KEM Research review, evaluation and interpretation (max. 4 pages + annex)
TITLE Effect of pressure maintenance by fluid injection on seismic risk
Title acronym

Long term fluid migration,
leakage risks and monitoring
strategies

Initiator

EZK

Project owner

Mariska Schimmel

Contact person

Karin van Thienen-Visser

Max. duration

X man month

KEM subtheme

Seismicity

Est. Budget

XX range in euros

KEM project code

KEM-24

Contractors

See below

Research category

ST/MT

Contractors

second

Risk instr. Innovation

yes

Contractors

third

KEM Quality review
Description of the scientific quality of the results (team, research method, research results, quality of the products, …), if needed
external review result (project evaluation text website)
The objectives of this project were twofold: i) to investigate and identify the best method of modelling and quantifying the
effect of fluid injection on the expected seismicity risk profile, ii) to find a reliable method to assess possible additional seismic
risks associated with fluid injection itself (as it might locally trigger seismicity) and to determine which factors, circumstances or
injection configuration and volumes may increase the probability of seismicity and vice versa. Injection fluids to be considered
were N2, CO2 and water, or a mixture of natural gas and one of the injected fluids. Also, different injection scheme options,
including reusing existing production facilities, had to be considered. The overall objective was to assess whether the net effect
of fluid injection on the seismic risk profile can be positive and to give recommendations to optimise the risk reduction at
minimum injection costs. Specific research questions of the study were as follows:
1. What are possible injection scenarios for pressure maintenance during production and for minimising the pressure difference
across the field once production has stopped?
2. What are the reservoir stresses and pressures for various fluid injection scenarios (defined in question 1) around wells and
over the entire gas field?
3. How to adapt the Groningen seismic model for injection for pressure increase as well as pressure decrease?
4. What is the effect of fluid injection on the overall seismic risk?
5. What is the effect of fluid injection on seismicity near injection wells?
6. What is the most optimal configuration of injection wells?
7. What is the overall effect of fluid injection on seismicity and what are the recommendations for the Groningen HRA model?
The project was contracted to a consortium of Fugro and Dynafrax companies, with Fugro in charge of the project
management. The project started in August 2020 and the final reports were submitted in August 2022.
The project consisted of five main work packages: i) Modelling of production-induced seismicity in Groningen gas field using 2D
hydro-mechanical coupled discrete element modelling, ii) Modelling of various fluid injection scenarios in the Groningen gas
field and induced seismicity using 2D hydro-mechanical coupled discrete element modelling, iii) Modelling of multiphase/gasmixture injection using TOUGH3, iv) Modelling of fluid injection induced fault activation using 3D hydro-mechanical coupled
discrete element model, v) Induced seismic hazard and risk assessment (HRA). In addition to the main tasks, a preliminary task
was added as WP0: literature review and compilation of input data/parameters for Groningen gas field modelling.
The research team expertise covered all areas of the project quite well. The research team members were knowledgeable and
skilled in their research areas and invested time and effort in all work packages. However, not all research questions and
modelling tasks could be addressed (see below). The research tools that were originally mentioned in the proposal (namely
PFC3D and TOUGH3) for numerical modelling could not be used due to computational costs. As a result, some of the original
research questions (see above) have been only partially answered or have remained unanswered. Relevant conclusions and
recommendations are provided based on (somewhat incomplete) results. The amount of work performed by the research team
has been high. The final reports are well written and well structured.
Optional: Confidential comments and recommendation for EZK and SodM (KEM only)

KEM Evaluation of the results
Evaluation whether the research questions are addressed adequately (questions answered, precision and uncertainties on
outcomes, potential consequences on current practice addressed, ..) (project evaluation text website)

