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 theEuropean 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

Directorate-General for Climate Action2024

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 theauthors, and the European Commission is not liable for any consequence stemming from the reuse of thispublication.
© European Union, 2024

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 meansthat 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 ................ 93.1. Legislative context ................................................................................. 93.2. 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
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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 statementthat 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 competentauthority;●

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 giveindications on how it can be implemented. They should therefore facilitate a correct anduniform 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 ofJustice 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-definedterms used in the CCS Directive.

8

3. Characterising the storage complex and surroundingarea
The goal of the process to characterise the storage site, storage complex andsurrounding area is to assess the containment, capacity1, injectivity and monitorability,
and also to demonstrate that storage activities pose no significant risk to human healthor the environment. The risk assessment process (see GD 1) provides the foundation fordata collection, site characterisation and site selection. Site-specific risks will beidentified 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, depletedhydrocarbon fields, CO2 mineralisation, coal seams and other settings, such as still activehydrocarbon fields, in case of EHR, or geothermal systems, in the case where theobjective 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 tointerpret 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 storageof CO2 in their territory must assess the storage capacity available in parts or in the wholeof 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 thepotential 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 tocharacterise and assess the potential storage complex and surrounding area. It requires
operators to complete the assessment according to best practice at the time of theassessment. 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 CO

2 within storage sites relying on
buoyancy/structural trapping can be assessed using the CO 2 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 theoperation to avoid CO2 exsolution prior to mineralisation, without causing a negativeenvironmental impact. From a project perspective, the emphasis is generally on verifying
sufficient capacity for project volumes, rather than estimating the overall available capacityof the selected storage site.

9

1.

Data collection.
o2.

Three-dimensional (3D) static geological modelling.
o3.

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

Using the data collected in Step 1, a three-dimensional static geologicalearth 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.

Characterising storage dynamic behaviour, sensitivity and risk assessment.
o

The process to characterise storage dynamic behaviour must be basedon 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.

oMultiple simulations must be carried out to identify the sensitivity of theassessment to assumptions made about particular parameters. The
simulations must be based on altering parameters in the static geologicalearth 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 theirnational 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, orother 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 theMember State's territory, a dual approach is often taken.
Depleted fields
Depleted hydrocarbon fields are those fields for which it has been demonstrated thathydrocarbon reserves have been depleted (either fully or partially) or the remaining
resources may no longer be recovered/developed economically. For depleted fields, thecapacity 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 evaluationof 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 ofdepletion and reinflation may not permit re-pressurisation to initial reservoir pressure.
This may also be necessary if reinflation could lead to higher reservoir pressures thanvirgin 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, thestorage efficiency factor will vary depending on the nature of the reservoir andcorresponding connected aquifer (i.e. the hydraulic unit), where a field with strong aquiferpressure 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 orpressure recharge).
Aquifer sites
Aquifer sites are often not depleted and CO2 injection operations generally increasepressure significantly above virgin pressure (unless active pressure management iscarried out using brine production). The storage capacity for such sites is constrained byallowable pressure build-up without fracturing the rock, or causing activation of faults, aswell 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 andIndustry 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 thatcan 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 andcompressibility 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. Hydraulicunits may have been mapped by national geological surveys, and the net thickness canbe derived from well logs of wells that penetrate the hydraulic unit. Similarly, porosity andn/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 theworld. 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 allowablepressure constraints and specific modelling for the hydraulic unit, it is recommended thatthe 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 theextent of the hydraulic unit containing the aquifer used for geological storage (allowingpressure 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 scaleof 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 andassessment of the potential storage complex and surrounding area. If access to thesedata 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 theexistence 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 hydraulicallyconnected 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 thestorage complex discussed in the following subsections. The data must be taken frommultiple sources including well logs, well cuttings analyses, core samples and analyses,
fluid samples, injection and production tests, outcrop studies, seismic/geophysicalsurveys 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 thestorage site may be defined by the maximum extent of the injected CO2 stream in thereservoir throughout the project lifecycle. In depleted hydrocarbon fields, the vertical andlateral 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 porousand permeable injection zone comprised of sandstone or carbonate. In sites whichleverage 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 toachieve 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, eachwith 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 determinedby the vertical extent of the storage reservoir(s). The lateral extent should include thedomain 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 isprovided 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 thestorage site. It is recommended that protected groundwater is clearly identified andexcluded from the definition of the storage complex. Caprocks and seals are not requiredcomponents of the mineralisation storage complex definition if it is demonstrated thatthey 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 (seeFigure 1). Member States may also include specific requirements or descriptions to
consider in the surrounding area. The surrounding area will be determined on a sitespecific basis, and will be based on a risk assessment. The recommended geologic
considerations for the surrounding area include areas of increased pressure, horizonscontaining 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:oa diagram indicating the vertical and lateral extent of the storage site,
indicating the vertical and lateral extent of key horizons within the site;oproperties of the injection zone rock such as lithology, thickness,mineralogy, porosity, permeability, facies types and distributions;oproperties of the caprock, where applicable, such as lithology, thickness,mineralogy, porosity, permeability, facies types and distributions;ofaults and fractures: location, orientation, fault throw, transmissibility,
fracture distribution (also see Section 3.3.5);ocapacity estimates.

