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

Abstract

In recent years, the development of distributed acoustic sensing (DAS) technology has enabled direct monitoring of subsurface strain during hydraulic fracturing (HF) operations. Most low-frequency (<1 Hz) DAS (LFDAS) signals exhibit strain or strain-rate patterns that are characteristic of propagating tensile hydraulic fractures. However, mixed-mode fault reactivation, consisting of shear slip (mode II) with a tensile opening (mode I), can occur in cases where propagating hydraulic fractures intersect preexisting natural fractures or faults. We have developed anomalous LFDAS signals that were observed during an HF operation, with characteristics that differ from typical tensile hydraulic fractures. Anomalous characteristics include the onset of an asymmetric compression-extension doublet after pumping was terminated. The location of the signals, coupled with the image log and seismic data, suggests that mixed-mode reactivation occurred on a preexisting fault. We use a simplified numerical model based on the displacement discontinuity method (DDM) to simulate the anomalous LFDAS signal. Results find that the first-order characteristics of the anomalous signal can be approximated as an initial tensile fault opening (mode I) followed by a shear slip (mode II) on a fault. We determine the approach of using DDM to investigate mixed-mode fault reactivation during HF operations.