KEM-19b SW aquifer depletion and seismicity of post-production Groningen

The Groningen gas field has been produced  since the 60’s and has caused many induced seismic events. Next to the Groningen gas field, some of the surrounding aquifers are also depleting in pressure. These aquifers may extend quite a bit beyond the gas reservoir. One of such aquifers is the southwestern aquifer, a region where small-scale seismicity has been observed in recent times, likely caused by the depletion of the Groningen gas field and the resulting pressure depletion in the aquifer. The specific objective of this research is to focus on this southwestern aquifer and look into the pressure depletion as well as the future seismic activity originated in the aquifer. This project is a follow-up of the KEM-19 study that was completed recently.

In KEM-19, a regional model was constructed that included the southwestern aquifer of the Groningen gas field. However, this model proved to be too coarse and the results for induced seismicity too general. The project will be focusing on improving the existing regional model into a local model of the southwestern aquifer. Special focus is on the characterization of smaller and larger scale faults including fault orientation and throw to be able to include them in a geological model. For the pressure modelling, the improved geological model should be used as well as additional information on the small gas fields in the Lauwerssea trough and, if needed, the Annerveen gas field. This will significantly improve the quantification of the current pressure depletion and its evolution in the future, including uncertainties. Additionally, further work needs to be performed on the geomechanical analysis of this aquifer, taking into account a possible geothermal project in the southwestern aquifer and the long-term expectations of induced seismicity. The overall goal is the long-term safety of the region after cessation of gas production.

This project focuses on the following research questions:

1.      What will be the pressure depletion of the southwestern aquifer taking into account the depletion of Groningen gas field as well as the smaller gas fields in the Lauwerssea trough?

2.      What are the locations of smaller faults, and their orientation and throw in the southwestern aquifer?

3.      What is the expected induced seismicity in the coming decades? Also addresssing: What is the seismic hazard? How does this compare to the seismic hazard of the Groningen gas field?

4.      Would a possible geothermal project impact the estimates for induced seismicity? If yes, to what extent?

5.      To what extent is the  area of influence of a geothermal project changed by pressure depletion in the southwestern aquifer? Also addressing : What areas are more or less prone to induced seismicity due to a geothermal project? At what distance from a geothermal doublet are the effects due to the activity negligible?

KEM RESEARCH QUESTION

The project was executed and reported in 2 parts:

Part I study: Evaluation of post-abandonment fluid migration and seismic hazard assessment in the southwest aquifer of Groningen.

This part included the following activities:

A thorough petrophysical characterization of Slochteren Formation (porosity, permeability, and net-to-gross ratios) were evaluated using log data from numerous wells. -

High-resolution maps of faults in the study area have been prepared.

Porosity and permeability estimates of various formations and trends with depth were obtained. - Custom porosity-permeability relationships were developed for fields where sufficient data were available.

An ensemble of static geological models of the area, integrating well log data, formation properties, and structural interpretations, were set up and an ensemble of dynamic reservoir simulation models to estimate GIIP (Gas Initially in Place) and pressure history were developed.

Satisfactory match of simulation results with historical production and pressure data from wells in different fields: Roden, Vries North, Pasop, Boerakker, Faan, as well as east side of the model (Groningen side) was achieved.

Both the Ground Motion Model and the Seismic Source Model for the Groningen area have been extended to include the SW aquifer. This helps to assess ground motion and potential seismic hazard.

Quantification of uncertainties in GIIP estimates and reservoir behavior was done using ensemble modeling and pore volume multipliers.

This resulted in a Part I REPORT of 156 pages and substantial Part I ANNEXES on static and dynamic modelling of the area, as well as seismicity analysis and seismic hazard assessment, after abandonment of gas production. 

Part II study on Seismicity estimates for potential (geothermal) subsurface operations. This included the following activities:

Seismic Hazard Modeling Framework: A quantitative framework is developed based on the Coulomb Failure Function (Δ𝐶𝐹𝐹) and Shear Capacity Utilization (SCU) indicator to assess fault reactivation potential.

The tool SRIMA (Seal and Reservoir Integrity Mechanical Analysis) has been used to calculate the response of a reservoir layer, which is sandwiched between a seal and a base layer, to the injection of return geothermal water. It is used to test the sensitivity of pressures, temperatures, induced stresses and reactivation potential of faults to various input parameters.

Pressure and Temperature Effects: The impact of thermal contraction (due to injection of cooler fluids) and pressure depletion on vertical strain and stress changes is modeled. Combined effects of thermal and pressure perturbations are shown to significantly alter the Coulomb failure conditions on faults.

Sensitivity and Scenario Analyses: Multiple scenarios are tested involving different reservoir depths (e.g., 3000 m), thicknesses, permeability values, and injection conditions (e.g., 25-year injection at 300 m³/hr). Depleted and undepleted conditions are considered to understand stress contrasts with the surrounding formations.

Spatial Assessment of Seismic Risk: The radius of influence for critical SCU values is calculated for different time horizons (1, 5, and 25 years). SCU values and stress changes are mapped as a function of distance and time, providing insight into spatial risk propagation. -

Probabilistic Inputs: Stochastic input ranges are defined for key geomechanical parameters (e.g., Young’s modulus, cohesion, Biot coefficient, friction angle) to accommodate geological uncertainties. Quality of the products The deliverables were presented in two final reports. -

This resulted in a Part II REPORT of 46 pages on seismicity estimates for future geothermal operations.

The project has been evaluated by the KEM panel.

PROJECT EVALUATION
The research has been carried out in a systematic, well-structured, and scientifically sound manner. The study shows that while pressure in the aquifer will continue to decline, the risk of noticeable earthquakes there is very low—especially when compared to the Groningen gas field itself. Only a few small events are expected in the coming decades.

The research also assessed how future geothermal operations in the southwestern aquifer might affect underground pressures, stresses, and fault movements. Probabilistic studies show that the highest probability of seismic event lies just North of city of Groningen. Nevertheless, it will be necessary to assess the magnitude and spatial footprint of pressure and temperature changes and include them in updating the seismicity model and provide updated seismic hazard assessment. These results help policymakers and regulators by confirming that the southwestern aquifer itself does not pose a major seismic risk. However, they highlight the need to keep monitoring ground movement and to carefully assess risks before starting geothermal projects.

Results of this project help policymakers and regulators by confirming that the southwestern aquifer itself does not pose a major seismic risk. However, they highlight the need to keep monitoring ground movement and to carefully assess risks before starting geothermal projects. The findings and methods from this project will be used in future decision-making and are a valuable addition to the toolbox for managing subsurface activities in the Netherlands. 

The Ministry of Climate and Green Growth refers to the KEM-19b in a Letter to Parliament to the project on seismicity in Groningen.