. This involves removing the casing along a section of the wellbore and 
placing cement to cover the exposed formations with a cement-to-rock contact across 
the borehole. 

While  this  decommissioning  procedure  may  reduce  the  risk  of  well  leakage,  it  is  not 
required by any of the CO2  storage-specific standards listed above. The need to carry 

13 Pressure sufficient to cause the flow of formation fluids along permeable conduits into 

environmental or economic resources above the storage complex. 

24 

 
 
out an intervention to place a bridge plug in existing wells based on a risk assessment 
of existing well barriers in the well, however, may be considered recommended practice 
for wells that are accessible or are to be decommissioned during the CO2 storage project. 
This  risk  assessment  should  factor  in  the  potential  long-term  exposure  to  reactive 
chemistries that can degrade casing and cement, and predicted changes to matrix stress 
and compaction or elongation strains along the wellbore. 

The  2022  OEUK  Guidelines:  Well  Decommissioning  for  CO2  storage  state  that  the 
storage site should be separated from the seabed or surface by two independent well 
barriers. However, it also indicates that wells without two well barriers may be accepted 
(without the requirement to perform well intervention or remediation) if a risk assessment 
concludes that the well leakage risk is insignificant, or it is demonstrated that a single 
well barrier ‘is as effective and reliable as the two barriers and is an appropriate method 
to achieve the objectives that two barriers would otherwise have provided’. 

If  the  risk  assessment  concludes  that  a  storage  site  is  suitable  for  CO2  storage  (per 
Article 4(4) of CCS Directive) contingent on the remediation of certain wells, then it is 
important that  the  storage  operator  can  demonstrate  a  high  chance  of  success  of the 
remediation  operation  (see  Section  7.5  of  DNV-RP-J203  (2021)).  Section  7.7  of  ISO 
27914:2017  provides  guidance  on  (recompletion  and)  workover  of  wells.  Workover 
operations  can  represent  a  very  substantial  cost  and  will  therefore  normally  be  done 
following  the  issuance  of  the  storage  permit  and  financial  investment  decision.  The 
storage permit may be granted contingent on the successful execution of the workover 
operation. 

The holder of a storage permit is responsible for ensuring adequate integrity of all wells 
during the life of the project until responsibility is transferred to the Member State, or the 
competent authority withdraws the storage permit. Following transfer or storage permit 
withdrawal, the competent authority is responsible for ensuring well integrity, including 
any  risk  mitigation  (e.g.  monitoring  or  well  remediation).  Competent  authorities  and 
storage operators should define and agree before the start of injection how any liability 
not  transferred  to  the  competent  authority  under  Article  18(1)  of  the  CCS  Directive  is 
dealt with for decommissioned wells. 

3.3.8.  Domains surrounding the storage complex that may 

be affected 

Annex  I  (Step  1 (h))  to the  CCS  Directive  indicates that  the  domains  surrounding  the 
storage complex that may be affected by the storage of CO2  in the storage site must be 
documented. The domains include the deep geosphere, the shallow geosphere, (marine 
or  surface)  biosphere,  and  the  atmosphere  (see  Table  1).  Subsurface  modelling  and 
other evaluation may be used to determine which domains may be affected. 

The  impacts  may  include  CO2  migration  into  oil  and  gas  reservoirs,  contamination  of 
groundwater  consumable  to  humans,  pressure  interactions  with nearby operations,  or 
potential  impacts  to  geothermal  operations.  The  documentation  should  include  site- 
specific characteristics and location, such as the location of the storage site, onshore or 
offshore. Additional groundwater and hydraulic connectivity considerations are outlined 
in Section 3.3.2, Hydrogeology. 

25 

 
 
 
Table 1: Definition of domains surrounding the storage complex. 

Domain 

Definition 

Atmosphere 

On the surface, the compilation of gases that make up the environment surrounding 
the storage complex. 

(Marine) 
biosphere 

The seawater column and the seabed for offshore projects, or the surface of the Earth 
including plant and animal life for onshore projects. 

Shallow 
geosphere 

Shallow subsurface down to the formation above the ultimate or secondary caprock. 
Includes groundwater zones above the injection zone. 

Deep geosphere 

Includes the injection zone, caprock(s), seal(s), and geologic formations below the 
storage complex. 

3.3.9.  Population distribution 

The location and concentration of populated areas above and adjacent to the storage 
site will be an important aspect to consider, particularly during the risk assessment and 
uncertainty  analysis.  The  competent  authority  will  need  to  consider  the  likelihood  of 
leakage combined with the potential impact prior to approval of a site. 

The local terrain (e.g. flat-lying, or low-lying valleys adjacent to storage sites) should be 
factored into the considerations of impact in the event of a leakage, and whether there 
is  a  likelihood  for  leaking  CO2  to  disperse  or  concentrate.  The  impact  of  potential 
contamination of groundwater from CO2 leakage should also be considered. This should 
be evaluated using hydrogeological studies (see Section 3.3.2). 

There will be a need to examine data in the assessment that identify land holdings, tenure 
and potentially site access. 

3.3.10. Proximity to valuable natural resources 

Operators must document the proximity of a storage complex to valuable surface and 
subsurface  natural  resources,  such  as  Natura  2000  areas  (under  Council  Directive 
79/409/EEC  of  2  April  1979  on  the  conservation  of  wild  birds14  and  Council  Directive 
92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and 
flora15), groundwater and hydrocarbons). 

The purpose of this documentation is to identify risks related to the exposure of natural 
resources  to  CO2 
leakage.  These  considerations  should  be  included  in  the  risk 
assessment (see GD 1), including potential exposure to leakage in the monitoring plan 
(see Chapter 5). 

14  https://eur-lex.europa.eu/legal-content/en/TXT/?uri=CELEX%3A32009L0147. 

15  https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A31992L0043. 

26 

 
 
 
 
 
 
 
 
 
 
3.3.11. Activities around the storage complex and possible 

interactions 

There are a range of potential competing activities around the storage complex that must 
be considered. Competition can arise from surface uses, pore space being used for other 
purposes,  and  the  fact  that  potential  leakage  of  CO2  may  affect  the  usability  of  other 
subsurface resources. 

Site  conditions  can  be  impacted  by  other  activities  surrounding  the  storage  complex, 
which are important to factor into the characterisation workflows. For example, pressure, 
temperature  and  chemical  changes  in  the  subsurface  due  to  nearby  operations  can 
impact  storage  site  conditions,  characterisation  outcomes,  injection  thresholds  and 
monitoring requirements. It is recommended that the competent authorities require, and 
establish  efficient  mechanisms  for,  information  sharing  amongst  operators  of  nearby 
activities to improve the characterisation process and risk management effectiveness: 

●  Groundwater. 

o  CO2 could contaminate groundwater resources. 

●  Geothermal. 

o  CO2  could impact the facilities both at the surface and in the subsurface 
thus increasing costs to the operators as they would have to strip off CO2 
and allow for changes in the materials of the infrastructure at a facility. 

o  CO2 

injection  could  increase  regional  reservoir  pressure  and  benefit 

geothermal production operations. 

●  Other CO2 storage sites. 

o  CO2 injection could lead to an increase in pressure in other storage sites, 
either  through  hydraulic  connections  or  through  leakage  into  other 
hydraulic units, reducing the capacity and injectivity of those sites. 

●  Other storage applications. 

o  This  includes  natural  gas  storage,  hydrogen  storage,  compressed  air 
storage,  and  other  subsurface  energy  storage  applications.  CO2  could 
reduce the pore space available for energy storage applications that may 
be used in conjunction with wind farms and other power generation. 

●  Offshore wind farms. 

o  Wind farms may provide access constraints for monitoring, maintenance 

and corrective action activities required at the storage site. 

●  Fishing. 

o  Fishing  activities  may  lead  to  access  constraints  for  monitoring, 
maintenance and corrective action activities required at the storage site. 

●  Subsea cables and pipelines. 

27 

 
 
o  Subsea  cables  and  pipelines  may  result  in  access  constraints  for 
monitoring,  maintenance  and  corrective  action  activities  required  at  the 
storage site. 

●  Salt mining and brine mining. 

o  CO2  could  impact  or  halt  mining  operations  by  creating  unsafe  mining 

conditions. 

●  Oil and gas field development. 

o  The CO2  plume could contaminate the production of hydrocarbons, thus 
increasing costs to the operators as they would have to strip off CO2  and 
allow for changes to the infrastructure at a facility. 

o  Conversely, CO2 injection could increase regional reservoir pressure and 

actually benefit oil and gas production. 

o  The  CO2  plume  could  reduce  the  pore  space  available  for  natural  gas 

storage reservoirs. 

●  Coal bed methane production. 

o  The CO2  plume could contaminate the production of hydrocarbons, thus 
increasing costs to the operators as they would have to strip off CO2  and 
allow for changes to the infrastructure at a facility. 

o  CO2  injection  could  increase  regional  reservoir  pressure  and  decrease 
production as methane can only be produced by producing the formation 
water to allow it to desorb off the coal. 

●  Coal mining. 

o  CO2  could have an impact on or halt coal mining operations and create 

unsafe mining conditions. 

●  Underground coal gasification. 

o  CO2 could extinguish underground coal gasification processes or limit the 

development of such operations. 

●  Energy/waste storage. 

o  CO2  injection  could  increase  regional  reservoir  pressure,  reducing  the 

injectivity of water or waste injection wells. 

o 

Injecting  CO2  or  other  fluid streams (such  as  wastewater  disposal)  can 
alter the geochemical makeup of reservoir fluids, resulting in dissolution 
or  precipitation  of  minerals  and  changes  to  fluid  movement  in  the 
reservoir. 

o  CO2 

injection  could  increase  regional  reservoir  pressure,  introducing 
leakage  pathways  that  compromise  energy  storage  or  waste  storage 
sites. 

The competent authority should provide the operator with timely guidance on which of 
the  items  listed  above  are  relevant  to  the  specific  storage  complex  at  the  time  of  the 
permit application. It will be in the final phases of the complex characterisation analysis 

28 

 
that  any  possible  interactions  can  be  identified,  and  an  assessment  can  be  made  to 
determine whether there is any likelihood that they pose a conflict of use. 

It is recommended that the assessment focus on known possible interactions at the time 
of the assessment, and not future activities that may take place at the site. The operator 
should document any such conflicts and include this documentation in the storage permit 
application sent to the competent authority. 

It is also recommended to maintain dialogue with the competent authority on any known 
planned activities surrounding the storage complex that could result in a conflict of use. 

There are monitoring practices and best practices to follow to reduce the likelihood of 
such conflicts. The most effective way to do so will be at the discretion of the authority 
that grants the permits to carry out subsurface and related activities in close proximity to 
each other, and/or to insist on agreements between operators of the various industries 
on how they conduct their activities and share information and planning. 