14

Figure 1. Illustrative example of the storage site, storage complex, surrounding area andleakage pathway in a saline aquifer or depleted oil and gas field.
2.

Storage complex:oa diagram indicating the vertical and lateral extent of the storage complex,
including the vertical and lateral extent of all horizons in the storage
complex;orock properties for key horizons: lithology, porosity, permeability,
thickness, facies types and distributions;ofaults and fractures: location, orientation, fault throw, transmissibility,
fracture distribution (also see Section 3.3.5);oleakage pathways: faults, fractures, permeable zones in caprock, capillarypressure estimates (for more details, see Section 3.3.8);opaleogeography, 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 thegroundwater7 intended for human consumption8, or necessary to avoid impacts on healthor the environment within the storage complex and surrounding area, and to prevent itfrom 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 thesubsurface during volcanic activity, mantle degassing, diagenesis, and geochemicalalterations of the rock formations over time. Baseline monitoring (see Chapter 4) can beused to characterise the chemical composition of groundwater prior to injection.
Some storage sites may propose CO2 injection into aquifers. which requiredemonstration that the site would pose no significant risk to human health or theenvironment, and that groundwater intended for human consumption is protected, withno CO2 injection being performed in such aquifers (also see Section 3.3.10). In thesecases, it is recommended that groundwater intended for human consumption is clearly
differentiated from groundwater that is not intended for human consumption and that ageologic 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 toensure that early corrective action can be taken if groundwater intended for humanconsumption is threatened (see Section 5). Injection into aquifers, however, may besubject 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/legalcontent/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 andwhether it is supplied from a distribution network, from a tanker, or in bottles orcontainers;(b) all water used in any food-production undertaking for the manufacture,processing, preservation or marketing of products or substances intended for humanconsumption 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 byjurisdiction.

3.3.3. Reservoir engineering and petrophysics
Reservoir engineering is the study of the characteristics and movement of fluids within asubsurface porous medium (reservoir or injection zone). Reservoir engineering is closely
linked to the dynamic modelling step of the site characterisation process. Information onreservoir engineering is used to evaluate the capacity, containment and injectivity of astorage 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 andtemperature 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 engineeringcharacterisation. For shared hydraulic units, it is the responsibility of the operator, as partof 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 thatboth sites can simultaneously meet the requirements of the directive’ (Article 8). If thestorage complex spans across national boundaries, multiple competent authorities willneed 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 physicaland chemical properties of the rock and its petrological (mineral composition)
characteristics and how these elements impact upon the development of and interactionof 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 evaluationof 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 andgeothermal fields, there may be plentiful reservoir engineering and petrophysical data
due to exploration and production activity. However, it is recommended to ensure thatdata available are representative of the ‘current’ site conditions, and not of the initialcondition or production condition. Production can impact porosity, permeability and fluid

9 ‘Hydraulic unit’ is defined as a hydraulically connected pore space where pressurecommunication 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 isalso a key determinant for storage capacity and sustained injectivity. The hydraulic unit
should therefore be mapped and described. This mapping should also describe otherknown activities within the hydraulic unit that may impact pressure within the storage site.

17

characteristics leading to misrepresentation of geologic conditions in 3D static anddynamic 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 whichminerals are likely to precipitate or dissolve. These processes have implications forassessing capacity, containment, injectivity and for monitoring the injected CO2 stream.
A geochemical analysis may identify the likelihood for minerals in a storage zone orcaprock to dissolve, which can potentially result in leakage. In some cases, geochemical
analysis can identify the propensity for mineralisation reactions to take place, leading tomineral storage of CO2. It may also identify the propensity of a geochemical system toresult in precipitation of solids, which may be another indicator of mineral trapping, orlead 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 thecontents 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 eachstorage complex. Geochemical sampling from appropriate and key intervals (e.g. thewater 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 ofpressure and temperature conditions, as well as chemical and mineralogical sampling ofrock, reservoir fluids and injected fluids.

