Abstract:

Water-enhanced geothermal system (EGS) and CO₂-EGS are currently the primary technologies for geothermal resource extraction in hot dry rock (HDR) reservoirs. The presence of natural fractures in high-temperature HDR reservoirs significantly affects the fracture propagation path, and the interactions between fracturing fractures and natural fractures are closely related to the fracturing fluid. However, compared to conventional hydraulic fracturing, the interaction patterns and mechanisms between fractures during supercritical CO₂ (ScCO₂) fracturing in HDR reservoirs remain unclear in existing studies. Moreover, there is a lack of comparative analysis on the fracture interaction patterns and mechanisms between hydraulic fracturing and ScCO₂ fracturing in the presence of natural fractures. A thermo-hydro-mechanical-damage coupled model is developed to simulate the interaction between fractures during water/ScCO₂ fracturing in a HDR reservoir, which is validated by experimental results. Numerical simulations are then conducted under different in-situ conditions (horizontal stress difference, natural fracture inclination angle, cementation strength) and operational parameters (injection temperature and injection rate). The results show that the interaction patterns can be categorized into three types: turning along natural fractures, offsetting and crossing, and penetration through natural fractures. Compared to low-temperature water injection, the thermal stress induced by low-temperature ScCO₂ reduces the initiation pressure and extension pressure by 17.86% and 0.9%, respectively, for every 10 °C decrease in injection temperature. Under high-temperature environments, ScCO₂ has significantly higher permeability than water, allowing it to more effectively diffuse into micro-pores and connect natural fractures. This leads to intersecting fractures under varying reservoir horizontal stress differences, natural fracture inclination angles and injection rate conditions during ScCO₂ fracturing. Additionally, the study further reveals that ScCO₂ fracturing is suitable for deep, high-temperature, naturally fractured geothermal reservoirs with moderate to low cementation strength, facilitating the development of complex fracture networks. In contrast, hydraulic fracturing is more appropriate for intervals characterized by high cementation strength or elevated stress differences, where strict control over fracture propagation paths is required. These findings provide theoretical guidance for the rational selection of fracturing fluids and optimization of fracture network morphology in geothermal reservoir stimulation.

Key words: natural fracture, hot dry rock, multi-field coupling, hydraulic fracturing, supercritical CO₂ fracturing