3.3.12. Proximity to the potential CO2 source(s) 

Annex I (step 1 (l)) to the CCS Directive indicates that the proximity of the CO2 source to 
the storage site must be documented, including estimates of the total potential mass of 
CO2 economically available for storage, and adequate transport networks. Site selection 
for geological storage of CO2 depends on the likely geologic storage volume and supply 
rates being identified at a very early stage in the selection process. Some sites may be 
commercially and technically viable at low rates of injection; others at high rates. Some 
sites will have large storage capacity; others only small. 

CO2  supply  issues  will  be  an  important  factor  in  the  size  and  nature  of  the  trapping 
mechanisms envisaged at any site and can impact the safety and containment of CO2. 
The geological characteristics will determine whether a site is compatible with injection 
requirements.  Reservoir  properties  and  modelling  activities  will  determine  the  CO2 
specification, numbers of wells, and be a factor in the commerciality of the storage site. 

Transport networks are an important consideration to ensure CO2  is safely delivered to 
the storage site in a condition that is compatible with the geological conditions (including, 
for  example,  the  fluid  phase,  the  chemical  makeup,  pressure  and  temperature). 
Transport  methods  can  have  significant  implications  on  site  development,  impacting 
injection infrastructure, capital requirements and project economics. 

Changing the CO2  supply conditions during a site characterisation process may affect 
the outcome of the process. While subsurface suitability of a storage site may have been 
proven,  it  may  not  be  possible  to  technically  or  commercially  meet  the  CO2  supply 
volumes and injection rates required. It is therefore recommended to ensure that limits 
have  been  tested  and  a  range  of  CO2  supply  conditions  given  as  part  of  the  site 
characterisation process. 

29 

 
 
 
3.4.  Building the three-dimensional static geological 

Earth model 

Building three-dimensional static geologic Earth model(s) is included in Step 2 in Annex 
1 to the CCS Directive. Step 2 consists of using the data collected in Step 1 to construct 
one or  more  three-dimensional  static  geological Earth  models  of  the  potential  storage 
complex,  including  the  caprock  (where  relevant,  see  Box  1)  and  the  hydraulically 
connected  areas  and  fluids.  This  should  be  done  using  numerical  reservoir  modelling 
software to characterise the complex in terms of the: 

1.  geological structure of the physical trap; 

2.  geomechanical,  geochemical  and  flow  properties  of  the  reservoir  overburden 
(caprock, seals, porous and permeable horizons) and surrounding formations; 

3.  fracture system characterisation and presence of any human-made pathways; 

4.  areal and vertical extent of the storage complex; 

5.  pore space volume (including porosity distribution); 

6.  baseline fluid distribution; 

7.  any other relevant characteristics. 

Box 1: Modelling the caprock. 

Annex I makes specific reference to a 'caprock' as a component in the 3D geological 
model.  This  term  refers  to  a  geological  formation  overlying  the  injection  zone  that 
effectively restricts the upward migration of free-phase CO2. For in the context of the 
purpose of the CCS Directive, the caprock should have sufficiently low permeability 
to deliver 'permanent containment of CO2  and prevent negative effects and any risk 
to the environment and human health' in the site and project- specific circumstances. 

Free-phase CO2  is buoyant and tends to migrate upward, making the presence of a 
caprock  an  important  component  of  containment  of  the  injected  fluid.  Conversely, 
dissolved CO2 in water (solubility trapping) is dense and usually sinks relative to native 
reservoir fluids. Mineralised CO2  is solid and does not migrate. 

In  sites  that  rely  on  trapping  mechanisms  where  the  CO2  does  not  exist  in  a  free- 
phase,  such  as  solubility  trapping  and  mineralisation,  our  interpretation  is  that  a 
caprock is not an explicitly necessary component of the storage complex and the 3D 
model. 

In these cases, the model(s) would need to provide sufficient evidence to demonstrate 
that  the  injected  CO2  remains  in  a  non-free  phase for  the  duration  of  the  injection, 
closure, and post-closure periods and that pressures associated with the injection of 
CO2  does  not  drive  CO2-bearing  fluids  to  the  atmosphere,  the  water  column  or  to 
domains where negative effects to the environment may occur. 

A risk management approach is recommended during the construction of the 3D geologic 
model(s) (e.g. ISO 27914:2017, DNV-RP-J203). Scenarios that reduce uncertainty and 
risk should be evaluated and considered, including sensitivities for input variables. Many 

30 

 
 
 
modelling  approaches may  be  used (i.e.  deterministic,  probabilistic),  and  the  selected 
methodology  should  be  completed  according  to  best  practices  at  the  time  of  model 
construction. 

The operator may need to consider models at different scales, such as basin/regional, 
subregional, or local (storage complex) level to adequately manage the address site risks 
adequately. Multiple models may also be required to assess different processes, such 
as a geochemical and geomechanical models, and should be integrated into a cohesive 
interpretation of the subsurface. 

Multiple  data  sources,  as  discussed  in  Section  3.3,  will  be  used  as  input  into  the 
construction  of the  geological  models.  The model(s) should contain sufficient  detail  to 
determine  the  geologic  and  fluid  properties  related  to  fluid  flow,  phase  changes, 
geochemical impacts and differences in capacity, injectivity and containment. The error 
associated with interpretations should also be assessed, such as the error involved in 
upscaling,  contouring  or  averaging.  For  instance,  a  probability  range  for  the  storage 
volume  and  closure  area  could  be  given  based  on  uncertainties  in  the  reservoir 
parameters  and  in  the  seismic  velocity  model  used.  Geostatistical  methods  are 
recommended to be used to populate rock and fluid properties away from control points, 
and  calibrated  to  measured  data.  The  use  of  analogue  data  is  recommended  to 
supplement  or  validate  measured  data,  where  available  and  appropriate.  Where 
fractures are a significant consideration related to capacity, containment, injectivity or a 
monitoring  plan,  it  will  also  be  necessary  to  build  a  fracture  distribution  model  that 
includes an assessment of multiple possible distributions and properties of the fractures. 

During the various stages of appraisal and development of a storage site, new and more 
reliable data will be acquired that affect critical matters associated with storage complex 
characterisation. There will be a need to iteratively update and review the geological and 
reservoir simulation modelling throughout the process of both the characterisation of a 
storage complex and during its operational phases. It will also be necessary to maintain 
and  manage  data  to  be  able  to  compare  iterations  of  the  site  characterisation  as  it 
evolves over the lifespan of the storage site. 

3.5.  Dynamic modelling 

3.5.1.  Characterisation of the storage dynamic behaviour 

When  building  the  dynamic  model(s)  (e.g.  ISO  27914  or  DNV-RP-J203),  it  is 
recommended that operators take a risk management approach. Scenarios that reduce 
uncertainty and risk should be evaluated and considered, including sensitivities for input 
variables.  Many  modelling  approaches  may  be  used,  including  deterministic  and 
probabilistic approaches. The selected methodology should follow best practice at the 
time  of  model  construction.  The  operator  may  need  to  consider  models  at  different 
scales,  such  as  basin/regional,  subregional,  or  local  (storage  complex)  level  to 
adequately address site risks. The dynamic model(s) will be used to demonstrate that 
injection operations pose no significant risk to human health and the environment. 

31 

 
 
 
 
Annex I, Step 3.1 specifies the requirements for storage dynamic modelling (see Table 
2).  The  modelling  scenarios  should  integrate  evaluations  of  capacity,  containment, 
injectivity, well placement and design, and monitorability. Additional factors may also be 
considered  depending  on  the  geologic  setting,  trapping  mechanism(s)  and  best 
practices. For example, it may be important to model the elevated pressure region over 
time when assessing the nature of CO2  flow in the reservoir, phase behaviour, the risk 
of fracturing the storage formation and caprock, and possibly the monitoring area. It may 
be  important  to  model  the  extent  of  the  CO2  to  identify  the  storage  complex  and  the 
monitoring area. 

Dynamic models may include a suite of approaches and methodologies, such as fluid 
transport  and  pressure  evolution,  thermodynamic  modelling,  reaction  kinetics,  and 
geochemical  fate  and  reactive  transport  models.  It  will  also  be  important  to  model 
pressures within and around the storage complex. For example, where solubility trapping 
is a primary containment mechanism, pressure modelling will be important to ensure the 
site conditions are suitable for long-term storage. Relative permeability is an important 
component to consider in the dynamic behaviour of a storage site as it has a significant 
impact on the distribution of CO2 in the subsurface. In all cases, the modelling must factor 
in  geologic  heterogeneity  and  account  for  a  range  of  possible  scenarios  (geologic 
uncertainty). Several iterations of modelling may be required to refine the models and to 
understand geologic uncertainty, which may incorporate outcomes of dynamic modelling 
into updated static models, and then revising dynamic outputs accordingly. 

History matching workflows are recommended to calibrate CO2  injection model results, 
where data are available.  Sources  of  these  data may include production/injection  test 
data, CO2  injection history, or fluid production histories from depleted field sites. 

Table 2: Characterisation of the storage dynamic behaviour. 

Factors  listed  in  Annex  1,  Step  3.1,  of  CCS  Directive:  Characterisation  of  the  storage  dynamic 
behaviour 

At least the following factors must be considered: 

– possible injection rates and CO2 stream properties; 

– the efficacy of coupled process modelling (how different individual effects in the simulator(s)) interact, such 
as interactions between plume migration, geomechanics, and geochemistry); 

– reactive processes (how the injected CO2 reacts with in-situ minerals feedback in the model); 

– the reservoir simulator used (multiple simulations may be required in order to verify certain findings); 

– short and long‐term simulations (to establish CO2 fate and behaviour over decades and millennia, including 
the rate of dissolution of CO2  in water). 

The dynamic modelling process must provide insight into: 

– pressure and temperature of the storage formation as a function of the injection rate and the accumulative 
volume of injection over time; 

– areal and vertical extent of injected CO2 stream and the elevated pressure zone, vs time; 

32 

 
 
 
– the nature of CO2 flow in the reservoir, including phase behaviour; 

– CO2 trapping mechanisms and rates (including spill points and lateral and vertical seals); 

– secondary containment systems in the overall storage complex; 

– storage capacity and pressure gradients in the storage site; 

– the risk of fracturing the storage formation(s) and caprock; 

– the risk of CO2 entry into the caprock; 

– the risk of leakage from the storage site (for example, through abandoned or inadequately sealed wells); 

– the rate of migration (in open‐ended reservoirs); 

– fracture sealing rates; 

–  changes  in  formation(s)  fluid  chemistry  and  subsequent  reactions  (for  example,  pH  change,  mineral 
formation) and inclusion of reactive modelling (kinetics, reactive transport, thermodynamics) to assess affects; 

– displacement of formation fluids; 

– increased seismicity and elevation at surface level. 

3.5.2.  Sensitivity characterisation 

Annex  I  Step  3.2  states  that  ‘multiple  scenarios  should  be  modelled  to  identify  the 
sensitivity  of  the  assessment  of  assumptions  made  about  particular  parameters. 
Significant sensitivities shall be taken into account in the risk assessment’. 

