Periodic Reporting for period 4 - GEoREST (predictinG EaRthquakES induced by fluid injecTion)
Periodo di rendicontazione: 2023-08-01 al 2025-01-31
Fluid injection in deep geological formations is proliferated to utilize underground resources with the objective of reducing CO2 emissions through harnessing of renewable geothermal energy, eliminating CO2 emissions from the hard-to-abate industry, and to store energy. These activities, which are useful to mitigate the climate emergency, may, however, induce seismicity. If felt, induced earthquakes have a very negative effect on public perception, which has led to the cancellation of several projects, like the enhanced geothermal systems at Basel (Switzerland) and Pohang (Korea Republic), and the Castor underground gas storage (Spain). The causes of induced seismicity are vaguely understood, especially the reason why the largest-magnitude earthquake usually occurs after the stop of injection, like in the three mentioned examples of project cancellation. To date, operators have not counted with effective tools to anticipate to felt earthquakes. Therefore, forecasting injection-induced earthquakes is a big challenge that must be overcome to effectively deploy geo-energies to significantly reduce CO2 emissions and thus mitigate climate change and reduce health issues related to air pollution. The objective of this project is to develop a novel methodology to predict and mitigate induced seismicity. We propose an interdisciplinary approach that integrates coupled thermo-hydro-mechanical-seismic processes that occur in the subsurface as a result of fluid injection to understand and forecast induced seismicity. The methodology, based on new analytical and numerical solutions, addresses the specific objectives of (1) understanding the processes that lead to induced seismicity by model testing of specific conjectures, (2) improving and extending subsurface characterization to reduce prediction uncertainty, and (3) using the resulting understanding and site specific knowledge to forecast and mitigate induced seismicity. Project developments have been tested and validated against fluid-induced seismicity at the CO2 storage site of Decatur (Illinois, USA), the Castor underground gas storage (Spain), and the enhanced geothermal systems at Basel (Switzerland) and Pohang (Korea Republic), which have been chosen because of their diverse characteristics in order to generalize project developments.
We have significantly advanced the understanding of the triggering mechanisms that induce seismicity during and after the stop of injection. We have demonstrated that the classical conceptualization in which pore pressure controls induced seismicity may serve to control post-injection seismicity when injecting into a single fracture/fault, but it fails for fracture networks, which is the typical case in the subsurface. As a result, a change in the paradigm is necessary, in which not only pore pressure changes should be considered, but also poromechanical stresses, cooling-induced stress changes, static stress transfer due to seismic and aseismic slip, and deformation-induced pore pressure changes.
We have shed light to the intriguing and counterintuitive phenomenon of post-injection seismicity. Some faults undergo stability improvement during injection because of compressional poromechanical stress induced by the expansion of fractures and intact rock caused by pressurization. After the stop of injection, pore pressure rapidly drops around the injection well, causing poromechanical stress relaxation and thus vanishing the stabilizing effect, which may lead to reactivation of those faults. As a result, bleeding-off the well may be contraindicated because the more pronounced pressure drop around the well further destabilizes distant faults.
We have identified geologic carbon storage as a technology with low induced seismicity risk, while deep geothermal systems have a larger potential to induce moderate earthquakes. The reason for this is that, in conventional geologic carbon storage, CO2 is injected into sedimentary rock, which are relatively deformable, whereas stiff, brittle crystalline rock is typically found at the depths necessary to find temperature high enough to generate electricity. The stiff crystalline basement accumulates more stress than the softer shallow sedimentary rock, making it critically stressed and prone to induce seismicity. By developing a state-of-the-art numerical model, we have been able to reproduce the spatio-temporal evolution of the monitored seismicity at the enhanced geothermal system of Basel. By developing a hybrid forecasting model that uses the effective stress changes computed numerically to estimate the seismicity rate and using statistical seismology, we have identified stimulation protocols that would have prevented the largest-magnitude earthquake after the stop of injection. We have also found that poromechanical stress changes significantly influenced the induced earthquake at Pohang, and that limiting the injection pressure would have improved fault stability in the post-injection period. To derisk geo-energies, we have advanced subsurface characterization of both porous and fractured rock interpreting laboratory experiments and by designing a long-term field test at the Mont Terri underground rock laboratory.