3.3.5. Geomechanics
Geomechanics is the study of forces (stresses) and deformation in the subsurface andthe evaluation of potential failure (breaking) of rock fabric, which is one of the root causesof the creation of permeable pathways that can result in leakage. Knowledge of the stresscondition 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 geologicalstorage of CO2 changes the stress condition in the subsurface both within and outside
the reservoir. A changing temperature also impacts the stress condition in thesubsurface. Geomechanical analyses assess how the stress conditions change due toprior 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 prescribedpressure gradients and/or and thresholds for rock fracture gradients or minimum total
principal stress may be required by the competent authority), and injectivity (asgeomechanical considerations constrain injection rates, pressure and temperature). It isclosely linked with thermal modelling (temperature changes can result in fractures) andgeochemical analysis (for example, dissolution can weaken rock and precipitation canstrengthen rock). Operators are recommended to consider taking an integrated approachto these subjects.
Geomechanics analyses require characterisation of the geological structure (geometryof 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 ofbore hole instability and losses), well testing (e.g. extended leak-off test and step-ratetest), core analyses and various rock mechanical testing, and seismic interpretation offormations, 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 characterisechanges in the stress as injection proceeds, because previous production and plannedstorage 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 onchanges 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 criticallystressed pre-existing faults to identify induced seismicity potential and otherconsiderations.
For depleted oil and gas reservoirs, geomechanical analyses are critical to identify stresschanges 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 createdthrough injection. The stress condition will change from the production phase to theinjection phase in these projects, but a return to the pre-production stress state isunlikely. 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 ofa 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 toassess injection pressure limits as it is not biased by near-wellbore stress variations andwellbore 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 sustainableinjection rates for individual wells. The operator should communicate clearly to thecompetent authority the basis used to assess the injection pressure limits, includingreference depths as applicable and expected safety margins. Site-specificgeomechanical assessments, testing and modelling should be used to assess theappropriate thresholds for the site.
There are a range of thresholds and approaches proposed (for example, in US EPAClass 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 aseismic 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 naturalgeologic (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 humanactivities, typically associated with industrial operations such as mining, hydraulicfracturing (fracking), geothermal energy extraction and reservoir-induced
seismicity from large water impoundments. These earthquakes occur as a resultof changing the stress conditions within the Earth’s crust. The seismic events are
directly linked to human interventions and would not have occurred naturally inthe absence of such activities.

●

Triggered seismicity refers to earthquakes that are indirectly influenced byhuman activities but are primarily caused by natural stress conditions in the
Earth’s crust. These earthquakes occur when pre-existing faults or stresses in20

the Earth’s crust are affected by external factors, such as seismic waves fromlarge distant earthquakes, volcanic activity or changes in fluid pressure due toactivities 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 releaseof stress accumulated naturally over time.
Fault movement and seismic waves can be initiated due to increased subsurfacepressure or sometimes due to geochemical processes or subsurface temperaturechanges. 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 (seeGD 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 fromgeomechanical 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 andtechniques. Seismicity may occur also on invisible faults (e.g. faults with little throw thatare 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 posea safety hazard that increases with increasing moment magnitude. Operators arerecommended 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 associatedwith the seismic hazard depend on the potential damage, which is expected to bedifferent for onshore and for offshore geological storage sites for CO2. Sitecharacterisation 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 of21