Any assumptions about a specific parameter that could create a risk of leakage and affect 
human  health  or  the  environment  should  be  considered  a  significant  sensitivity. 
Assumptions  or  interpretations  of  parameters  that  impact  estimates  of  capacity, 
injectivity, or monitorability may also be considered significant sensitivities. 

Sensitivity characterisation workflows may include tornado plots showing the sensitivity 
of  different  parameter  alterations  on  key  metrics  of  interest,  scenario  evaluations, 
probabilistic assessments that evaluate which scenarios or assumptions are reasonable, 
as well as the likelihood of those assumptions occurring. 

3.5.3.  Risk assessment 

The  process to  characterise  the  storage  site,  including  3D  static  geological  modelling 
and  dynamic  modelling,  is  closely  tied  to  the  risk  assessment  process  as  these 
components  aid  in  the  identification  and  sensitivity  analysis  of  leakage  pathways, 
secondary effects of CO2  storage, and risks to human health and the environment. 

Guidance document 1 provides detailed guidance on carrying out risk assessments in 
accordance with the requirements of Step 3.3 in Annex I to the CCS Directive. This is 

33 

 
 
 
 
 
without  prejudice  of  compliance  with  other  EU  regulations,  for  instance  the 
Environmental Impact Assessment Directive or the Nature Directive. 

34 

 
4.  CO2 composition 

4.1.  Legislative context 

Recital (27) in the CCS Directive states that ‘it is necessary to impose on the composition 
of the CO2  stream constraints that are consistent with the primary purpose of geological 
storage, which is to isolate CO2  emissions from the atmosphere, and that are based on 
the  risks  that  contamination  may  pose  to  the  safety  and  security  of  the  transport  and 
storage network and to the environment and human health’. 

Article 12 of the CCS Directive, which covers the criteria for CO2  streams for geological 
storage, sets out the requirements operators must follow with the aim of ensuring that 
the above objective will be met. It sets three requirements. 

1.  A CO2 stream must consist ‘overwhelmingly of carbon dioxide’. 

2.  No ‘waste or other matter’ may be added to the CO2  stream for the purpose of 

disposing this waste or other matter underground. 

3.  In addition to CO2, the CO2 stream may contain: 

a.  incidental substances associated with the (CO2  emission) source (this is 
dependent on the used feedstock and the industrial process), the capture 
process; or injection process; and 

b.  trace  substances  that  may  be  added  to  assist  in  monitoring  and 

verification of CO2  migration. 

Concentrations of such incidental and added substances must, however, be below levels 
that would: 

● 

● 

● 

adversely  affect  the  integrity  of  the  storage  site  or  the  relevant  transport 
infrastructure provided the CO2  composition is acceptable to all stakeholders; 

pose a significant risk to the environment or human health; or 

breach the requirements of applicable EU legislation. 

Member States must ensure that operators accept CO2  streams for storage only if their 
composition is analysed, including an analysis of the presence of corrosive substances, 
and a risk assessment has been carried out indicating that the levels of incidental and 
trace  substances  in  the  CO2  stream  are  acceptable,  as  defined  above.  In  addition, 
Member States must keep a register of the quantities and properties of the CO2 streams 
delivered and injected, including the composition of these streams. 

Several references provide information on the impact of enhanced CO2  concentrations 
in ambient air on human health and environment (e.g. Benson et al., 2002). 

35 

 
 
 
4.2.  CO2 stream consists overwhelmingly of carbon 

dioxide 

The ‘CO2  stream’ is a flow of substances that results from CO2  capture processes. The 
actual  CO2  concentration  is  only  one  of  several  factors  that  may  contribute  to  risk  to 
human health or the environment. Streams with, for instance, 70% CO2 and 30% N2 may 
have certain risk considerations, whereas streams with similar or lower percentages of 
H2S could have more significant risk implications. 

Therefore,  competent  authorities  must  assess  the  trade-off  between  the  cost  of 
additional CO2  stream purification, and the cost of managing risks to human health, the 
environment,  storage  sites  and  transport  infrastructure.  The  requirements  to  manage 
acceptable levels of concentrations of ‘non-CO2 constituents’ align with this objective and 
will be further discussed in Section 4.4. 

4.3.  No waste or other matter may be added for the 

purpose of disposal 

The intent of this requirement is to clarify that it is not permitted to co-inject ‘waste or 
other matter’ with the CO2  stream that can create an additional risk to human health or 
the environment. Here ‘waste’ means the substances defined as waste in Article 3(1) of 
Directive 2008/98/EC. 

The  co-injection  of  ‘other  matter’  can  be  allowed  if  this  is  necessary  to  deliver  safe 
geological storage of carbon dioxide under Article 1 and the following requirements are 
met: 

● 

● 

the CO2 stream is overwhelmingly carbon dioxide; and 

the  concentrations  of  added  substances,  as  defined  in  the  relevant  storage 
permit, are below levels that would adversely affect the integrity of the storage 
site  or  the  relevant  transport  infrastructure,  pose  a  significant  risk  to  the 
environment  or  human  health,  or  breach  the  requirements  of  applicable  EU 
legislation. 

Additional substances may be necessary for operational and monitoring purposes. They 
may include, for example, tracers added to monitor the behaviour of the injected stream, 
substances  added  to  condition  the  CO2  stream  (e.g.  to  treat  or  inhibit  scale),  or 
substances added to alter the properties of the CO2  stream in the reservoir (e.g. to alter 
viscosity  or  density).  These  substances  are  permissible  if  they  are  added  for  the 
purposes of ensuring the safe and effective injection of CO2, and provided they meet the 
requirements of Article 12 of the CCS Directive (see Section 4.4). 

Mineralisation  projects  have  demonstrated  that  CO2  can  be  stored  safely  provided  all 
injected  CO2  is  fully  dissolved  in  water  and maintained  in  dissolved  form  until  CO2  is 
mineralised through geochemical reactions with the host reservoir  rock.  This  typically 

36 

 
 
 
 
involves injecting approximately 25 parts water per part CO2  injected16. ‘Mineralisation’ 
CO2  storage operations are also permissible under the CCS Directive since the water 
injected is not considered to be part of the CO2  stream. 

4.4.  Acceptable concentrations of incidental and added 

substances 

Article 12 of the CCS Directive covers the acceptable level of non-CO2  constituents in 
the CO2 stream, i.e. constituents that stem from the ‘source, capture or injection process’ 
or substances added to help monitor and verify CO2  migration. Concentrations of such 
incidental or added substances are permitted if the requirements of Article 12(1)(a-c) are 
met. 

The  requirements  under  Article  12(1)(c)  mean  that  the  concentration  of  pollutants 
regulated  under  the  Integrated  Pollution  Prevention  and  Control  Directive,  the  Large 
Combustion Plants Directive or the Industrial Emissions Directive must comply with the 
limit  values  and  other  requirements  of  those  Directives,  including  the  use  of  best 
available techniques. 

To ensure compliance with Article 12(1)(a-b) and Article 12(3)(a), competent authorities 
must ensure that each operator carries out an analysis of the composition of the CO2 
stream and a risk assessment of potential impacts of the CO2 stream (including incidental 
or  added  substances)  to  ‘the  integrity  of  the  storage  site  or  the  relevant  transport 
infrastructure’ or the ‘environment or human health’. The risk assessment should include 
assessment of the potential for CO2 stream compositions outside nominal specifications. 
Variations in CO2  compositions may occur in multiple situations, including: 

● 

transient composition variations due to start-up or shut-down of capture facilities; 

●  mixing  of  CO2    streams  from  multiple  sources,  in  transport  vessels  or 

infrastructure, or prior to injection; 

●  CO2 shipping with boil-off or different cargo during ship operations. 

This also implies that the location and frequency of measurement of CO2  composition 
should consider the potential for such variations. 

The CCS Directive does not lay down specific requirements regarding the locations of 
CO2  stream measurement, nor the frequency of measurements. It is recommended that 
the locations and frequency of measurements of the CO2 stream are informed by the risk 
assessment, to ensure that the CO2  stream composition is within safe operational limits 
based on system design. This also implies that there may be a trade-off between more 
frequent  measurements  or  for  instance  measurements  on  each  wellhead  and  a  more 
robust  system  design  or  other  risk  controls  to  manage  the  risk  to  storage  integrity, 
transport infrastructure, the environment and human health. 

16 Snæbjörnsdóttir, S.Ó., Sigfússon, B., Marieni, C. et al. (2020), ‘Carbon dioxide storage 

through mineral carbonation’., Nature Reviews Earth Environ 1, 90-102. 

37 

 
 
 
 
 
 
Constituents in the CO2 stream can generally be divided into three categories: 

1. 
SOX, Glycol, and H2  (can cause embrittlement); 

constituents that can have an impact on corrosion, such as O2, H2O, NOX, 

2. 
NOX, SOX, and Hg; 

constituents that can pose a concern for human safety, including CO, H2S, 

3. 
including: 

constituents that could affect the physical properties of the CO2  stream, 

o  H2O – the risk of hydrates; 

o 

inert  gases  -  impact  on  vapour  pressure  during  the  liquid  phase  and 
impact on critical pressure during the dense phase; 

o  non-corrosive constituents that  could  precipitate as  solids  – leading to 

flow assurance problems; 

o  non-corrosive constituents that could freeze and cause blockage issues. 

Therefore, the risk assessments to address Article 12(1) (a) and (b) of the CCS Directive 
should factor in each of the constituents and their potential to affect the integrity of the 
storage site and transport infrastructure or represent a significant risk to the environment 
or  to  human  health.  Various  references  describe  issues  related  to  components  or 
impurities  of  a  CO2  stream  (DNV-RP-F104;  ISO  27921;  ZEP  and  CCSA  2022,  and 
others). 

Operators also need to keep a register of the quantities and properties of the CO2 stream 
delivered  and  injected,  including  the  composition  of  those  streams.  While  pipeline 
operators are likely to impose CO2 stream composition standards to protect the physical 
integrity and flow characteristics of the pipes, the competent authority must approve the 
composition of the CO2 stream as it affects pipeline integrity and storage integrity as part 
of the storage permit. 

In determining an acceptable composition of the CO2  stream, operators and competent 
authorities  could  consider  optimising  the  composition  across  the  integrated  capture, 
transport  and  storage  chain.  Overall  optimisation  involves  examining  the  trade-off 
between  reducing  components  in  a  CO2  stream  across  the  value  chain,  the  cost  of 
removal of components, designing the infrastructure to tolerate the composition of the 
stream and ensuring the integrity of the storage site is not adversely affected. 

In the event of significant irregularities in the CO2  stream composition during operation, 
corrective measures would have to be taken on a case-by-case basis. This would include 
an  analysis  of  the  causes  of  the  irregularity  and  the  impact  of  the  injection  of  the 
inappropriate stream into the storage site. 

