资源环境科技发展态势分析平台

Earthquakes induced during hydraulic fracturing operations have occurred in a number of locales. However, in situ studies aimed to discern the triggering mechanism remain exclusively statistical in their nature. Here we calculate the fault slip tendencies of 11 hydraulic fracturing induced earthquakes in a historically aseismic area using a recently constructed quantitative model for the in-situ stresses. It is shown that the ambient pore pressures of the nearby Duvernay unconventional reservoirs can provide enough Delta P-f triggering fault movement. The local fluid pressures acting on the fault could readily be increased above the critical value if hydraulic connection exists between the fault and a propagating hydraulic fracture. The critical pressures necessary to induce slip are estimated using a probabilistic model that incorporates uncertainties of stress and fault's mechanical properties. These critical pressures are greater than the expected hydrostatic pressure but less the pore pressures of nearby unconventional reservoirs.

Plain Language Summary Hydraulic fracturing operations within the Duvernay Formation, a major hydrocarbon source rock in the Western Canada Sedimentary Basin, have been linked to induced earthquakes primarily on the basis of statistical analysis. Here we attempt to examine whether faults in the area are prone to slip, causing earthquakes, using well-constrained values for the state of stress in the Earth's crust at the locations of the induced earthquakes. Perhaps most interestingly, the fault planes, along which slip occurred, could not have remained immobile if the fluid pressures directly acting on the faults are the same as the anomalously high pore fluid pressures measured within the Duvernay Formation itself. However, the faults have been historically quiescent and this suggests that the fluid pressures acting on them must be lower. This might be accomplished if pressures are relieved by fluid migration to overlying hydrocarbon reservoirs. Further, this suggests that the earthquakes are triggered by elevated fluid pressures communicated to the preexisting faults.