KEM-15 Seismic risks due to coolng effects in Geothermal systems

This research question aims at improving the understanding geothermal water injection under various conditions with different volume and rate regimes and the risk assessment method of geothermal injection wells. This should help SodM in their inspection task as well as the sector to optimise production plans, estimating the life time of the doublets and assessing the time needed to establish thermal and hydrostatic equilibrium again. It is considered to be important to develop better guidelines and tools to as initial conditions and being able to define safe operating procedures of geothermal injection (and Production) wells.

The research specifically aims to address the following research questions: (1) In which generic geometries (or scenarios) of reservoir + fault(s) and a doublet in a given far-field stress state are there elevated risks of fault instability? (2) Under what parameters are faults within reach of the thermal front radiating from the injection well, within the lifetime of a typical geothermal system (i.e., before thermal breakthrough at the producer well)? (2) What is the influence of fault permeability in relation with the reservoir permeability on the size of the fault plane that is cooled down due to the injection of cold water? and (4) How does the stress tensor change on various spots of a fault plane trough time with the development of a cooled patch?
What is the magnitude of seismic events that can be expected due to combined thermal cooling effects?

KEM RESEARCH QUESTION

The output of this project consisted a main report with 12 annexes and inc ludes recommendations and guidelines regarding operational parameters for safe cold-water injection into geothermal reservoirs.

KEM-15 MAIN REPORT (ANNEXES A1, A2a and A2b, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12)

Key parameters were investigated within their expected maximum variability and ranked as listed below with respect to their influence on induced seismic hazard: (1) The orientation of existing faults with respect to the in situ stress field is a key parameter, as some faults may already be close to their critical state. (2) Whether or not a fault will slip in an unstable manner is controlled by the sign of the a-b fault constitutive parameter, where negative a-b values indicate a propensity for unstable slip. Unfortunately, there is no easy way to estimate the value of this parameter for a given subsurface fault. (3) Another key parameter is the distance between the injection well and a given fault. Roughly speaking, if the distance between the injection well and the fault is greater than the distance between the injection and production wells, there is unlikely to be a risk of cooling-induced seismicity on that fault. (4) The risk of induced seismicity is greater in locations that have higher rates of natural seismic activity, as quantified by the seismogenic index. Fortunately, the seismogenic index is low throughout the Netherlands. (5) The risk of induced seismicity also depends on the properties of local matrix rock. The risk is greater for rocks that are stiffer, less permeable, and which have higher thermal expansion coefficients; seismic risk is reduced for softer, more permeable rocks with low thermal expansion coefficients. (6) Stress changes leading to seismic hazard are related to injection rate and pressure and the temperature of the injected fluid. By reducing injection pressure and rate and increasing the re-injection temperature, seismicity can be reduced. (7) Finally, the influence of (injection) fluid properties such as density and viscosity on seismic risk is minor.

The project has led to the following recommendations, which are supported by the data and the modelling studies:

1. Given the variability of subsurface rock and fault properties and in situ stresses, it is necessary to conduct an a priori site specific seismic risk assessment for each proposed geothermal project, to evaluate the natural seismic hazard and the potential for induced seismicity.

2. If the potential location is found to have an elevated risk of induced seismicity, then more detailed modelling, perhaps using the methods employed in this study, should be carried out.

3. Once geothermal energy production has commenced in a given field, a local seismic monitoring network should be set up to monitor any induced seismicity that may occur.

4. If a seismic event (or peak ground acceleration, etc.) is observed that exceeds a pre-determined threshold (i.e., a “traffic light system”), injection rates can be decreased (or production rates increased), and the temperature of the injected fluid can be increased, in order to mitigate further induced seismicity. The success of these mitigation methods can be assessed by monitoring the induced seismic activity.

The project has been evaluated in 2023, which have lead to some corrections in the reports,. The final versions are linked in this webpage..

KEM PROJECT EVALUATION

Overall, it can be concluded that the researchers have addressed the goals of this project in a thorough manner, using appropriate tools, and have interpreted their results so as to reach useful conclusions and recommendations.

As it regards a hazard analysis due to geothermal activities, given the issues summarized above, the conclusions should be considered with caution, and possibly require further analyses to be consolidated. The project concludes that if these policies are followed, it is likely that geothermal energy production in the Netherlands can continue to be carried out. However, the remarks given on the hazard analysis part should always be considered.

SodM has issued a Dutch summary and interpretation their website, which can be accessed using this link.