38 

 
5.  Monitoring 

This section provides guidance on the allocation of monitoring responsibilities. While the 
operator  is  responsible  for  monitoring  the  storage  site  operations  and  surrounding 
environment  under  Article  13(1)  of  the  CCS  Directive,  the  competent  authorities  in  all 
relevant Member States will carry joint responsibility for transboundary monitoring. Data 
sharing is encouraged to ensure that adjacent storage sites are accurately characterised 
and monitored, especially across borders. 

One of the aims of monitoring is to demonstrate that the injected CO2 is contained in the 
storage site. Monitoring activities will need to take place within the storage site as well 
as within the storage complex to identify any early warning signs of a potential leakage. 
Monitoring  may  also  need  to  take  place  in  the  surrounding  area  to  prevent  negative 
effects to the environment or human health. 

A risk assessment should be carried out to determine the significance of associated risks. 
The results should then feed into the design of the monitoring of the storage complex 
and surrounding environment under Article 13 of the CCS Directive. 

5.1.  Legislative context 

Monitoring  is  required  by  Article  13  of  the  CCS  Directive  to  ensure  the  safety  of 
geological storage. The requirements are as follows. 

●  Article 13(1): Member States shall ensure that the operator carries out monitoring 
of the injection facilities, the storage complex (including where possible the CO2 
plume), and where appropriate the surrounding environment for the purpose of: 

a)  comparison  between  the  actual  and  modelled  behaviour  of  CO2  and 

formation water, in the storage site; 

b)  detecting significant irregularities; 

c)  detecting migration of CO2; 

d)  detecting leakage of CO2; 

e)  detecting  significant  adverse  effects  for  the  surrounding  environment, 
including in particular on drinking water, for human populations, or for users 
of the surrounding biosphere; 

f)  assessing the effectiveness of any corrective measures taken pursuant to 

Article 16; 

g)  updating the assessment of the safety and integrity of the storage complex 
in the short- and long-term, including the assessment of whether the stored 
CO2  will be completely and permanently contained. 

●  Article 13(2). The monitoring shall be based on a monitoring plan designed by 
the operator pursuant to the requirements laid down in Annex II, including details 
on  the  monitoring  in  accordance  with  the  guidelines  established  pursuant  to 
Article 14 and Article 23(2) of Directive 2003/87/EC, submitted to and approved 

39 

 
 
 
 
by the competent authority pursuant to point 6 of Article 7 and point 5 of Article 9 
of  this  Directive.  The  plan  shall  be  updated  pursuant  to  the  requirements  laid 
down in Annex II and in any case every five years to take account of changes to 
the assessed risk of leakage, changes to the assessed risks to the environment 
and human health, new scientific knowledge, and improvements in best available 
technology. Updated plans shall be re-submitted for approval to the competent 
authority. 

Annex II sets out the criteria for establishing and updating the monitoring plan and for 
post-closure  monitoring.  Directive  2003/87/EC  is  the  EU  ETS  Directive  establishing  a 
scheme for greenhouse gas emission allowance trading within the EU. Article 14(1) of 
the EU ETS Directive sets out that the Commission must adopt guidelines that describe 
detailed arrangements concerning the monitoring and reporting of emissions for activities 
included  in  Annex  I  to  the  Directive.  These  rules  are  set  out  in  the  Monitoring  and 
Reporting Regulation (MRR) 2018/2066/EU, Annex IV, Section 23. Article 14(2) of the 
CCS Directive sets out that Member States must ensure the emissions are reported in 
accordance with the guidelines. 

The aim of this section is to provide operators and competent authorities with guidance 
on  interpreting  the  requirements  for  the  monitoring  plan  under  Annex  II,  and  on  the 
requirements for monitoring and reporting in accordance with the MRR. 

5.2.  Establishing the monitoring plan 

5.2.1.  Approach 

The initial monitoring plan must be part of the storage permit approved by the competent 
authority.  The  competent  authority  is  obliged to ensure that  the  operator  monitors the 
injection  facilities,  the  storage  complex  (including  where  possible  the  injected  CO2 
stream  and  its  migration),  and  where  appropriate  the  surrounding  environment  during 
the operational phase and after closure until transfer of responsibility. If any leakages or 
significant  irregularities  are  detected,  monitoring  must  be  intensified  as  required  to 
assess the attribution/origin of the CO2, the scale of the problem and the effectiveness 
of corrective measures. 

The main objectives and purpose of monitoring are to confirm containment of CO2, to 
alert  of  any  increased  leakage  risk,  to  identify  any  leakage  if  it  occurs  and  significant 
irregularities, and to verify the behaviour of the injected CO2. The competent authority 
should  ensure  that  all  monitoring  activities  are  based  on  site-specific  plans  that  have 
been  agreed  and  approved  by  it  as  part  of  the  storage  permit  and  following  the 
requirements laid down in Annex II to the CCS Directive. The monitoring plan must be 
updated regularly and at least every five years. 

Monitoring and associated monitoring plans are recommended to: 

● 

be risk-based, linked to the risks identified in the site characterisation process 
and the overall risk assessment; 

● 

be specific to the storage site, storage complex, and trapping mechanism(s); 

40 

 
 
 
 
●  be sufficiently extensive to cover the storage complex (including where possible 
the  CO2  plume),  migration  and  behaviour  of  formation  fluids  and  where 
appropriate the surrounding environment; 

● 

● 

● 

ensure that monitoring is linked to preventive and corrective measures; 

use technology based on the best available at the time of design; 

include  regular  and  routine  reporting  of  monitoring  data  and  interpretations  of 
results; 

●  be  regularly  updated  to  take  account  of  changes  to  the  assessed  risks  to  the 
environment and human health, new scientific knowledge, and improvements in 
best available technology; 

● 

● 

● 

set  detection  limits  such  that  significant  irregularities  and  leakages  can  be 
identified and corrected before they lead to negative impacts on human health 
and the environment, including consideration for uncertainties; 

be ready to be operational on or before the first day of injection; 

include redundancies and/or contingency plans for high-risk scenarios. 

The  starting  point  for  developing  and  updating  any  monitoring  plan  is  an  adequate 
characterisation  and  risk  assessment.  Guidance  on  both  risk  assessment  and  site 
characterisation are covered in GD 1 and Section 3 of this document. Following the CO2 
storage lifecycle risk framework, the risk assessment will result in site-specific criteria for 
monitoring  requirements.  It  may  include  threshold  values  for  adopting  a  range  of 
preventive or corrective measures, some of which will include monitoring plans. 

The CCS Directive requires that competent authorities ensure that the injection facilities, 
storage complex and surrounding environment are monitored for the purposes listed in 
Article 13(1) (see Section 5.1). Annex II to the Directive also specifies that the monitoring 
plan must in any case include continuous or intermittent monitoring of the following items, 
considered mandatory: 

● 

fugitive emissions of CO2 at the injection facility; 

●  CO2 volumetric flow at injection wellheads; 

●  CO2 pressure and temperature at injection wellheads (to determine mass flow); 

● 

● 

chemical analysis of the injected material; 

reservoir  temperature  and  pressure  (to  determine  CO2  phase  behaviour  and 
state). 

In addition to the above list, continuous or intermittent monitoring of the pressure in the 
injection wellbore annulus is recommended. 

The Directive does not specify the measurement methods or technologies that should 
be considered or used for monitoring. It does, however, provide some general guidance 
on the technologies to consider and use as appropriate (see Annex II to CCS Directive): 

● 

technologies that can detect the presence, location and migration paths of CO2 
in the subsurface and at surface; 

41 

 
● 

● 

technologies  that  provide  information  about  pressure-volume  behaviour  and 
areal/vertical  distribution  of  the  injected  CO2  fluid  to  refine  numerical  3D 
simulation to the 3D geological models of the storage formation; 

technologies that can provide a wide areal spread in order to capture information 
on  any  previously  undetected  potential  leakage  pathways  across  the  areal 
dimensions  of  the  complete  storage  complex  and  beyond,  in  the  event  of 
significant irregularities or migration of CO2  out of the storage complex. 

This  can  be  achieved either  by measuring the  absence  of  any  leakage through  direct 
detection methods, or by verifying indirectly that the CO2  is behaving as expected in the 
reservoir  based  on  static  and  dynamic  modelling  and  updating,  corroborated  by 
monitoring data. The main challenge for measuring the absence of any leakage consists 
of spatial and temporal coverage of the monitoring method, i.e. ‘Where and when do we 
need to monitor in order to be sure that no leakage occurs’. The strategy should therefore 
be based on the risks identified. 

For indirect model-based monitoring, the emphasis is more on scenario confirmation. As 
long as predictive models behave in line with the monitoring data, the understanding of 
both  the  processes  occurring  and  the  behaviour  of  the  storage  complex  can  be 
considered sufficient. If there are deviations, the causes of the deviations must be found. 
If  there  is  a  significant  deviation  between  observed  and  predicted  behaviour,  the  3D 
model must be recalibrated to reflect the observed behaviour as described in Chapter 2 
of this GD and in GD 3. If, however, the deviations fall well beyond the uncertainty ranges 
of the predictive models, then additional monitoring and possibly preventive or corrective 
measures may need to be taken. 

It is also important to check the methods and techniques against the main objectives and 
different elements of the storage system and monitoring plan at the specific site. Figure 
2  provides  an  overview  of  the  possible  components  of  a  monitoring  plan  but  it  is  not 
prescriptive.  The  plan’s  elements,  objectives  and  technologies  should  be  site-specific 
and risk-based; they are also likely to vary through the project life cycle. 

At present, there is no technical measurement that provides a full quantitative analysis 
of CO2 leakage. Therefore, a portfolio of methods is likely to be required to adapt to each 
specific storage complex. The methods and plan should also cover the formation waters 
and brine within the storage complex and in surrounding units that may be impacted by 
injection or leakage. 

Figure 2: Monitoring plan components. 

42 

 
Operational 

Plume 

Pathways 

Injection well 
control 
Pressure & 
temperature 
Composition 
Quantification 

Caprocks 
Faults & 
fractures 
Wells 
Aquifers 

Calibrate models 
Migration 
Kinetics 
Trapping 
mechanisms 
Trapping 
efficiency 
Pressure 
Water behaviour 

Environmental 
(leakage) 

Detection of 
suspected leak or 
anomaly 

Attribution of 
anomaly 
Leak 
quantification 
Emissions/ 
ETS impact 
Safety & 
environmental 
impacts 

The above components can be factored into the plan in terms of: 

● 

operational monitoring (which must meet the mandatory requirements); 

●  monitoring the plume, including: 

o 

tracking the injected CO2 and its movement; 

o  water/brine  behaviour,  properties  and  movement  resulting  from  CO2 

injection; 

●  monitoring pathways for potential leakage identified by risk assessment, i.e.: 

o  caprocks; 

o 

faults and fractures; 

o  wells (and well integrity); 

o  overlying aquifers. 

