Abstract
Coupled poroelastic stressing and pore-pressure accumulation along pre-existing faults in deep basement contribute to recent occurrence of seismic events at subsurface energy exploration sites. Our coupled fluid-flow and geomechanical model describes the physical processes inducing seismicity corresponding to the sequential stimulation operations in Pohang, South Korea. Simulation results show that prolonged accumulation of poroelastic energy and pore pressure along a fault can nucleate seismic events larger than Mw3 even after terminating well operations. In particular the possibility of large seismic events can be increased by multiple-well operations with alternate injection and extraction that can enhance the degree of pore-pressure diffusion and subsequent stress transfer through a rigid and low-permeability rock to the fault. This study demonstrates that the proper mechanistic model and optimal well operations need to be accounted for to mitigate unexpected seismic hazards in the presence of the site-specific uncertainty such as hidden/undetected faults and stress regime.
Highlights
Coupled poroelastic stressing and pore-pressure accumulation along pre-existing faults in deep basement contribute to recent occurrence of seismic events at subsurface energy exploration sites
Recent experimental results including direct fluid injection into a natural fault[27] and U.S DOE (Department of Energy) geothermal stimulation activities in the Sanford underground testing facility reveal that aseismic processes modeled by a rate-dependent friction law can be used to identify a precursor to seismic slip[27] and the locations of seismic events can be directly monitored to delineate creation of a hydraulic fracture and additional reactivation of pre-existing structures
We elaborate the following critical questions; (1) what are the physical mechanisms inducing moderate to large earthquakes (Mw ≥ 3) after pausing/terminating well operations? and (2) how do geological and operational parameters affect spatio-temporal patterns of seismic events? We examine the poroelastic response of the basement fault to sequential stimulation operations that were motivated at the Pohang ehanced geothermal system (EGS) site using a three-dimensional (3-D) coupled simulation with mechanistic analyses (Figs. 1 and S1 and Table 1)
Summary
Coupled poroelastic stressing and pore-pressure accumulation along pre-existing faults in deep basement contribute to recent occurrence of seismic events at subsurface energy exploration sites. Pore-pressure accumulation along conductive faults has been considered as the principal mechanism for inducing seismicity[3,15] in which diffusive propagation of pressure plumes is essential, but controlled by hydraulic connectivity from faults to the fluid-injection reservoir Another primary mechanism is the poroelastic stressing in which the volumetric changes of the pressurized zone perturb the stress field of the surrounding rock by transmitting elastic forces to longer distances even beyond the hydraulically affected region such as distant and disconnected basement faults[16,17,18,19]. Additional simulations with various geological and operational factors will emphasize the importance of physical characterization of faults and surrounding basement and adequate injection-extraction operations to mitigate the risk of induced seismicity prior to and/or during subsurface energy activities
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