Frictional Stabilities on Induced Earthquake Fault Planes at Fox Creek, Alberta: A Pore Fluid Pressure Dilemma

TL;DR

This study investigates how pore fluid pressures influence fault slip stability during hydraulic fracturing, showing that reservoir pressures can trigger earthquakes if hydraulic connections exist, with critical pressures estimated probabilistically.

Contribution

It introduces a quantitative model to assess fault slip tendencies and pore pressure effects in induced earthquakes, incorporating uncertainties in stress and fault properties.

Findings

Pore pressures in reservoirs can trigger fault slip.

Critical pressures for slip are higher than hydrostatic but lower than reservoir pressures.

Fault stability depends on hydraulic connectivity and pore pressure levels.

Abstract

Earthquakes induced during hydraulic fracturing operations have occurred in a number of locales. However, in-situ studies aimed to discern the triggering mechanism remains exclusively statistical in their nature. Here, we calculate the fault slip-tendencies of eleven hydraulic fracturing induced earthquakes in a historically aseismic area using a recently constructed quantitative model for in-situ stresses. It is shown that the ambient pore pressures of the nearby Duvernay unconventional reservoirs can provide enough Pf triggering fault movement. The local fluid pressures acting on the fault could readily be increased above the critical value if a hydraulic connection exists between the fault and a propagating hydraulic fracture. The critical pressures necessary to induce slip, is estimated using a probabilistic model that incorporates uncertainties of stress and fault mechanical properties. These critical pressures are greater than expected hydrostatic pressure but less the pore pressures of nearby unconventional reservoirs.