● 

environmental monitoring for leaks out of the storage complex towards, at or near 
the surface, on land or offshore: 

o  detection of suspected leakage anomaly; 

o  attribution of leakage anomaly; 

o  quantification of leakage; 

o  accounting  and  quantifying  emissions  from  the  storage  complex  for 
surrender  of  emissions  trading  allowances  for  any  leaked  emissions 
under EU ETS Directive 2003/87/EC; 

o  safety and environmental impacts. 

Although  the  CCS  Directive  does  not  specify  the  well  design  requirements,  Member 
States are recommended to define design requirements based on industry best practice. 

43 

 
 
 
 
 
 
 
 
Well design best practices have been established to limit the risk of leakages, blow-outs, 
and to protect fresh waters. 

There are also special considerations to be made when selecting materials in CCS wells 
tied to the acidic nature of CO2  in the presence of water, and to the phase behaviour of 
CO2  streams in different pressure and temperature systems (IEAGHG 2018, Iyer et al 
2022, US EPA 2012). 

5.2.1.1.  Monitoring technology & scientific status 

Operators and competent authorities should take account of the state of development of 
the  technologies  considered  and  whether  it  is  proven  commercial  technology, 
developmental or at the research stage. This should also factor in whether the method 
and specific technique is proven for use in CO2 storage and/or other relevant applications 
(e.g. oil and gas, hydrogeology, environmental monitoring, geothermal and mining). 

The design of monitoring plans at the time of the storage permit should reflect current 
best practice and the state of technology. When the monitoring plan is reviewed at a later 
stage, it should incorporate any learning and experience from actual storage projects, 
new scientific knowledge, and improvements in best available technology. 

5.2.1.2.  Overall monitoring limitations 

Each monitoring method has limitations to its potential application, sensitivity and use in 
CO2  storage, and there may be limitations in the applicability of specific methods at any 
given site. Monitoring methods and technologies should be demonstrated to be suited to 
the site-specific conditions and monitoring goals through analogue examples and/or site- 
specific testing. 

The additional considerations are relevant to operators and competent authorities as part 
of any monitoring strategy: 

●  Which methods are relevant for the specific site? 

●  What is the resolution of monitoring in detecting leakage? 

●  How accurately can leakage be quantified? 

●  What volumes of CO2 can be resolved in the plume or geosphere? 

● 

If continuous monitoring is considered,  what is the operational lifespan of the 
system? 

●  Does the monitoring system provide data in real time, accessible and usable also 

by third parties (e.g. researchers, citizens)? 

●  Does the monitoring system allow early warning? 

44 

 
 
 
 
 
5.2.2.  Scope and format of monitoring plans 

5.2.2.1.  Storage complex summary 

The  starting  point  for  developing  the  monitoring  plan  is  the  site  characterisation, 
modelling and risk assessment. The general requirements for both site characterisation 
and risk assessment are given in the other chapters of this GD. The monitoring plan, in 
turn, must be related to the corrective measures plan. 

The  storage  complex  may  be  described  differently  depending  on  the  trapping 
mechanism(s) used and the storage type. See Section 3.3.1 for a discussion on the site- 
specific considerations for the storage complex. 

The site-specific information in the monitoring plan should include: 

● 

location  and  geographical  considerations 
considerations, population centres, land use and potable aquifers); 

(e.g.  onshore/offshore, 

local 

●  overview  of  site/complex  location  and  geological  characterisation,  including 
including 

trapping  mechanism(s);  summary  of 

identified  risks, 

reservoir, 
pathways and potential impacts; 

● 

● 

● 

expected plume behaviour over time from simulation modelling; 

identification of leakage pathways in relation to the plume migration; 

justification for selecting each technology or methodology to tackle the specific 
risks. 

5.2.2.2.  Defining the monitoring area 

The storage complex and the surrounding area combined will normally encompass the 
monitoring area (see definition for surrounding area in GD 1). The monitoring area should 
be  sufficiently  extensive  to  cover  possible  routes  to  the  surface  through  identified 
potential  pathways for the specific  sites,  as these  may  be  offset from  the storage  site 
(Figure 3). This may include areas in hydraulic communication (the hydraulic unit) where 
negative impacts can occur, even if located at a distance outside the storage complex. 

If  CO2  migrates  from  a  storage  reservoir  (a)  via  an  undetected  fault  into  porous  and 
permeable  reservoir  rock  (b),  it  may  be  transported  by  buoyancy  towards  the  ground 
surface at point (c). This may result in the emission of CO2 at the ground surface several 
kilometres from the site itself at an unknown time in the future. 

Modelling  should  be  used to project the  extent  of  potential  impacts  of  CO2  storage  in 
saline aquifers, such as pressure increases and formation water displacement so that 
monitoring  plans  can  be  designed  to  tackle  these  impacts  as  necessary.  Monitoring 
should also be considered to detect CO2 movement into aquifer formations between the 
main storage reservoir and the surface, especially for the protection of drinking water. 

45 

 
 
 
 
 
Figure 3: Illustration of potential leakage from a storage site (IPCC). 

In some cases, different operators working in proximity may have overlapping monitoring 
footprints  or  alternative  uses  of  the  subsurface.  This  overlap  in  monitoring  footprints 
should not, however, extend to the areas where CO2  plumes can be expected to occur. 
Where  this  occurs,  the  operators  and  competent  authorities  should  work  together  to 
ensure the monitoring plan can be effectively implemented. 

5.2.2.3.  Plan description 

A monitoring plan drawn up by the operator must meet the following requirements under 
the CCS Directive. 

The monitoring plan must detail the monitoring to be carried out at the main stages of 
the project, including baseline, operational and post-closure monitoring. 

The following must be specified for each phase: 

● 

● 

the parameters monitored; 

the monitoring technology employed and justification for technology choice; 

●  monitoring locations and spatial sampling rationale; and 

● 

the frequency of application and temporal sampling rationale. 

At a minimum, the plan must include continuous or intermittent monitoring of the following 
items, under Annex II: 

● 

fugitive emissions of CO2 at the injection facility; 

46 

 
 
 
 
 
 
●  CO2 volumetric flow at injection wellheads; 

●  CO2 pressure and temperature at injection wellheads (to determine mass flow); 

● 

● 

chemical analysis of the injected material; 

reservoir  temperature  and  pressure  (to  determine  CO2  phase  behaviour  and 
state). 

Additional parameters are likely to be needed in the monitoring plan, depending on the 
storage  type,  the  trapping  mechanism  and  site-specific  risks.  Mineralisation  sites,  for 
example, may require geochemical sampling and geochemical modelling to accurately 
evaluate  containment  and  conformance.  Box  2  provides  additional  considerations  for 
monitoring plans related to mineralisation sites. 

These measurements and related triggers or thresholds are very important in providing 
early  indications  of  any  anomalous  behaviour  and  enable  preventive  measures  to  be 
taken before any leakage occurs. 

The choice of monitoring technology must be based on the best practice available at the 
time  of  design.  The  following  options  should  be  considered  and  used  as  appropriate 
(Annex II): 

● 

● 

● 

technologies that can detect the presence, location and migration paths of CO2 
in the subsurface and at the surface; 

technologies  that  provide  information  about  pressure-volume  behaviour  and 
areal/vertical distribution of the CO2  plume to refine numerical 3D simulation of 
the 3D geological models of the storage formation established under Article 4 and 
Annex I to the CCS Directive; 

technologies that can provide a wide areal spread in order to capture information 
on  any  previously  undetected  potential  leakage  pathways  across  the  areal 
dimensions  of  the  complete  storage  complex  and  beyond,  in  the  event  of 
significant irregularities or migration of CO2  out of the storage complex. 

In addition to the technology considerations highlighted in the CCS Directive, techniques 
for water sampling and analysis are also recommended. Saline water from the injection 
zone, reacting to higher pressures and imperfections in the caprock or well cement, can 
breach  the  caprock  endangering  shallower  drinking  water  sources  and  indicating  a 
pathway whereby the CO2  plume might later escape. 

Therefore, it may also be appropriate to monitor for relevant chemical changes, such as 
the salinity (resistivity), pressure and temperature of fluids above the caprock to detect 
such fluid movements. Water sampling and analysis may also need to be monitored and 
factored into geochemical modelling. 

Different categories of monitoring can be set out: 

●  Mandatory  monitoring  (for  all  sites).  Some  parameters  to  be  monitored  are 
mandatory in the CCS Directive. These parameters are important for operational 
monitoring,  and  will  provide  some  important  and  continuous  measurements 
relating  to  plume  behaviour  and  potential  leakage  pathways  (e.g.  formation 
pressure data). 

47 

 
●  Required  (site-specific) monitoring.  Site-specific monitoring activity  is  designed 
to  collect  evidence  for  containment  in  the  reservoir  and  to  demonstrate  the 
integrity  of  seal,  fault  and  wells  at  the  specific  site.  This  will  build  on  the  risk 
assessment during site characterisation in order to manage site-specific risks and 
uncertainties. 

●  Optional contingency monitoring. Contingency monitoring systems are only used 
in  the  event  of  irregularities  or  system  failures.  Under  the  CCS  Directive,  a 
‘significant irregularity’ is defined as ‘…any irregularity in the injection or storage 
operations or in the condition of the storage complex itself, which implies the risk 
of a leakage or risk to the environment or human health’. 

Box 2: Monitoring plan description for mineralisation sites. 

Mineralisation sites may initially rely on solubility trapping initially, as it can may take 
two years or more for the injected CO2 to fully mineralise. Ensuring the CO2  remains 
dissolved in water is therefore essential to demonstrating containment. Pressure and 
temperature  monitoring  of  the  injection  zone  is  recommended  to  be  a  significant 
component of monitoring at mineralisation sites to ensure the CO2 remains trapped in 
solution, unless a secondary (i.e. structural) trapping mechanism is also present. 

5.2.2.4.  Baseline surveys 

The scope of baseline surveys will depend on the availability and type of data over the 
specific site, storage complex and surrounding area, including data acquired before and 
during site characterisation and as part of any environmental impact assessment. 

Baseline measurements may be considered as follows: 

● 

● 

● 

● 

formation  gas  and  fluid  characteristics  in  the  storage  reservoir,  surrounding 
complex  and  formations  that  might  be  affected  by  potential  leakage,  including 
aquifers; 

background CO2 emissions at surface or sea floor; 

surface and near-surface environmental surveys; 

seabed,  surface  or  near-surface  baseline  surveys  to  define  any  pre-existing 
leakage indicators; 

●  ground surface surveying, e.g. where ground movement monitoring is expected 

to be beneficial and/or in areas of ground movement risk; 

● 

natural seismic activity (earthquakes). 

