Stress field inversion using downhole fiber-optic distributed acoustic sensing array during hydraulic fracturing in shale gas reservoir

ABSTRACT We present a case study using a full-section fiber-optic distributed acoustic sensing (DAS) array to monitor microseismicity during hydraulic fracturing and provide insights into subsurface stress variations. Leveraging DAS data’s high spatial resolution and migration-based techniques, we identify 163 micro earthquakes, 46 of which were selected for moment tensor and stress inversion analysis. Independent inversions explain the source mechanisms of events in fracturing stage 8, showing consistency with in situ stress measurements and alignment with the regional stress field, as validated by the World Stress Map and the pilot hole. Seismic events are categorized into three swarms based on their focal mechanism characteristics. Swarm 1 exhibits significant stress anomalies, likely driven by rapid fluid-induced fracturing which alters local stress conditions. In contrast, Swarms 2 and 3 shows stress alignments with regional trends, indicating shear failure along preexisting faults. We also use focal mechanism tomography (FMT) to estimate pore fluid pressure thresholds for each swarm. Swarm 2 exhibits the lowest excess pore pressure (1.24 MPa), suggesting that high fluid pressure is prone to enhance the fracturing of preexisting faults and induce earthquakes. Our findings provide new insights into the role of hydraulic fracturing in induced seismicity, demonstrating that stress anomalies arise from complex fracture geometries and dynamic pore pressure variations. This study highlights the potential of integrating DAS monitoring with stress inversion and FMT to advance our understanding of shale reservoir geomechanics and induced seismicity.