CO2 Fracturing in Pre-conditioned Reservoir Rocks Maximizes Geothermal Extraction Potential While Minimizing Induced Seismicity

Geothermal energy offers a reliable low-carbon pathway for decarbonization by providing continuous heat and baseload power. Enhanced geothermal systems (EGS) further expand this potential by enabling energy recovery from deep, otherwise inaccessible low-permeability crystalline rocks. However, wider deployment remains constrained by challenges in achieving effective reservoir stimulation while minimizing induced seismicity. These limitations largely stem from an incomplete understanding of fluid-driven fracturing mechanisms and fluid–rock interactions in crystalline reservoir rocks. Recently, CO 2 -based fracturing fluids and rock pre-conditioning techniques, which introduce distributed pre-existing damage into the rock mass, have emerged as promising stimulation strategies. Here, we numerically investigate fluid-driven fracturing of granite, a representative EGS reservoir rock, using a coupled hydro-grain-based model (Hydro-GBM) that explicitly captures grain-scale heterogeneity. Different stimulation scenarios were systematically examined by combining fracturing fluids (high-viscosity water versus low-viscosity CO 2 ) with contrasting rock conditions (intact versus pre-conditioned). The underlying grain-scale mechanisms governing fracture development and induced micro-seismicity were also analyzed. The results show that low-viscosity CO 2 reduces breakdown pressure and promotes more distributed fracture development, while rock pre-conditioning accelerates crack initiation, increases fracture complexity, and lowers the intensity of induced seismic responses. Among all scenarios, the hybrid strategy combining CO 2 injection with rock pre-conditioning produced the most favorable outcome, generating the most extensive fracture network while yielding the lowest peak micro-seismic magnitude. Compared with conventional water fracturing in intact rock, this approach increased crack counts by 69% and reduced peak micro-seismic energy by approximately 25%. The enhanced fracture development primarily results from the activation of grain-boundary cracking and pre-existing damage, driven mainly by stress redistribution during fracture propagation and fluid leak-off, with elevated matrix pore pressure playing a secondary role. These findings provide mechanistic insights for designing safer and more effective EGS stimulation strategies. More broadly, the proposed hybrid approach offers a pathway toward improved geothermal energy recovery while reducing water demand, mitigating seismic risk, and enabling potential subsurface carbon storage.