The  extent  of  sampling  for  a  baseline  measurement  will  depend  on  the  risk  to  be 
considered and site-specific conditions. For example, there may be seasonal variability 
in  CO2  concentrations  at  the  surface  or  sea  floor,  and  baseline  surveys  may  want  to 
consider sampling in all seasons and various types of attribution monitoring (see Box 3). 
The  characterisation  of  natural  seismic  activity  in  a  site  or  region  may  require  data 
collection over a longer timeframe. 

48 

 
 
 
 
Depleted  hydrocarbon  fields  may  require  data  acquisition  to  demonstrate  a  ‘new’ 
baseline after production operations cease. Such data acquisition needs to be managed 
as described in Section 3.3. 

Box 3: Attribution monitoring. 

The aim of attribution monitoring is to differentiate naturally occurring CO2  from CO2 
that  originated  from  storage  operations.  Natural  processes,  such  as  the  decay  of 
organic  matter,  dolomitisation,  volcanic  activity/migration  of  magmatic  CO2  through 
dikes  and  sills,  wildfires  or  other  process  can  generate  CO2.  This  is  a  key 
consideration  in  baseline  monitoring  to  distinguish  natural  CO2  from  leaked  CO2. 
Geochemical  monitoring  methods  including  tracers  can  sometimes  be  used  to 
attribute CO2  to its source (Zero Emissions Platform 2022 and Dixon et al 2015). 

5.2.2.5.  Detailed plan format 

A  template  for  a  monitoring  plan  is  proposed  in Table  3.  The  plan  should  include  the 
following. 

●  Parameters to be monitored (e.g. Column 1). These parameters follow both from 
the mandatory monitoring obligations as stipulated by the CCS Directive and from 
the risk assessment. The latter parameters will be highly site-dependent. 

●  The  technique  that  will  be  used  to  measure  the  parameter  (Column  2),  with  a 
more  detailed  description  of  the  technique  provided  outside  the  table.  Site- 
specific  issues  especially  need  to  be  clarified  in  an  accompanying  text.  For 
example, the description would encompass the acquisition parameters. 

●  The category of monitoring: mandatory, required, contingency (Column 3). 

●  Frequency of measurement (Column 4). 

●  Spatial coverage (Column 5) of the data acquisition planned in each phase of the 
project (pre-injection, injection and post-injection including long-term stewardship 
after  transfer  of  responsibility).  The  rationale  behind  the  monitoring  strategy 
should be described in an accompanying text. 

●  The  expected  accuracy of  the  monitoring  method  and  of  expected  values  that 

indicate normal behaviour (Column 6). 

●  Threshold  alert  values  where  predicted  normal  behaviour  stops  and  where 
potentially  anomalous  measurements  occur  (Column  7).  The  assessment  of 
threshold alert values is closely linked to the outcomes of the site characterisation 
and  risk  assessment  should  follow  storage permit  award,  as well  as  execution 
and  synthesis  of  baseline  monitoring.  As  long as  the  measured  values remain 
below the threshold values (Threshold 1), no actions are required (green column). 
If  the  values  exceed  the  threshold  values,  specific  preventive  actions  may  be 
defined and adopted when the alert value is exceeded. This stage is considered 
as an increased alert phase, where behaviour starts to deviate from expectations. 
For example, it could lead to recalibration of the models and to more stringent 
measures  if  the  behaviour  persists.  The  triggering  of  alerts  might  also  be 

49 

 
 
 
 
dependent  on  how  measurements  taken  at  different  locations  and  times 
corroborate or contradict each other. 

●  Contingency  values  and  actions  (Column  8).  If  the  monitoring  measurements 
values exceed the identified threshold coloured red, which would indicate either 
a significant irregularity or leakage, the highest alert phase starts and immediate 
actions  (or  corrective  measures)  are  required,  as  specified  in  the  second  sub- 
column. 

Table 3 provides an indicative example, not a minimum requirement for monitoring all 
sites, as monitoring plans should be site-specific and tied to the risk assessment. The 
example  provided  shows  several  parameters  in  the  template  such  as  injection  rate, 
injected gas composition and fault integrity. Note that more than one monitoring method 
may be selected for each parameter. At the operator’s discretion, the table can be further 
subdivided  to  describe  the  different  risks  and  elements  to  be  monitored  (for  example 
operations,  caprock,  well  leakage  pathway,  plume).  Monitoring  techniques  and 
frequency should be established in accordance with best practice at the time of drafting 
the monitoring plan or update. 

The operator should also connect the chosen monitoring methods to the risks identified 
in the risk assessment and provide a rationale for the choice of method. It should list the 
risks identified in the risk assessment analysis. It should relate the chosen monitoring 
method to the risks they address, recognising that one method can cover more than one 
risk and that a single risk may require more than one method. It should describe why 
each method is considered appropriate to manage a specific risk from both a technical 
and a cost-efficiency point of view. 

50 

 
Table 3: Proposed monitoring plan template with example information. Additional information may be included in the monitoring plan as described throughout Section 5, such as an 
explanation of how monitoring ties to risk treatment, a justification for the choice of each technology or methodology to address specific risks, and the requirements set by the 
competent authority or local jurisdictions. 

Category of monitoring 

Project phase and frequency 

Normal situation 

Alert value 

Mandatory 

Required 

Contingency 

Pre-inj. 

Inj. 

Post-inj. 

Long-term 
stewardship 

Expectation 
value 

Accuracy 

> Threshold 
1 

Action** 

Contingency value 
(significant irregularity) 

> 
Threshold 
2 

Contingency 
measure*** 

Parameter 
to be 
monitored* 

Technique 
adopted 

Injection rate 

Flow meter 

Pressure 

Pressure device 

Temperature 

Thermometer 

Injected gas 
composition 

Gas samples 

Fault 
integrity 

Well integrity 

Microseismic 
monitoring 

Repeated  3D 
seismic; fibre- 
optic sensors 

Aqueous 
chemistry (CO2, 
pH) 

Annular 
pressure 

Wireline logging 

Optical well 
logging 

Cement bond 
logging 

Geophones 
behind the 
casing of a well 

Baseline 
data 

Baseline 
data 

Baseline 
survey 

x 

x 

x 

x 

x 

x 

x 

x 

x 

x 

Cont. 

Cont. 

Cont. 

Every year 

Cont. 

Cont. 

Every year 

Location 

Wellhead 

Wellhead + 
downhole 

Wellhead + 
downhole 

Larger than 
hydrostatic 
pressure 

Micro seismic 
monitoring of 
seal 

Larger than 
fracturing 
pressure 

Stop 
injection 

Allowed 
fluctuations 

Adapt gas 
composition, 
reduce 
injection rate 

Adapt gas 
composition, 
stop 
injection 
temporarily 

Cont. 

Wellhead 

Defined % 

Order of 
years, 
based on 
modelling 

Roughly 
yearly 

Order of 
few months 

Order of 
few months 

Order of 
few months 

Order of 
few months 

Possible 
survey 
after 
several 
years 

Possible 
survey after 
several 
years 

Fault area 

No signal 
changes 

Signal 
change in 
the seal 

Wellbore 

t.b.d. 

Wellbore 

t.b.d. 

Wellbore 

t.b.d. 

Wellbore 

t.b.d. 

t.b.d. 

t.b.d. 

t.b.d. 

t.b.d. 

x 

Baseline 
data 

Cont. 

Cont. 

Injection 
well 

No events 
in caprock 

Events in 
the caprock 

Signal 
change 
above the 
seal 

t.b.d. 

t.b.d. 

t.b.d. 

t.b.d. 

Cement 

Large 
events in 
the caprock 

Stop 
injection 

*Follows on from the risk assessment 

** To be decided by the operator, examples are updating model, additional monitoring 

*** By operator, examples are stop injection, back-production, well workover, contingency monitoring 
Note: This table is not intended to represent a full monitoring plan, but to show example information to illustrate how the table should function. The numbers and data do not represent real site-specific values. Information in 
actual monitoring plans must be detailed and adapted to national requirements in place. 

51 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
5.2.3.  Plan implementation, reporting and performance 

management 

5.2.3.1.  Reporting and documentation 

According to the CCS Directive, the operator must report the results of the monitoring to 
the competent authority at a frequency to be  determined by the authority, but at least 
once  a  year  until  transfer  of  responsibility.  Monitoring  is  part  of  the  wider  reporting 
requirements that must include: 

● 

● 

● 

all results of the monitoring, including information on the monitoring technology 
employed; 

the  quantities  and  characteristics  of  the  CO2  streams  delivered  and  injected, 
including composition of those streams, in the reporting period; 

any  other  information  the  competent  authority  considers  relevant  to  assess 
compliance  with  permit  conditions  and  increasing  the  knowledge  of  CO2 
behaviour in the storage site. 

5.2.3.2.  Data retention and ownership 

There  CCS  Directive  does  not  lay  down  any  specific  provisions  on  data  retention  or 
ownership. each Member State may choose to develop policies, laws and regulations 
governing  who  has  access  to  and  the  right  to  use  the  monitoring  data  and  who  is 
responsible for the long-term preservation of such data. In general, Article 14(4) of the 
CCS Directive encourages competent authorities to collect and keep all information that 
they  consider  relevant  to  assess  compliance  with  storage  permit  conditions  and 
increasing the knowledge of CO2  behaviour in the storage site. 

Such pooled data could be used to better characterise regional geology, monitor regional 
effects  of  injection  (e.g. basin-wide  pressure  build-ups)  and  develop  better  monitoring 
technologies and  practices.  It  is  possible that  policies could call  for  some  or  all  of the 
monitoring data to be treated as confidential business information for a set period of time 
after it is collected. After that period has expired, the data would be made public. 

The policies regarding ownership and use must balance the project developers’ rights to 
retain proprietary data with the public need for transparency and openness about results 
and the social value of pooling of data across sites. The public value of data access in 
order to accelerate and disseminate learning about storage given the importance of rapid 
CCS  deployment  should  also  be  factored  in.  Sharing  data  would  help  to  improve  the 
modelling,  and  contribute  to  better  risk  assessments,  while  providing  valuable 
information for research and innovation activities. 

The policies on data retention may also consider the obligations of the site operator to 
retain  both  raw  monitoring  data  and  processed  data  for  specific  periods  of  time. 
Presumably much of the processed monitoring data will have to be retained to create the 
operating  history  needed  when  responsibility  is  transferred  (see  GD  3).  The  policies 

52 

 
 
 
 
 
might also cover who within the government would retain copies of the monitoring data 
submitted by the operator during the injection and the post-closure pre-transfer period. 
Policies are also needed regarding the long-term retention of monitoring data after the 
transfer of responsibility phase. 

All  such  policies  might  consider  exactly  what  data  are  to  be  retained,  the  format 
(including use of a consistent coordinate reference system) and the type of media to be 
used for data storage and back-up. 

5.2.3.3. 

Interpreting monitoring results and site performance 

Annex II to the CCS Directive sets out the following requirements for interpretation and 
updating. 