The deliverables are presented in three final reports: a 35-page report on literature review and input data compilation, a 121page report on numerical modelling of pressure evolution during reservoir depletion and for various injection scenarios, and a
64-page report on the effect of fluid injection on seismic hazard. In addition, an umbrella report in two versions, Dutch and
English, has been provided. This umbrella report provides a useful summary of the research objectives, methodologies, results,
conclusions, and recommendations. Results presented in the three reports are briefly described below.
1. The presented literature review is not so much about the review of literature on fluid injection techniques and/or case
studies, but it is a compilation of literature that contained input data/parameters for Groningen gas field modelling. It provides
a useful compilation of data that was needed for the numerical modelling. While many essential data was collected, there are
input data/parameters, specially related to faults properties and seismic modelling, that are simply not available. This has had
adverse effect on modelling capabilities.
2. The numerical study report is well written and well-structured. Preparation and selection of input data has been done
carefully. Unfortunately, not all necessary data, specially parameters needed for reliable modelling of seismic effects, could be
obtained. The report contains accounts of 2D modelling of the entire Groningen gas field (using PFC2D), for the depletion
period until 2020, and for determining future seismicity for a period of 30 years after stopping production either at 2010 or
2020. Results of depletion period are compared to observed data. An acceptable match is found between the simulated and
observed seismicity for the time period between 1990 and 2020 for the seismic events with ML larger than 2.0. This, however,
seems to have been possible at the expense of getting too high-pressure values compared to observed pressures, as the
simulated pressure distribution doesn’t match the observed one. Results of PFC2D modelling shows that after stopping
production considerably fewer seismic events will occur.
Effect of fluid injection on future seismicity has been simulated using PFC2D code. First, a comparison is made between
injecting CO2 versus injecting N2. The simulation considers injection of fluid in the east central region only for a period of 15
years after production stop in 2020 (all other regions no production and no injection), and then simulates pressure evolution
and seismicity for another 15 years after total shut in. Results showed that CO2 injection does not lead to significant pressure
increase whereas N2 does so specially near injection points. On the other hand, N2 injection results in larger induced seismicity
in areas of pressure increase. It was concluded that N2 injection would be a more effective measure for increasing the reservoir
pressure in highly depleted areas. Therefore, four hypothetical injection scenarios were defined based on injecting N2. The
scenarios differed in terms of different combination of injection in some regions and continued production in some other
regions, or injection in some regions and total stop of production. In all scenarios, it was found that the largest magnitude
seismic event occurred at the Loppersum region.
The only three-dimensional simulation of fluid injection scenarios that could be carried out was the simulation of fluid pressure
distribution using TOUGH3 code. Two hypothetical scenarios of injection CO2 or N2 were studied. Findings corroborated results
of PFC2D modelling. That is, the reservoir pressure increases (as intended for pressure maintenance purposes) is significantly
more when N2 is injected.
3. The seismic hazard report contains a good review of previous regional seismic hazard studies, the methodology for
probabilistic seismic hazard analysis, a summary of the characteristics of the developed seismic catalogue as well as an induced
seismicity catalogue for four different injection scenarios, details of two seismotectonic models and an induced seismicity
model based on seismic catalogues for four different injection scenarios, separate ground motion prediction equations for
natural and induced seismicity, and finally results of probabilistic induced seismicity modelling. The study considered the four
hypothetical scenarios of injecting N2 that were defined in PFC2D modelling. A seismic occurrence distribution has been
determined for each one of the four injection scenarios and for two periods of time (injection time only or injection with a shutin period), and the results are used to estimate the seismic hazard. In all cases, it was found that the seismic occurrence
distribution shows higher seismic activity levels than the natural seismotectonic zone. However, induced seismic activity hazard
will be still very low. It was also found that the induced change in seismic hazard will occur for a short period only (1, 5 or 10
years depending on the scenario chosen) and then seismic hazard returns to its natural baseline. This modelling work is sound
but its value is limited by the input data.
Optional: Confidential comments and recommendation for EZK and SodM (KEM only)

KEM interpretation of the outcome
The interpretation of the results (consequences on methods/data to be used in practice, con risk instrument modules, on
inspection procedures and operator procedures, ..) (project evaluation text website)
Based on the results of this study, answers to the original research questions can be summarized as follows:
1. Regarding possible injection scenarios for pressure maintenance during production, it is not possible to give a definite
answer. Injection should be avoided near the Loppersum area because of the stress-criticality of Loppersum fault system and
near production wells in order to avoid sharp pressure variations.
Regarding possible injection scenarios after stopping production, it is concluded that seismic activities will decrease sharply
after shut in anyway and injection will not lead to any significant reduction of seismicity.
2. Regarding the evolution of reservoir stresses and pressures for various fluid injection scenarios, no conclusive
recommendation can be made.

