Study on transient flow and heat transfer characteristics of CO₂ fracturing in hot dry rock reservoirs

Supercritical CO₂ fracturing offers advantages such as reduced initiation pressure and the formation of complex fracture networks, making it one of the effective methods for hot dry rock reservoir stimulation. Addressing the challenges of unclear fluid flow and heat transfer characteristics within fractures during supercritical CO₂ fracturing in high-temperature hard rocks, and the difficulty in predicting CO₂ phase behavior, a coupled flow-heat transfer model for supercritical CO₂ fracturing in geothermal reservoirs was established. This model reveals the influence patterns of pump column and reservoir parameters on the flow and heat transfer characteristics within the fractures. The study indicates that CO₂ rapidly transitions to a supercritical state within the first 1–5 m of the fracture entrance. Its density and viscosity decrease by approximately 50% at a fracture length of 25 m before stabilizing. Parameter sensitivity analysis indicates that injection flow rate, rock strength, and injection temperature significantly influence the fracturing effectiveness. Increasing the injection flow rate from 0.04 m³/s to 0.1 m³/s raised the injection pressure by only 0.2 MPa (an increase of 0.66%), while extending the fracture length by 78 m (an increase of 73.2%). Simultaneously, CO₂ demonstrated superior fracture generation capability in high-strength reservoirs. Increasing the rock elastic modulus from 40 GPa to 60 GPa resulted in a 15 m increase in fracture length (an increase of 8.44%). Injection temperature was increased from 0 ℃ to 30 ℃, resulting in a 12 m increase in fracture length (an increase of 6.87%). The reservoir temperature had less than a 2% impact on injection pressure and fracture length, rendering it negligible. These findings provide theoretical support for designing supercritical CO₂ fracturing process parameters and offer significant guidance for advancing the efficient development of geothermal resources in hot dry rocks.