Offset well pressure transient caused by fault slip constrains fault zone hydraulic properties in EGS stimulation

Meng Li, Taeho Kim, Eric M. Dunham, Sireesh Dadi, Paul Segall, Aleksei Titov, Richard Gibson, & Shanna Chu

Submitted August 30, 2026, SCEC Contribution #15440, 2026 SCEC Annual Meeting Poster #TBD

Safe and successful enhanced geothermal systems (EGS) development requires managing induced seismicity and aseismic slip, which depends crucially on understanding hydraulic fracture-fault interactions and evolution of fault zone hydraulic properties. We investigate these processes by modeling a unique dataset, integrating injection schedules, wellhead pressure, stress, and microseismicity, from multi-stage stimulation of horizontal wells at Project Cape. Seismicity begins when hydraulic fractures intersect with the fault, providing fluid pressure sources that drive slip. Pressure increases rapidly in a cased but unperforated well, offset ~300 m from the stimulated well, when slip opens a hydraulic connection to the pressurized fault near the heel of the well, constraining the timing, location, and lower bound of fault slip. However, negligible seismic slip occurs at the time of pressure increase, demonstrating at least several cm of aseismic slip.

Our model simulates fluid transfer from hydraulic fracture-fault intersections into the fault zone and then to the offset well. We constrain fault diffusivity, transmissibility (permeability times thickness), and overpressure by fitting pressure in the offset well. One interpretation of the data suggests a low-permeability fault zone that is pressurized by 15-25 MPa around fracture-fault intersections by the hydraulic fractures. These act as pressure sources for ~1 day after shut-in before fracture closure due to leak-off. Pressurized regions experience shear slip and possible opening or dilation. Gradual pressure decline over 10 days comes from pressure diffusion along the fault with an estimated diffusivity of ~0.01 m²/s. Fault transmissibility (~10⁻¹⁶ to ~10⁻¹⁵ m³) is constrained by pressure recovery following fluid discharge events at the offset wellhead. Transmissibility increases 2-3 times over several days, suggesting permeability enhancement from slip.

Our approach showcases the use of offset well pressure to constrain fault zone pressurization and hydraulic properties where downhole instrumentation is unavailable.

Key Words
induced seismicity, enhanced geothermal system, fluid injection

Citation
Li, M., Kim, T., Dunham, E. M., Dadi, S., Segall, P., Titov, A., Gibson, R., & Chu, S. (2026, 08). Offset well pressure transient caused by fault slip constrains fault zone hydraulic properties in EGS stimulation. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)