Numerical modeling of low-frequency distributed acoustic sensing signals for mixed-mode fracture activation

In recent years, the development of distributed acoustic sensing (DAS) technology has enabled direct monitoring of subsurface strain during hydraulic fracturing operations. Most low frequency (<1Hz) DAS signals (LFDAS) exhibit strain or strain-rate patterns that are characteristic of propagating Mode-I (tensile) hydraulic fractures. However, in cases where pre-existing natural fractures or faults exist in proximity to operations, mixed-mode failure is possible, consisting of shear slip (Mode-II) plus dilation. Yet, the characteristics of mixed-mode LFDAS signals are poorly understood. We present a numerical simulation approach based on the Displacement Discontinuity Method to perform forward modeling of the mixed-mode LFDAS signals during the initial reactivation stages for a critically stressed fault. Our novel workflow provides a practical tool to perform fast forward modeling to characterize fault reactivation during hydraulic fracturing operations.