seismic monitoring stations supported by geologic surveys can identify large-moment
magnitude earthquakes that impact broad areas and pose a hazard to communities andinfrastructure. Additional measurement and monitoring systems are often required to beinstalled at CO2 storage sites to identify smaller-scale seismic activity that can jeopardise
containment. More precise measurement, for example using wellbore geophysicalmonitoring 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 sitespecific, 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 themonitoring area and should be set in concert with the competent authority and theoperator. Seismic monitoring is recommended to begin as early as feasible before CO 2
injection starts. Some approaches to date have included a traffic light system indicating
increasingly stringent operational constraints as seismic activity increases. This can bebased 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-madepathways
Predicting the presence of leakage pathways for the potential migration of CO2 out of thetargeted storage formations is a vital component of the characterisation process. It is
included explicitly in the containment evaluation and it links to risk assessment, sitecharacterisation, CO2 composition, monitoring and corrective action.
The project’s description of the storage complex will form the basis and context for thisevaluation, and will need to factor in the trapping mechanism(s). All leakage scenariosthat 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 tooverlying and adjacent geological formations outside the targeted storage formation, ora conduit to the surface. The risk associated with some natural pathways will differ bytrapping 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 delineationof 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 pertinentearthquakes 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 exsolvingfrom 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 tocompaction due to previous production activities (see Section 3.3.5).
Human-made pathways
Human-made pathways include wellbores and boreholes. As part of the characterisationof the storage complex, data should be collected to inform the status and condition of allexisting 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 shouldbe 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 toenvironmental 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 – Wellqualification, 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 inexisting (legacy) wells12. All the above guidance documents describe a risk-based
approach to assess the need for risk mitigation, including monitoring, intervention andremediation. Neither document is prescriptive regarding the materials or wellabandonment design, for example.
Allowing a risk-based, non-prescriptive approach is particularly important to enable costeffective storage in depleted hydrocarbon fields. Depleted hydrocarbon fields may have
many legacy wells within the storage complex and surrounding area. While all wells willneed 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 165301:2017, 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 trappingby well barriers. For instance, if a well can be demonstrated to not be exposed to freephase CO2 or other buoyant fluids, e.g. if all injected CO2 is fully dissolved in the storagesite, 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 thefollowing 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 stipulaterequirements for each of these elements.
For CO2 storage projects, specific considerations should be given to the chemical andmechanical 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 CO2saturated formation fluid’; and●‘evaluate the predicted pH and chemical composition of the fluids in contact withthe 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 modellingshould be carried out to evaluate the mechanical aspects of well integrity.
A particular consideration related to the potential chemical or mechanical degradation ofcasing and cement is the potential for debonding of casing and cement, or corrosion ofcasing 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 andplacing cement to cover the exposed formations with a cement-to-rock contact acrossthe 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 assessmentof existing well barriers in the well, however, may be considered recommended practicefor 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 stressand compaction or elongation strains along the wellbore.
The 2022 OEUK Guidelines: Well Decommissioning for CO2 storage state that thestorage site should be separated from the seabed or surface by two independent wellbarriers. 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 CO 2 storage (perArticle 4(4) of CCS Directive) contingent on the remediation of certain wells, then it isimportant that the storage operator can demonstrate a high chance of success of theremediation 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 thecompetent 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 andstorage 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 isdealt 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 thestorage 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, (marineor surface) biosphere, and the atmosphere (see Table 1). Subsurface modelling andother evaluation may be used to determine which domains may be affected.
The impacts may include CO2 migration into oil and gas reservoirs, contamination ofgroundwater consumable to humans, pressure interactions with nearby operations, or
potential impacts to geothermal operations. The documentation should include sitespecific 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.
DomainDefinition

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 thestorage complex.

3.3.9. Population distribution
The location and concentration of populated areas above and adjacent to the storagesite will be an important aspect to consider, particularly during the risk assessment anduncertainty analysis. The competent authority will need to consider the likelihood ofleakage 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 befactored 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 shouldbe 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 andsubsurface 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 andflora15), 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 otherpurposes, and the fact that potential leakage of CO2 may affect the usability of othersubsurface 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 canimpact storage site conditions, characterisation outcomes, injection thresholds andmonitoring requirements. It is recommended that the competent authorities require, andestablish efficient mechanisms for, information sharing amongst operators of nearby
activities to improve the characterisation process and risk management effectiveness:●

Groundwater.
o●●

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●Wind farms may provide access constraints for monitoring, maintenanceand corrective action activities required at the storage site.

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

Other storage applications.
o●

CO2 could contaminate groundwater resources.

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

●

Salt mining and brine mining.
o

●

●

●

o

The CO2 plume could contaminate the production of hydrocarbons, thusincreasing costs to the operators as they would have to strip off CO 2 andallow for changes to the infrastructure at a facility.

oConversely, CO2 injection could increase regional reservoir pressure andactually 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, thusincreasing costs to the operators as they would have to strip off CO 2 andallow 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 formationwater to allow it to desorb off the coal.

Coal mining.
CO2 could have an impact on or halt coal mining operations and create
unsafe mining conditions.

Underground coal gasification.
o●

CO2 could impact or halt mining operations by creating unsafe mining
conditions.

Oil and gas field development.

o●Subsea cables and pipelines may result in access constraints formonitoring, maintenance and corrective action activities required at thestorage site.

CO2 could extinguish underground coal gasification processes or limit thedevelopment of such operations.

Energy/waste storage.
oCO2 injection could increase regional reservoir pressure, reducing theinjectivity of water or waste injection wells.

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

o

CO2 injection could increase regional reservoir pressure, introducing
leakage pathways that compromise energy storage or waste storagesites.

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 thepermit 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 timeof 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 ofsuch conflicts. The most effective way to do so will be at the discretion of the authoritythat grants the permits to carry out subsurface and related activities in close proximity toeach 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 tothe 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 becommercially and technically viable at low rates of injection; others at high rates. Somesites 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 CO 2.
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 tothe 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 affectthe 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 sitecharacterisation 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 hydraulicallyconnected 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 geologicalmodel. 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 acaprock 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 freephase, such as solubility trapping and mineralisation, our interpretation is that acaprock is not an explicitly necessary component of the storage complex and the 3Dmodel.
In these cases, the model(s) would need to provide sufficient evidence to demonstratethat 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 ofCO2 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 andrisk should be evaluated and considered, including sensitivities for input variables. Many30