3. The question of how to adapt the Groningen seismic module for injection to account for pressure increase or decrease
cannot be answered.
4. Regarding the effect of fluid injection on the overall seismic risk, as mentioned above, fluid injection seems to be not
necessary or preferable. In case of opting for injection, the effect will obviously depend on type of injection, injection rate, and
location.
5. Regarding fluid injection effect on seismicity near injection wells, no significant seismicity is expected. additional study will be
needed.
6. The question of most optimal configuration of injection wells is not really relevant according to this study, as injection has
either no effect or is not preferable.
7. Regarding the overall effect of fluid injection on seismicity, no conclusive statement can be made. The fluid injection may be
positive during significant production as average depletion and compaction can be stopped.
The main reason that conclusive answers to above questions could not be provided is that the main code used for numerical
studies and its underlying assumptions proved not to be quite suitable for simulating Groningen field. The codes PFC2D and
PFC3D are based on the Discrete Element Modelling (DEM) approach. DEM simulates the interactions of large pieces of rock
with each other and their relative movement. Each rock element is assumed to be spherical with diameters ranging from 180 m
to 260 m. DEM is in principle suitable for modelling the stress field and relative movement of pieces rocks. Thus, fault behavior
and activation can be modelled based on DEM approach. However, at this scale DEM approach is not suitable for simulating
flow of fluids. The fluid flow in PFC2D and PFC3D is simulated as if it occurs in between large masses of rock (which are
modelling blocks of PFC2D and PFC3D). This does not represent realistic flow processes that occur within pores of the reservoir.
Moreover, as seismicity is significantly affected by the behavior of faults that mainly extend in the vertical direction, on needs
to employ a 3D model for this purpose. So, while PFC2D can be used for some preliminary study, it is not really suitable for
conclusive study of seismicity. One can argue that the very small thickness of reservoir compared to its horizontal thickness,
would justify the 2D assumption; this is, however, acceptable for modelling fluid flow and pressure distribution only and not for
seismicity modelling. So, for simulating faults and fault movements, the use of PFC3D would have been necessary. However,
due to limitation of PFC3D in modelling fluid flow, the best option would have been to couple TOUGH3 code with PFC3D, as was
originally planned. This option, however, proved to be unfeasible due to the high computational effort that was needed.
So, a conclusive judgement on the value of fluid injection for reservoir pressure maintenance and reducing seismic hazard
cannot be made yet. Additional research in this regard is recommended. In general, results of this project and its
recommendations are useful to the government and the regulators in formulating policies, and determining research priorities
related to mitigating measures for seismic risk reduction.
Optional: Confidential comments and recommendation for EZK and SodM (KEM only)

Closure text for the website
A summary in simple terms of the goal, the outcome and impact on mining policies or toolboxes of the research project (project
evaluation text website)
The objectives of this project were twofold: i) to investigate and identify the best method of modelling and quantifying the
effect of fluid injection on the expected seismicity risk profile, ii) to find a reliable method to assess possible additional seismic
risks associated with fluid injection itself (as it might locally trigger seismicity) and to determine which factors, circumstances or
injection configuration and volumes may increase the probability of seismicity and vice versa. Injection fluids to be considered
were N2, CO2 and water, or a mixture of natural gas and one of the injected fluids.
The deliverables are presented in three final reports: a report on literature review and input data compilation, a report on
numerical modelling of pressure evolution during reservoir depletion and for various injection scenarios, and a report on the
effect of fluid injection on seismic hazard. In addition, an umbrella report in two versions, Dutch and English, has been
provided. This umbrella report provides a useful summary of the research objectives, methodologies, results, conclusions, and
recommendations.
The research team members of Fugro and Dynafrax were knowledgeable and skilled in their research areas and invested time
and effort in all work packages. However, not all research questions and modelling tasks could be addressed (see below). The
research tools that were originally mentioned in the proposal (namely PFC3D and TOUGH3) for numerical modelling could not
be used due to computational costs. As a result, some of the original research questions have been only partially answered or
have remained unanswered.
Based on the results of this study, answers to the original research questions can be summarized as follows:
1.

Regarding the overall effect of fluid injection on seismicity no conclusive statement can be made. The fluid injection may
be positive during significant production as average depletion and compaction can be stopped. Regarding possible

2.

injection scenarios after stopping production, it is concluded that seismic activities will decrease sharply after shut in
anyway and injection will not lead to any significant reduction of seismicity.
Regarding possible injection scenarios for pressure maintenance during production, it is not possible to give a definite
answer. Injection should be avoided near Loppersum area because of the stress-criticality of Loppersum fault system and
near production wells in order to avoid sharp pressure variations.

The main reason that conclusive answers to above questions could not be provided is that the main code used for numerical
studies and its underlying assumptions proved not to be quite suitable for simulating Groningen field. The best option for
numerical modelling would have been to couple TOUGH3 code with PFC3D, as was originally planned or an alternative code
with similar capabilities. The question of how to adapt the Groningen seismic module for injection to account for pressure
increase or decrease can therefore not be answered.
Additional research on this topic is recommended to better address the original research questions.