Thermal Effect on the Breakdown Pressure of Granite in Supercritical CO2 Fracturing: Experimental Study and Model Validation

Summary Breakdown pressure (Pb) is a key parameter in supercritical carbon dioxide (SC-CO2) fracturing technology. A limited understanding of the thermal effects resulting from the difference between SC-CO2 and reservoir temperatures impedes accurate Pb prediction. In our work, fracturing experiments were conducted on granite to analyze the impacts of temperature and injection rate in terms of the thermal effects on Pb. Additionally, we establish an effective Pb prediction model that considers thermal effects, achieving a low error rate relative to the experimental data. As the temperature increases from 308 to 328 and 348 K, the experimental Pb decreases by 8.34% and 20.35%, respectively. The rising temperature reduces the viscosity and pressurization rate of SC-CO2, thereby increasing the stress caused by the infiltration effect, which is inversely related to Pb. As the temperature difference increases from 0 to 20 and 60 K, the experimental Pb decreases by 3.27% and 13.19%, respectively. The thermal stress induced by these temperature differences negatively impacts Pb, and increases in temperature difference enhance thermal stress and weaken the pore pressure at the characteristic length dw. As the injection rate increases from 30 to 50 and 70 mL/min, the experimental Pb increases by 4.84% and 18.38%, respectively. The weakened infiltration effect and reduced thermal stress at dw increase the Pb under faster injection rates. Increased temperature differences and lower injection rates significantly reduce the breakdown pressure of low-permeability rocks due to combined thermal stress and fluid infiltration but have relatively minor effects on high-permeability rocks.