Static fatigue effects on hydraulic fracture propagation in deep shale: Insights from true triaxial experiments and acoustic emission characterization

To explore practical and effective injection strategies for deep shale gas reservoirs, true triaxial hydraulic fracturing experiments were conducted on Lushan shale cubes (100 mm×100 mm×100 mm) pretreated with four static fatigue pressurization schemes: constant pressurization (CP), intermittent pressurization (IP), stepwise pressurization (SP) and intermittent stepwise pressurization (ISP). Acoustic emission (AE) monitoring was employed to capture fracture initiation and propagation in real time, and postfracturing microscopy and a 3D laser scanner were conducted to characterize the fracture morphology quantitatively. The results showed that static fatigue reduces the breakdown pressure by 4%–18% by promoting subcritical crack growth and fluid infiltration. SP and ISP achieving more substantial reductions than CP and IP because of their lower initial pore pressures near the borehole. Static fatigue mitigates fracturing-induced seismicity and suppresses early microcrack growth during the constant flow-fracture stage. Compared with sustained CP, stepwise (SP) or cyclic pressurization schemes (IP and ISP) redistribute AE activity from stress-concentrated stages to the static fatigue period, thereby smoothing the fracturing response and alleviating microseismic surges at breakdown. Constant-amplitude schemes (CP and IP) tend to induce shear-related microcracks that divert fracture paths and limit fracture propagation, whereas increasing-amplitude schemes (SP and ISP) promote tensile microcrack development and facilitate sustained crack extension. Both SP and ISP are recommended for deep shale stimulation. SP generates smoother and more branched fractures suitable for tight shale, whereas ISP produces more tortuous fracture networks better adapted to heterogeneous reservoirs. These findings provide valuable guidance for optimizing stimulation strategies in complex deep shale gas reservoirs.