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, suchas a geochemical and geomechanical models, and should be integrated into a cohesiveinterpretation of the subsurface.
Multiple data sources, as discussed in Section 3.3, will be used as input into theconstruction of the geological models. The model(s) should contain sufficient detail todetermine 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 arerecommended 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 tosupplement or validate measured data, where available and appropriate. Where
fractures are a significant consideration related to capacity, containment, injectivity or amonitoring 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 morereliable 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 andreservoir simulation modelling throughout the process of both the characterisation of astorage complex and during its operational phases. It will also be necessary to maintainand 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 isrecommended 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 andprobabilistic approaches. The selected methodology should follow best practice at thetime of model construction. The operator may need to consider models at different
scales, such as basin/regional, subregional, or local (storage complex) level toadequately 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 Table2). 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 bestpractices. For example, it may be important to model the elevated pressure region overtime when assessing the nature of CO2 flow in the reservoir, phase behaviour, the riskof 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 themonitoring area.
Dynamic models may include a suite of approaches and methodologies, such as fluid
transport and pressure evolution, thermodynamic modelling, reaction kinetics, andgeochemical 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 thesite conditions are suitable for long-term storage. Relative permeability is an importantcomponent 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 tounderstand 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, suchas 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, mineralformation) 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 thesensitivity 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 affecthuman 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 sensitivityof 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 modellingand dynamic modelling, is closely tied to the risk assessment process as thesecomponents 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 inaccordance with the requirements of Step 3.3 in Annex I to the CCS Directive. This is33

without prejudice of compliance with other EU regulations, for instance theEnvironmental 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 compositionof the CO2 stream constraints that are consistent with the primary purpose of geologicalstorage, 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 andstorage network and to the environment and human health’.
Article 12 of the CCS Directive, which covers the criteria for CO2 streams for geologicalstorage, sets out the requirements operators must follow with the aim of ensuring thatthe 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 isdependent on the used feedstock and the industrial process), the capture
process; or injection process; andb. trace substances that may be added to assist in monitoring andverification 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 theircomposition 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 andtrace 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 concentrationsin ambient air on human health and environment (e.g. Benson et al., 2002).

35

4.2. CO2 stream consists overwhelmingly of carbon
dioxideThe ‘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 tohuman 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 ofadditional CO2 stream purification, and the cost of managing risks to human health, theenvironment, storage sites and transport infrastructure. The requirements to manage
acceptable levels of concentrations of ‘non-CO2 constituents’ align with this objective andwill 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 orother 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 storagesite or the relevant transport infrastructure, pose a significant risk to theenvironment 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 alterviscosity 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 therequirements 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 ismineralised through geochemical reactions with the host reservoir rock. This typically36

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) aremet.
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 thelimit 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 CO 2
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 thatthe 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 morerobust 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.
constituents that can have an impact on corrosion, such as O2, H2O, NOX,
SOX, Glycol, and H2 (can cause embrittlement);2.
constituents that can pose a concern for human safety, including CO, H2S,
NOX, SOX, and Hg;
3.
constituents that could affect the physical properties of the CO2 stream,including:o

H2O – the risk of hydrates;oinert gases - impact on vapour pressure during the liquid phase andimpact on critical pressure during the dense phase;onon-corrosive constituents that could precipitate as solids – leading toflow assurance problems;onon-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 thestorage site and transport infrastructure or represent a significant risk to the environmentor 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, andothers).
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 physicalintegrity and flow characteristics of the pipes, the competent authority must approve thecomposition of the CO2 stream as it affects pipeline integrity and storage integrity as partof the storage permit.
In determining an acceptable composition of the CO2 stream, operators and competentauthorities 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 ofremoval of components, designing the infrastructure to tolerate the composition of thestream 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 theinappropriate stream into the storage site.

38

5. Monitoring
This section provides guidance on the allocation of monitoring responsibilities. While theoperator 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. Datasharing 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 thestorage site. Monitoring activities will need to take place within the storage site as wellas 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 ofgeological storage. The requirements are as follows.
●

Article 13(1): Member States shall ensure that the operator carries out monitoringof the injection facilities, the storage complex (including where possible the CO 2
plume), and where appropriate the surrounding environment for the purpose of:a) comparison between the actual and modelled behaviour of CO2 andformation 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 usersof 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 approved39

by the competent authority pursuant to point 6 of Article 7 and point 5 of Article 9of 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 tothe assessed risk of leakage, changes to the assessed risks to the environmentand human health, new scientific knowledge, and improvements in best available
technology. Updated plans shall be re-submitted for approval to the competentauthority.
Annex II sets out the criteria for establishing and updating the monitoring plan and forpost-closure monitoring. Directive 2003/87/EC is the EU ETS Directive establishing ascheme 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 andReporting 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 inaccordance with the guidelines.
The aim of this section is to provide operators and competent authorities with guidanceon interpreting the requirements for the monitoring plan under Annex II, and on therequirements 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 competentauthority. The competent authority is obliged to ensure that the operator monitors theinjection 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 orsignificant irregularities are detected, monitoring must be intensified as required toassess the attribution/origin of the CO2, the scale of the problem and the effectivenessof corrective measures.
The main objectives and purpose of monitoring are to confirm containment of CO2, toalert 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 havebeen agreed and approved by it as part of the storage permit and following therequirements 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 whereappropriate 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 ofresults;●

