Bleed-off control on post-injection seismicity in enhanced geothermal systems

Abstract

Deep geothermal reservoirs could provide widespread access to clean and renewable energy around the world. However, hydraulic stimulation of these reservoirs to create sufficient injectivity and heat extraction has frequently induced earthquakes during and, in particular, after reservoir stimulation, which raises public concerns. This study aims to provide a possible explanation for post-injection seismicity and understand how it responds to well bleed-off as a common industrial practice to control such seismic activity. To this end, we perform coupled hydromechanical simulations of reservoir stimulation in a conceptual model comprising a deep granitic reservoir intersected by a network of long fractures and a nearby, critically-stressed fault. We find a combination of mechanisms triggering post-injection seismicity with time delays of several months after stopping injection: (1) poroelastic stressing that transmits normal and shear stress and causes undrained pressure buildup on the fault, (2) fracture-dominated pore pressure migration toward the fault, and (3) long-lasting along-the-fault pressure diffusion toward pre-stressed fault patches, promoted by dilation-induced fault permeability changes. In this setting, bleed-off causes rapid pressure decline in the near-wellbore region but marginal pressure changes farther away. The resulting attenuations of pore pressure and shear stress on the fault plane may not be enough to prevent fault reactivation. Bleed-off may counterintuitively accelerate fault slip by rapid relaxation of normal stress on the fault, which not only brings the stress state closer to failure conditions, but also accelerates pore pressure diffusion along the fault by slightly increasing its permeability. We show that bleed-off can effectively control post-injection seismicity only if rupture initiates from a structure in close proximity and with sufficient hydraulic connection to the wellbore. Future research should be directed toward the optimization of stimulation and post-stimulation design in light of the involved triggering mechanisms and through effective combination with subsurface characterization to control post-injection seismicity.

Keywords

Enhanced geothermal system / Stimulation-induced seismicity / Post-injection earthquake / Triggering mechanisms / Coupled multi-physics modeling

Data availability

The input files and the FEM code CODE_BRIGHT used for numerical simulations in this study are publicly available at the institutional repository Digital. CSIC, which practices FAIR principles (https://doi.org/10.20350/digitalCSIC/16715).

CRediT authorship contribution statement

Iman R. Kivi: Writing - original draft, Visualization, Validation, Software, Resources, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Victor Vilarrasa: Writing - original draft, Validation, Supervision, Resources, Methodology, Funding acquisition, Formal analysis, Data curation, Conceptualization. Kwang-Il Kim: Writing - original draft, Validation, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation. Hwajung Yoo: Writing - original draft, Validation, Methodology, Investigation, Formal analysis, Data curation. Ki-Bok Min: Writing - original draft, Validation, Supervision, Funding acquisition, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

I.R.K. and V.V. acknowledge support by the PCI2021-122077-2B project funded by MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR. I.R.K. also acknowledges funding from the Engineering and Physical Sciences Research Council through the UKRI Postdoc Guarantee Award THMC4CCS (Grant No. EP/X026019/1). V.V. also acknowledges funding from the European Research Council under the European Union’s Horizon 2020 Research and Innovation Program through the Starting Grant GEoREST under Grant agreement No. 801809. IMEDEA is an accredited “Maria de Maeztu Excellence Unit” (Grant No. CEX2021-001198, funded by MICIU/AEI/10.13039/501100011033). K.I.K. acknowledges support by the Innovative Technology Development Program for High-level waste management of the National Research Foundation of Korea funded by the Korean government (Ministry of Science and ICT) (Grant No. 2021M2E3A2041312). K.-B.M. and H.Y. were supported by a grant from the Human Resources Development program (No. 20204010600250) of the Korea Institute of Energy Technology Evaluation and Planning, funded by the Ministry of Trade, Industry, and Energy of the Korean Government.

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