Investigation of fracture propagation dynamics during multi-stage water injection shearing in fault-fracture reservoirs

Abstract

Activation of water injection-induced shear in hot dry rock reservoirs (commonly termed 'hydro-shearing') is a critical technique for enhancing permeability in enhanced geothermal systems, thereby significantly improving the efficiency of reservoir heat extraction. In this study, granite samples from geothermal reservoirs were utilized to fabricate filled jointed granite specimens, and the mechanical properties of water injection-induced shear in granite at various joint angles were examined. The experiments were conducted using a coupled mechanical-hydraulic shear testing system. Additionally, the FRACOD software was employed to simulate the evolution of key shear-enhancing fractures, including wellbore fracturing, joint penetration, and fault activation, in geothermal reservoir formations at different depths. The analysis focused on fracture development patterns, displacement fields, and stress fields at three different depth stages. By integrating a case study from the Yangbajing geothermal project in Tibet, this research investigated the effects of wellbore placement on stress fields, displacement fields, and acoustic emission energy in fracture-type thermal reservoirs. Based on the wellbore model at the 24-meter depth, a systematic sensitivity analysis was conducted to investigate the influence of four critical parameters, namely injection pressure, in-situ stress ratio, fault cohesion, and fault friction angle, on fault slip displacement. The findings indicate that the peak shear strength reduction of granite with different fracture dip angles under water pressure varies, with the 30-degree dip angle granite showing the greatest reduction due to its proximity to the shear failure angle. As burial depth increases, fracture propagation during borehole hydraulic fracturing, natural fracture activation, and fault shear stimulation becomes progressively restricted. Moreover, the wellbore placement significantly affects the response of fracture-type reservoirs, and direct injection into the fault yields superior shear stimulation effects. Injection pressure shows a strong positive correlation with fault slip, as does in-situ stress ratio, while fault cohesion and internal friction angle exhibit negative correlations. Notably, injection pressure emerges as the key factor, contributing 53.73 % to slip displacement variance. These findings provide essential insights for optimizing reservoir and wellbore construction in water-injected shear stimulation projects for hot dry rock geothermal exploitation.