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

set detection limits such that significant irregularities and leakages can be
identified and corrected before they lead to negative impacts on human healthand 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 sitecharacterisation 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 formonitoring requirements. It may include threshold values for adopting a range ofpreventive 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 monitoringplan 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 andstate).

In addition to the above list, continuous or intermittent monitoring of the pressure in theinjection 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 guidanceon 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 andareal/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 ofsignificant 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 thereservoir based on static and dynamic modelling and updating, corroborated bymonitoring data. The main challenge for measuring the absence of any leakage consistsof spatial and temporal coverage of the monitoring method, i.e. ‘Where and when do weneed 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 ofboth the processes occurring and the behaviour of the storage complex can beconsidered 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 3Dmodel must be recalibrated to reflect the observed behaviour as described in Chapter 2of this GD and in GD 3. If, however, the deviations fall well beyond the uncertainty rangesof 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 anddifferent 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 notprescriptive. The plan’s elements, objectives and technologies should be site-specificand 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 eachspecific 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 byinjection or leakage.
Figure 2: Monitoring plan components.

42

Operational

Plume

Injection wellcontrolPressure &temperature
Composition
Quantification

Calibrate models
Migration
Kinetics
Trappingmechanisms
Trapping
efficiency
Pressure
Water behaviour

Pathways
Caprocks
Faults &fractures
Wells
Aquifers

Environmental(leakage)
Detection ofsuspected leak or
anomaly
Attribution of
anomaly
Leakquantification
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;owater/brine behaviour, properties and movement resulting from CO2
injection;

monitoring pathways for potential leakage identified by risk assessment, i.e.:ocaprocks;ofaults and fractures;owells (and well integrity);ooverlying aquifers.

environmental monitoring for leaks out of the storage complex towards, at or near
the surface, on land or offshore:odetection of suspected leakage anomaly;oattribution of leakage anomaly;oquantification of leakage;oaccounting and quantifying emissions from the storage complex for
surrender of emissions trading allowances for any leaked emissions
under EU ETS Directive 2003/87/EC;osafety 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 al2022, 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 laterstage, 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 tothe site-specific conditions and monitoring goals through analogue examples and/or sitespecific testing.
The additional considerations are relevant to operators and competent authorities as partof 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 thesystem?

●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, inturn, 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 sitespecific considerations for the storage complex.
The site-specific information in the monitoring plan should include:●

location and geographical considerations (e.g. onshore/offshore,considerations, population centres, land use and potable aquifers);●

overview of site/complex location and geological characterisation, including
reservoir, trapping mechanism(s); summary of identified risks, including
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 specificrisks.

5.2.2.2.

local

Defining the monitoring area

The storage complex and the surrounding area combined will normally encompass themonitoring 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 andpermeable reservoir rock (b), it may be transported by buoyancy towards the groundsurface 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 CO 2 storage insaline aquifers, such as pressure increases and formation water displacement so thatmonitoring plans can be designed to tackle these impacts as necessary. Monitoringshould also be considered to detect CO2 movement into aquifer formations between themain 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 toensure 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 ofthe 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 andstate).

Additional parameters are likely to be needed in the monitoring plan, depending on thestorage type, the trapping mechanism and site-specific risks. Mineralisation sites, forexample, may require geochemical sampling and geochemical modelling to accurately
evaluate containment and conformance. Box 2 provides additional considerations formonitoring 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 betaken before any leakage occurs.
The choice of monitoring technology must be based on the best practice available at thetime 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 andareal/vertical distribution of the CO2 plume to refine numerical 3D simulation of
the 3D geological models of the storage formation established under Article 4 andAnnex 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 ofsignificant 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, canbreach the caprock endangering shallower drinking water sources and indicating apathway 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 detectsuch fluid movements. Water sampling and analysis may also need to be monitored andfactored into geochemical modelling.
Different categories of monitoring can be set out:●

Mandatory monitoring (for all sites). Some parameters to be monitored aremandatory 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. formationpressure data).

47

●

Required (site-specific) monitoring. Site-specific monitoring activity is designed
to collect evidence for containment in the reservoir and to demonstrate theintegrity 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 anduncertainties.