● 

● 

‘The  data  collected  from  the  monitoring  shall  be  collated  and  interpreted.  The 
observed  results  shall  be  compared  with  the  behaviour  predicted  in  dynamic 
simulation process of the 3D pressure-volume and saturation behaviour (of CO2) 
undertaken  in the  context  of  the  security  characterisation  pursuant  to  Article  4 
and Annex I Step 3’. 

‘Where there is a significant deviation between the observed and the predicted 
behaviour, the 3D model shall be recalibrated to reflect the observed behaviour. 
The recalibration  shall  be  based  on  the  data  observations from the  monitoring 
plan,  and  where  necessary 
the  recalibration 
assumptions, additional data shall be obtained’. 

to  provide  confidence 

in 

●  The  risk  assessment  for  the  site/complex  (steps  2  and  3  of  Annex  I)  ‘shall  be 
repeated using the recalibrated model(s) so as to generate new hazard scenarios 
and flux rates and to revise and update the risk assessment’. 

● 

‘Where  new  CO2  sources,  pathways  and  flux  rates  or  observed  significant 
deviations  from  previous  assessments  are  identified  as  a  result  of  history 
matching  and  model  recalibration,  the  monitoring  plan  shall  be  updated 
accordingly’. 

The  parameters  and  thresholds  used  to  flag  any  significant  irregularities  should  be 
described  and  planned  through  discussions  with  the  competent  authority  and  the 
operator. They should be clearly included in the monitoring plan. 

If a leakage is detected or significant irregularities occur that might lead to leakage, the 
operator must immediately inform the competent authority under the CCS Directive as 
well as the authority responsible under the ETS Directive. 

Under the CCS Directive, corrective measures must be implemented immediately and 
monitoring  carried  out  to  check  their  effectiveness.  The  operator  is  also  required  to 
implement the monitoring approach to quantify the leakage under the EU ETS. 

Quantification of the leaked CO2 will then be carried out according to the monitoring plan 
and reported annually. The monitoring and reporting guidelines for CCS set out various 
options for establishing when a leak started. The operator must provide evidence of the 
last point in time that the leakage was not detected. If this is not possible, reporting of 
the leakage might cover the whole timeframe since injection started. 

53 

 
 
 
The quantification approach will have been included in the monitoring plan, which must 
be approved by the competent authority under the EU ETS. This may warrant updating 
in light of any new information concerning the leak. 

After  corrective  measures  are  taken  and  the  leakage  can  no  longer  be  detected,  the 
leakage can be deleted as an emission source from the storage site's EU ETS permit. 

This process requires clear and timely communication between all authorities involved. 
It  requires  not  only  knowing  the  respective  contacts,  but  also  giving  guidance  on  the 
minimum  information  that  must  be  provided  and  giving  staff  members  an  appropriate 
level of training to be able to correctly interpret the information received. 

5.2.3.4. 

Inspections 

Inspections  are  required  under  Article  15  of  the  CCS  Directive.  Member  States  are 
required to establish a system of inspections, both routine and non-routine inspections 
of  the  storage  complex.  The  purpose  of  these  inspections  is  to  check  and  promote 
compliance with the CCS Directive and to monitor the effects on the environment and on 
human health. 

Inspections should be planned and implemented by people with the relevant knowledge 
and should cover all monitoring domains surrounding the storage complex (see Section 
3.3.8), and site-specific risks. 

Inspection  activities  may  include  site  visits,  auditing  of  record-keeping,  and  may  also 
include evaluations of initial and updated risk assessments, static and dynamic models, 
and monitoring plans to ascertain that there are no negative effects to the environment 
or human health. 

The scope and frequency of inspections is clearly stated in the CCS Directive (Article 15). 

● 

Inspections  should  include  activities  such  as  visits  of  the  surface  installations, 
including the injection facilities, assessing the injection and monitoring operations 
carried  out  by  the  operator,  and  checking  all  relevant  records  kept  by  the 
operator. 

●  Routine  inspections  must  be  carried  out  at  least  once  a  year  until  three  years 
after closure and every five years until transfer of responsibility to the competent 
authority has occurred. They must examine the relevant injection and monitoring 
facilities as well as the full range of relevant effects from the storage complex on 
the environment and on human health. 

●  Non-routine inspections must be carried out: 

o 

o 

o 

if the competent authority has been notified or made aware of leakages 
or significant irregularities pursuant to Article 16(1); 

if the reports pursuant to Article 14 have shown insufficient compliance 
with the permit conditions; 

to investigate serious complaints related to the environment  or human 
health; or 

54 

 
 
 
o 

in  other  situations  where  the  competent  authority  considers  this 
appropriate (e.g. after increased seismic activity). 

●  Following each inspection, the competent authority must prepare a report on the 
results  of  the  inspection.  The  report  must  evaluate  compliance  with  the  CCS 
Directive and indicate whether further action is necessary. The operator may be 
given the opportunity to review the report for inaccuracies regarding information 
provided  to  the  competent  authority  during  the  inspection,  although  the 
opportunity  may  also  be  made  available  to  the  operator  during  the  inspection 
process.  The  report  must  be  communicated  to  the operator  concerned  and  be 
made  publicly  available  in  accordance  with  relevant  EU  legislation  within  two 
months of the inspection. 

5.2.3.5.  Evaluation of performance 

The  comparison  and  evaluation  of 
performance of a CO2  storage site can consider performance in terms of: 

the  predicted  performance  and  measured 

● 

safety and environment (CCS Directive); 

●  effectiveness in emission reduction (ETS MRG); or 

● 

evaluating  the  performance  (by  the  operator,  competent  authority  and/or  an 
independent third party). 

Under the CCS Directive, the competent authority is responsible for evaluating overall 
performance.  Evaluation  under  the  CCS  Directive  covers  regular  evaluations  of 
monitoring data from different reports and the baseline measurement, comparisons with 
predictive  models  and  identifying  any  additional  risk  management  measures  needed 
during the injection and post-injection stages. 

5.3.  Updating the plan 

Monitoring plans must be ‘updated, at least every five years’, to account for changes to 
assessed risk of leakage, changes to the assessed risks to the environment and human 
health, new scientific knowledge and improvements in the best available technology. The 
initially installed monitoring system and related procedures may also need to be updated 
as a result. Competent authorities may set a more stringent frequency. 

The plans should also be updated as a matter of urgency in the event of a leakage or 
significant irregularities as changes in monitoring are likely to be required as part of the 
corrective measures and for the purposes of quantifying the leakage. 

5.4.  Post-closure monitoring 

Monitoring in the post-closure period will supplement the monitoring data acquired before 
and  during  injection  in  order  to  demonstrate  permanent  containment,  as  described  in 
Article 18(1)(a) and Article 18(2) of the CCS Directive. This includes data on: 

55 

 
 
 
 
 
 
 
a)  whether the actual behaviour of the injected CO2 complies with the modelled 

behaviour; 

the absence of any detectable leakage; and 

that the storage site is evolving towards a situation of long-term stability. 

b) 

c) 

Plans for post-closure monitoring will be based on the risk assessment, characterisation, 
and  monitoring  data  collected  up  until  closure.  They  may  require  additional  or  new 
(techniques,  frequency,  technologies)  data  collection  for  further  characterisation.  The 
post-closure monitoring plan must also provide the information needed for transferring 
responsibilities to the competent authority. Additional guidance on the requirements for 
site closure and post-closure monitoring are provided in GD 3. 

Depending on the specific operational history of the site, the intensity of the monitoring 
can be expected to fall over time, provided that the risk assessment indicates that risk is 
decreasing. Additional guidance on site evaluation criteria is provided in GD 1. 

56 

 
6.  Corrective measures 

6.1.  Legislative context 

Corrective  measures  are  actions,  measures  or  activities  taken  to  correct  significant 
irregularities, or to close leakages in order to prevent or stop the release of CO2 from the 
storage complex (Definition 19, CCS Directive). They are designed to ensure the safety 
and effectiveness of geological storage. Corrective measures are part of the overall risk 
management process designed to ensure the safety of geological storage and to manage 
the risks from leakage during the project life cycle. Under Article 7(7), operators must 
submit a corrective measures plan with the storage permit application for approval by the 
competent authority as part of the storage permit. 

Corrective measures are not required for the migration of CO2 within the storage complex 
unless  a  significant  irregularity  is  identified.  The  operator  must  support  analysis  of 
scenarios that would indicate a leakage, including prudent modelling and monitoring. 

Under Article 9(6) of the CCS Directive, the permit must also include the requirement to 
notify the competent authority in the event of a leakage or significant irregularities, with 
the approved corrective measures plan and the obligation to implement the corrective 
measures plan in the event of a leakage or significant irregularities under Article 16. 

Article 16 of the CCS Directive requires that Member States ensure: 

● 

● 

● 

that the operator of the storage site immediately notifies the competent authority 
in  the  event  of  leakage  or  significant  irregularities  and  takes  the  necessary 
corrective measures including measures to protect human health; 

the corrective measures must be taken as a minimum on the basis of a corrective 
measures plan submitted to and approved by the competent authority; and 

if  the  operator  fails  to  take  the  necessary  corrective  measures,  the  competent 
authorities must take these measures and recover the costs from the operator, 
including  by  drawing  on  the  financial  security  under  Article  19  of  the  CCS 
Directive. 

The general principles for the overall approach to corrective measures are quite similar 
to, and closely linked to the risk assessment and monitoring of the complex. Corrective 
measures are recommended to be: 

● 

● 

● 

risk-based; linked to identified risks from site and complex characterisation (and 
risk  assessment)  and  subject  to  the  limitations  of  available  technologies  (as 
discussed in GD 1); 

specific to the storage site and complex; 

suitable  for  use  to  tackle  leakage  or  significant  irregularities  from  identified 
leakage pathways and specific leakage mechanisms out of the storage complex 
and any leakage to the surface; 

57 

 
 
 
● 

closely  linked  to  monitoring  plans  and  monitoring  (covered  in  Section  5  of  this 
document),  including  identifying  triggers  for  the  use  of  corrective measures  by 
identifying any leakages or irregularities; 

●  used when there is any leakage or significant irregularities; and 

● 

ready to use when injection operations commence. 

The  plans  should  be  updated,  as  appropriate,  as  part  of  the  storage  permit  review 
required under Article 11 of the CCS Directive. 

6.2.  Relationship to monitoring and monitoring plan 

updates 

Monitoring  and  corrective  measures  are  closely  linked.  Operators  should  develop  the 
plans and activities in a holistic manner along with the risk assessment. The competent 
authority is recommended to seek to ensure close integration between these measures. 

Corrective  measures  must  be  deployed  in  the  event  of  leakages  or  significant 
irregularities,  which  are  usually  detected  through  monitoring  results,  interpreting 
monitoring data or during inspections. 

It is recommended that operators design monitoring plans to allow for early warning of 
significant irregularities. This enables early intervention to be taken through corrective 
measures  to  prevent  the  situation  escalating  and  to  reduce  the  risks  associated  with 
leakage from the storage complex. 