We have shed light to the intriguing and counterintuitive phenomenon of post-injection seismicity. Some faults undergo stability improvement during injection because of compressional poromechanical stress induced by the expansion of fractures and intact rock caused by pressurization. After the stop of injection, pore pressure rapidly drops around the injection well, causing poromechanical stress relaxation and thus vanishing the stabilizing effect, which may lead to reactivation of those faults. As a result, bleeding-off the well may be contraindicated because the more pronounced pressure drop around the well further destabilizes distant faults.
We have identified geologic carbon storage as a technology with low induced seismicity risk, while deep geothermal systems have a larger potential to induce moderate earthquakes. The reason for this is that, in conventional geologic carbon storage, CO2 is injected into sedimentary rock, which are relatively deformable, whereas stiff, brittle crystalline rock is typically found at the depths necessary to find temperature high enough to generate electricity. The stiff crystalline basement accumulates more stress than the softer shallow sedimentary rock, making it critically stressed and prone to induce seismicity. By developing a state-of-the-art numerical model, we have been able to reproduce the spatio-temporal evolution of the monitored seismicity at the enhanced geothermal system of Basel. By developing a hybrid forecasting model that uses the effective stress changes computed numerically to estimate the seismicity rate and using statistical seismology, we have identified stimulation protocols that would have prevented the largest-magnitude earthquake after the stop of injection. We have also found that poromechanical stress changes significantly influenced the induced earthquake at Pohang, and that limiting the injection pressure would have improved fault stability in the post-injection period. To derisk geo-energies, we have advanced subsurface characterization of both porous and fractured rock interpreting laboratory experiments and by designing a long-term field test at the Mont Terri underground rock laboratory.
The project has made significant progress beyond the state of the art. It has substantially advanced the understanding of the causes that induce seismicity, especially those of post-injection seismicity, contributing to change the paradigm of what induces seismicity. By considering combinations of poroelastic stress changes, static stress transfer due to seismic and aseismic slip, buoyancy of the injected fluid, and deformation-induced pore pressure changes, we have provided the best explanations to date of the causes of the induced seismicity at Basel and Castor.
The project has performed pioneering research on cooling-induced seismicity in deep geothermal systems, in particular, in supercritical geothermal systems. Supercritical geothermal systems are gaining significant attention lately because of their high potential to generate large amounts of non-intermittent electricity, in the order of 35-50 MWe per well. We have identified that cooling-induced seismicity may become the dominant factor in the long term, as the cooled volume around the injection well becomes large enough to start affecting the stability of distant faults. The research on supercritical geothermal systems has led to the proposal of an innovative concept of CO2 storage in these systems, where CO2 sinks because it becomes denser than supercritical water, significantly reducing CO2 leakage risk.
We have developed analytical solutions to assess the stress and strain changes along a fault as a result of reservoir pressurization, which are useful tools for performing quick estimations of fault stability. We have also developed a novel methodology, named the equivalent fracture layer, to upscale the fracture thickness in numerical models while maintaining an accurate solution. By using the equivalent fracture layer in a novel hybrid forecasting model of induced seismicity that has the novelty of including the relevant physics, we have been able to reproduce the largest-magnitude earthquake after the stop of injection at Basel. This model is a useful tool for operators to investigate stimulation protocols that could avoid inducing the largest-magnitude earthquake in the post-injection period while achieving permeability enhancement of most faults.
The project has performed pioneering research on cooling-induced seismicity in deep geothermal systems, in particular, in supercritical geothermal systems. Supercritical geothermal systems are gaining significant attention lately because of their high potential to generate large amounts of non-intermittent electricity, in the order of 35-50 MWe per well. We have identified that cooling-induced seismicity may become the dominant factor in the long term, as the cooled volume around the injection well becomes large enough to start affecting the stability of distant faults. The research on supercritical geothermal systems has led to the proposal of an innovative concept of CO2 storage in these systems, where CO2 sinks because it becomes denser than supercritical water, significantly reducing CO2 leakage risk.
We have developed analytical solutions to assess the stress and strain changes along a fault as a result of reservoir pressurization, which are useful tools for performing quick estimations of fault stability. We have also developed a novel methodology, named the equivalent fracture layer, to upscale the fracture thickness in numerical models while maintaining an accurate solution. By using the equivalent fracture layer in a novel hybrid forecasting model of induced seismicity that has the novelty of including the relevant physics, we have been able to reproduce the largest-magnitude earthquake after the stop of injection at Basel. This model is a useful tool for operators to investigate stimulation protocols that could avoid inducing the largest-magnitude earthquake in the post-injection period while achieving permeability enhancement of most faults.