●

Optional contingency monitoring. Contingency monitoring systems are only usedin 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 riskof 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 andtemperature monitoring of the injection zone is recommended to be a significant
component of monitoring at mineralisation sites to ensure the CO2 remains trapped insolution, 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 andduring 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-existingleakage 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 beconsidered 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 toconsider 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 datacollection 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 CO2that originated from storage operations. Natural processes, such as the decay oforganic matter, dolomitisation, volcanic activity/migration of magmatic CO2 throughdikes 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 toattribute 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 thefollowing.
●

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

●

The technique that will be used to measure the parameter (Column 2), with amore detailed description of the technique provided outside the table. Sitespecific 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 stewardshipafter 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 thatindicate normal behaviour (Column 6).

●

Threshold alert values where predicted normal behaviour stops and where
potentially anomalous measurements occur (Column 7). The assessment ofthreshold 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 be49

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 subcolumn.

Table 3 provides an indicative example, not a minimum requirement for monitoring allsites, 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 andfrequency 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 therisks 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 technicaland 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 thecompetent authority or local jurisdictions.
Parameter
to bemonitored*

Category of monitoring
TechniqueadoptedMandatory

Required

Project phase and frequency

Contingency

Pre-inj.

Inj.

Normal situation
Long-term
stewardship

Post-inj.

Injection rate

Flow meterx

Pressure

Pressure devicex

Baseline
data

Cont.

Cont.

Every year

Wellhead +downhole

Temperature

Thermometerx

Baseline
data

Cont.

Cont.

Every year

Wellhead +downhole

Injected gas
composition

Gas samples

x

Fault
integrity

Cont.

Location

Accuracy

Contingency value(significant irregularity)> Threshold1

Action**>Threshold2

Contingency
measure***Larger thanhydrostaticpressure

Micro seismic
monitoring ofseal

Larger thanfracturing
pressureStopinjection

Adapt gas
composition,
reduceinjection rate

WellheadCont.

Order ofyears,
based onmodelling

Expectation
value

Alert value

Possiblesurveyafter
severalyears

Possiblesurvey after
severalyears

Adapt gas
composition,stop
injection
temporarily

Wellhead

Defined %Allowedfluctuations

Fault area

No signalchanges

Signalchange inthe seal

Signalchangeabove theseal

Repeated 3D
seismic; fibreoptic sensorsx

Aqueouschemistry (CO2,pH)

xRoughly
yearlyAnnularpressurex

Order offew months

Wellboret.b.d.

t.b.d.

t.b.d.

Wireline logging

x

Order offew months

Wellboret.b.d.

t.b.d.

t.b.d.

Optical wellloggingx

Order offew months

Wellboret.b.d.

t.b.d.

t.b.d.

x

Order offew months

Wellboret.b.d.

t.b.d.

t.b.d.

CementInjectionwell

No events
in caprock

Events in
the caprock

Large
events in
the caprockStop
injection

BaselinesurveyWell integrity

Cement bondlogging
Microseismic
monitoring

Geophones
behind thecasing of a wellx

Baseline
data

Cont.

Cont.

*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 orownership. 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 thatthey consider relevant to assess compliance with storage permit conditions andincreasing 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 themonitoring data to be treated as confidential business information for a set period of timeafter 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 toretain 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 inorder 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 themodelling, 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 toretain 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 theoperating 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 datasubmitted 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 thetransfer 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 andupdating.
●‘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 predictedbehaviour, 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 to provide confidence in the recalibration
assumptions, additional data shall be obtained’.

●

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 scenariosand 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 theoperator. They should be clearly included in the monitoring plan.
If a leakage is detected or significant irregularities occur that might lead to leakage, theoperator 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 andmonitoring 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 planand 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 thelast 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 updatingin light of any new information concerning the leak.
After corrective measures are taken and the leakage can no longer be detected, theleakage 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 theminimum 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 inspectionsof the storage complex. The purpose of these inspections is to check and promotecompliance 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 environmentor 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 theoperator.

●

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 competentauthority 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:oif the competent authority has been notified or made aware of leakages
or significant irregularities pursuant to Article 16(1);oif the reports pursuant to Article 14 have shown insufficient compliancewith the permit conditions;oto investigate serious complaints related to the environment or humanhealth; or54

oin other situations where the competent authority considers thisappropriate (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 theopportunity 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 twomonths of the inspection.

5.2.3.5.

Evaluation of performance

The comparison and evaluation of the predicted performance and measured
performance of a CO2 storage site can consider performance in terms of:●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 ofmonitoring data from different reports and the baseline measurement, comparisons withpredictive 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 toassessed risk of leakage, changes to the assessed risks to the environment and humanhealth, 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 orsignificant irregularities as changes in monitoring are likely to be required as part of thecorrective 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 modelledbehaviour;b)the absence of any detectable leakage; andc)that the storage site is evolving towards a situation of long-term stability.

