Leveraging negative pore pressure to constrain post-injection-induced slip of rock fractures

Negative pore pressure caused by unconventional energy production may offer insights into predicting and mitigating post-injection-induced seismicity. Here we presented triaxial shear experiments on sawcut, filled, and natural fractures under positive, zero, and negative pore pressures. The results show that negative pore pressure leads to an increase in the peak strength of the sawcut and filled fractures while reductions in post-peak stressing rate and fracture permeability, inhibiting unstable slip. Our analysis reveals that both the normalized stressing rates per net vented volume in the laboratory and per net production volume in the field decrease during the early stages of fluid extraction and stabilize at zero with long-term extraction. Notably, negative pore pressure develops as the normalized stressing rates approach zero, indicating its role as an indicator for constraining post-injection-induced slip of rock fractures. These findings align well with the observed correlation between net production volume and seismic activity at the Salton Sea Geothermal Field. Our study suggests that monitoring pore pressure can be an alternative method to predict the risk of post-injection-induced earthquakes, particularly in localized regions with isolated fractures experiencing high production volume and limited fluid replenishment.