In addition, monitoring will be used to assess the effectiveness of corrective measures, 
and  additional  monitoring  activities  may  be  required  in  the  event  of  any  leakage  or 
significant irregularities. 

Ultimately,  the  corrective  measures  will  need  to  be  specific  to  the  actual  leakage  or 
significant  irregularity,  taking  account  of  the  precise  location,  nature  and  the  specific 
situation  and  circumstances  in  which  the  leak  occurred.  The  risks  associated  with 
performing the corrective measures should also be factored in as there may be additional 
related threats or consequences. A flexible approach is needed to update and change 
the plan according to each specific situation. 

Rapid  and  effective  interaction  between  the  competent  authority  and  the  operator  is 
recommended during the response and when implementing corrective measures. It may 
require  strong  technical  expertise  in  drilling,  well  engineering  and  geosciences. 
Specialist consultants may also be involved. The authorities are recommended to assess 
available expertise within their organisations and, where required, to draw on external 
expertise. 

58 

 
 
 
6.3.  Responsibilities during project phases 

The operator must prepare an initial assessment of the corrective measures and submit 
this as part of the storage permit application for approval by the competent authority. 

Corrective measures may be used at any stage in the life cycle after the storage permit 
is awarded. These may be additional to or different from those laid out in the corrective 
measures plan. 

It  is  expected  that  corrective  measures  will  be  used  mostly  during  the  operations 
(injection)  phase  and  post-closure  pre-transfer  phase.  After  transfer  of  responsibility, 
corrective measures may still be required, although the likelihood is reduced from then 
on as the site evolves towards a condition of long-term stability. 

After the transfer of responsibility or the withdrawal of the storage permit, all activities 
are the direct responsibility of the Member State. After the transfer of responsibility, the 
purpose and aims of corrective measures are similar to earlier stages. However, because 
of the requirements around stability of the injected CO2  and containment for transfer of 
responsibility,  it  is  expected  that  there  will  be  little  requirement  for  any  corrective 
measures unless there is unexpected leakage or irregularities. 

6.4.  Scope and format of corrective measures plan 

Corrective measures plans are recommended to be based on the risk assessment, which 
is  in  turn  site-specific  and  closely  related  to  the  site  and  complex  characterisation. 
Potential and contingent corrective measures are recommended to be described for each 
of the main risks identified during each phase of the project lifecycle. 

Site characterisation (see Section 3 of this document) is recommended to guide the risk 
assessment, which informs the monitoring plan and in turn the corrective measures plan. 
As a result, the corrective measures plan will link to the specific risks of a storage site, 
storage  complex  and  the  surrounding  area,  including  considerations  such  as  the  CO2 
storage setting, trapping mechanism(s), location and other uses of the site. 

Monitoring plans should link directly to the corrective measures plan, indicating specific 
monitoring triggers, alert thresholds and the timing for deploying corrective measures. 
For  each  identified  key  risk  (see  GD  1)  in  each  project  lifecycle  phase,  the  following 
components are recommended to form part of the corrective actions plan: 

● 

a description of the significant irregularity that would prompt a corrective measure 
and rationale for the identified thresholds; 

●  monitoring method(s) to identify the significant irregularity and rationale for their 

use; 

● 

a description of the corrective measure(s) to be taken and the rationale for their 
use; 

●  a timeframe for response and completion of the corrective measure(s); 

59 

 
 
 
 
●  a description of contingent corrective actions. 

The operator must develop an initial proposed corrective measures plan before injection 
operations begin. As the site-specific risks may change over time, (some risks may be 
mitigated or new risks may arise) the corrective measure plan will also require updating 
as the project progresses (see Article 11 of the CCS Directive). Threshold values that 
trigger the need for corrective measures may be set after baseline data is acquired. In 
addition, the viability and cost of corrective measures depends on the nature of the risks 
and pathways, and there may be few options for corrective measures for certain kinds of 
risks. In some cases, only very generic measures such as reducing reservoir pressure 
or stopping injection may be proposed. 

It  is  recommended  to  maintain  close  dialogue  and  contacts  between  the  competent 
authority and the operator during the development of corrective measures plans, in order 
to further specify the definition and the triggers for deploying the plan. 

The competent authority is recommended to set minimum requirements tailored to the 
CO2  storage  setting  and  associated  risks.  These  may  include,  for  example,  lowering 
injection  rates  or  changes  to  injection  schedules,  reservoir  pressure  management, 
environmental remediation and sealing the identified leakage. 

6.5.  Documentation and reporting 

The CCS Directive specifies the need for the operator to submit a corrective measures 
plan as part of the application for a storage permit which is subject to approval by the 
competent authority. The plans should be updated, as appropriate, as part of the storage 
permit review required under Article 11 of the CCS Directive. 

Reporting  on  corrective  measures  is  also  covered  by  the  requirements  to  report 
monitoring under the CCS Directive. Under Article 14, the operator is required to report 
the results of the monitoring to the competent authority at a frequency to be specified by 
the competent authority (at least once a year) until transfer of responsibility. 

The occurrence of significant irregularities, leakages, and implementation of corrective 
measures will determine the extent to which corrective measures are reported. Where 
these have occurred, the competent authority is recommended to ensure that reporting 
includes documentation of the corrective measures taken, and that the operator has met 
its obligations to assess the effectiveness of corrective measures taken, in accordance 
with the monitoring requirements in Article 13. The operator should also report updates 
to the corrective measures plans and assumptions (i.e. risk, measures and methods). 

This information might also be used to review and update the corrective measures plan, 
as appropriate, alongside the review of the storage permit (see below). For example, if 
new information provides sufficient evidence about specific leakage risks, such as where 
injected CO2  has encountered specific wells or faults without evidence of irregularities, 
then the plan for corrective measures could be refined. 

The  competent  authority  is  recommended  to  ensure  that  the  effectiveness  of  any 
corrective measures taken during the project life cycle is reviewed at the time of transfer 
of responsibility in order to identify any outstanding corrective action. 

60 

 
 
 
6.6. 

Interpreting the results and performance of 
corrective measures 

It is important to assess the effectiveness of any corrective measures taken to determine 
whether leakage or significant irregularities have been sufficiently mitigated. In addition, 
Article 13(f) of the CCS Directive specifies that monitoring must be carried out to assess 
the effectiveness of any corrective measures taken. 

The operator is recommended to integrate an assessment of corrective measures with 
the assessment of monitoring results (see Section 5 of this document). In particular, the 
results  and  performance  of  corrective  measures  are  recommended  to  be  reviewed  in 
order to meet the requirements specified in Annex II (step 1.2) to the CCS Directive. 

Based on the revised risk assessments and the updated monitoring plans, the corrective 
measures plans should also be revised accordingly. 

6.7.  Inspections 

As described in recital 26, in the event of leakages or significant irregularities and thus 
when corrective measures are implemented, the competent authority must carry out non- 
routine inspections and the costs incurred in carrying out these non-routine inspections 
must be recovered from the operator. These non-routine inspections should also assess 
the  effectiveness  of  the  corrective  measures  implemented.  As  such,  monitoring  and 
modelling reviews may be included. 

6.8.  Updates to the corrective measures plan 

Although  there  are  no  formal  requirements  for  routine  and  regular  updates  to  the 
corrective measures plans in the CCS Directive, they should be updated, as appropriate, 
as part of the storage permit review required under Article 11 of the CCS Directive. 

These  updates  would  take  account  of  new  information  from  the  project  including  the 
results  of  monitoring  and  updates  to  the  site  characterisation,  the  assessed  risk  of 
leakage,  changes  to  the  assessed  risks  to  the  environment  and  human  health,  new 
scientific knowledge and improvements in best available technology. 

With  increased  knowledge  about  the  storage  site,  previously  identified  risks  might  be 
considered  irrelevant  or  new  risks  might  emerge  for  which  corrective  measures  are 
needed. Over the lifetime of a storage site, new techniques and technology for corrective 
measures might emerge or the approach might change. 

61 

 
 
 
 
 
 
7.  References 

California Low Carbon Fuel Standard, California Code of Regulations Title 17, Section 
95480. 

Dixon, T. and Romanak, K (2015), ‘Improving monitoring protocols for CO2  geological 
storage with technical advances in CO2  attribution monitoring,’ International Journal of 
Greenhouse Gas Control, Vol 41. 

DNV-RP-F104  (2021),  Recommended  Practice  on  the  Design  and  Operation  of  CO2 
Pipelines. 

DNV-RP-J203  (2021),  Recommended  practice  on  the  Geological  storage  of  carbon 
dioxide. 

Grigoli, F., Cesca, S., Priolo, E., Rinaldi, A., Clinton, J., Stabile, T., Dost, B., Fernandez, 
M., Wiemer, S., and Dahm, T. (2017) ‘Current challenges in monitoring, discrimination, 
and  management  of  induced  seismicity  related  to  underground  industrial  activities:  A 
European perspective,’ AGU Reviews of Geophysics, Vol 10.1002/2016RG000542. 

IEAGHG (2018), Well engineering and injection regularity in CO2 storage wells. 

Iyer et al (2022), ‘A review of well integrity based on field experience at carbon utilization 
and storage sites,’ International Journal of Greenhouse Gas Control, Vol 113. 

ISO 27914 (2017), International standard on carbon dioxide capture, transportation and 
geological storage - Geological storage. 

ISO  27921  (2020),  Technical  report  on  Carbon  dioxide  capture,  transportation,  and 
geological storage, Cross cutting issues, CO2  stream composition. 

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Getting in touch with the EU 

In person 

All over the European Union there are hundreds of Europe Direct centres. You can find 
the  address  of  the  centre  nearest  you  online  (european-union.europa.eu/contact- 
eu/meet-us_en). 

On the phone or in writing 

Europe Direct is a service that answers your questions about the European Union. You 
can contact this service: 

– 
– 
– 

by freephone: 00 800 6 7 8 9 10 11 (certain operators may charge for these calls), 
at the following standard number: +32 22999696, 
via the following form: european-union.europa.eu/contact-eu/write-us_en. 

Finding information about the EU 

Online 

Information about the European Union in all the official languages of the EU is available 
on the Europa website (european-union.europa.eu). 

EU publications 

You can view or order EU publications at op.europa.eu/en/publications. Multiple copies 
of  free  publications  can  be  obtained  by  contacting  Europe  Direct  or  your  local 
documentation centre (european-union.europa.eu/contact-eu/meet-us_en). 

EU law and related documents 

For access to legal information from the EU, including all EU law since 1951 in all the 
official language versions, go to EUR-Lex (eur-lex.europa.eu). 

EU open data 

The portal data.europa.eu provides access to open datasets from the EU institutions, 
bodies  and  agencies.  These  can  be  downloaded  and  reused  for  free,  for  both 
commercial and non-commercial purposes. The portal also provides access to a wealth 
of datasets from European countries. 

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