Evolution of fracture network, permeability and induced seismicity during fatigue hydraulic fracturing

Cyclic hydraulic fracturing (CHF) shows potential in reducing induced seismicity compared to conventional hydraulic fracturing (HF). However, controlling mechanisms that potentially limit induced seismicity but still enhance permeability during CHF remain unclear. We develop a novel time- and stress-dependent damage representative of fatigue crack growth through a coupled hydromechanical model using the block-based discrete element method (DEM). This new framework addresses the challenges in modeling CHF by simultaneously considering discrete fracture network, hydromechanical coupling, fatigue and in-situ stresses. Matching pressurization cycles-to-failure data in laboratory experiments confirms the contribution of sub-critical crack growth in the reduced breakdown pressures in CHF. Modeling fluid injections into a fractured reservoir with contrasting far-field stress ratios of 1.17 and 1.40 shows that CHF mainshocks are smaller than those by conventional HF. While HF induces seismicity primarily through the creation of new fractures, CHF generates seismicity predominantly from multiple small shear reactivations – these dissipate energy progressively and thereby reduce mainshock magnitude. CHF enhances permeability by creating a more connected fracture network than HF. Far-field stress ratio influences permeability by directing fracture growth orientations, and larger stress ratio leads to a higher proportion of shear fractures. This study provides new quantitative insights into the mechanisms of CHF in reducing induced seismicity while increasing effectiveness in elevating permeability.