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 forsite 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 isdecreasing. 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 thestorage 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 thecompetent 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, withthe 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 competentauthorities 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 (andrisk assessment) and subject to the limitations of available technologies (asdiscussed 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 thisdocument), including identifying triggers for the use of corrective measures byidentifying 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
updatesMonitoring and corrective measures are closely linked. Operators should develop theplans and activities in a holistic manner along with the risk assessment. The competentauthority 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 ofsignificant irregularities. This enables early intervention to be taken through corrective
measures to prevent the situation escalating and to reduce the risks associated withleakage 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 orsignificant irregularities.
Ultimately, the corrective measures will need to be specific to the actual leakage orsignificant irregularity, taking account of the precise location, nature and the specificsituation and circumstances in which the leak occurred. The risks associated withperforming 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 isrecommended 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 thenon 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, thepurpose and aims of corrective measures are similar to earlier stages. However, becauseof 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, whichis in turn site-specific and closely related to the site and complex characterisation.
Potential and contingent corrective measures are recommended to be described for eachof 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 specificmonitoring 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 theiruse;●

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

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 thattrigger 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 ofrisks. 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 competentauthority and the operator during the development of corrective measures plans, in orderto further specify the definition and the triggers for deploying the plan.
The competent authority is recommended to set minimum requirements tailored to theCO2 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 measuresplan as part of the application for a storage permit which is subject to approval by thecompetent 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 reportmonitoring 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 accordancewith 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 ofcorrective 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 withthe assessment of monitoring results (see Section 5 of this document). In particular, theresults and performance of corrective measures are recommended to be reviewed inorder 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 nonroutine 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 andmodelling reviews may be included.

6.8. Updates to the corrective measures plan
Although there are no formal requirements for routine and regular updates to thecorrective 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 theresults of monitoring and updates to the site characterisation, the assessed risk ofleakage, changes to the assessed risks to the environment and human health, newscientific knowledge and improvements in best available technology.
With increased knowledge about the storage site, previously identified risks might beconsidered irrelevant or new risks might emerge for which corrective measures areneeded. 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 geologicalstorage with technical advances in CO2 attribution monitoring,’ International Journal ofGreenhouse 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 utilizationand storage sites,’ International Journal of Greenhouse Gas Control, Vol 113.
ISO 27914 (2017), International standard on carbon dioxide capture, transportation andgeological storage - Geological storage.
ISO 27921 (2020), Technical report on Carbon dioxide capture, transportation, andgeological storage, Cross cutting issues, CO2 stream composition.
Offshore Norge ARMA 22-559 and ARM 2016-887.
Porter, R., Striolo, A., Mahgerefteh, H., and Walker, J. (2018), 'Addressing the risks of
induced seismicity in subsurface energy operations', Wires Energy and Environment, Vol
8:2.
Snæbjörnsdóttir, S.Ó., Sigfússon, B., Marieni, C. et al. (2020), ‘Carbon dioxide storage
through mineral carbonation’., Nature Reviews Earth Environ 1, 90-102.
Thibeau, S., Bachu, S., Birkholzer, J., Holloway, S., Neele, F., and Zhou, Q. (2014),'Using Pressure and Volumetric Approaches to Estimate CO2 Storage Capacity in Deep
Saline Aquifers', Energy Procedia 63, pp. 5294-5304.
Thorsteinsson, H. and Gunnarsson, G. (2014), 'Induced Seismicity-Stakeholder
Engagement in Iceland', GRC Transactions, Vol. 38.
US Class VI UIC Regulation 40 CFR 146.
US EPA Underground Injection Control Program, Class VI Well Construction Guidance,(2012).

62

Willacy, C., van den Bogert, P., van Dedem E., Blokland, J-W., ‘Constraining fault
geomechanics using elastic waveform inversion and dynamic rupture modelling’ (2019)
EAGE Annual conferences and Exhibition, London, UK.
Zero Emissions Platform (ZEP) and CCSA (2022), Network Technology Guidance forCO2 transport by ship.
Zero Emissions Platform (2022), Experience in developing CO2 storage under the
Directive on the geological storage of carbon dioxide.
Zhang, H., Eaton, D., Li, G., Liu, Y., and Harrington, R. (2016), 'Discriminating induced
seismicity from natural earthquakes using moment tensors and source spectra', JGR
Solid Earth, Vol 121:2, p 972-993.
Zoback, M. (2012) ‘Managing the seismic risk posed by wastewater disposal’, Earth.

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Finding information about the EU
OnlineInformation 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 theofficial 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 bothcommercial and non-commercial purposes. The portal also provides access to a wealthof datasets from European countries.

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