The effects of temperature on supercritical CO2 induced fracture: An experimental study

Supercritical CO2 seems to be the most promising non-aqueous fracturing fluid in the unconventional reservoir because it can increase the oil and gas production, save water resources and lower environmental impacts. The temperature effects induced by CO2 injection remain poorly understood in supercritical CO2 fracturing. Hence, we conducted experiments using artificial cubic specimens under the experimental temperatures from 25 degrees C to 100 degrees C. The CO2 temperature variation during the testing and the post-experiment fracture morphology were analyzed. In addition, we proposed a fracture mechanics model coupled with thermo-elastic stress to predict the breakdown pressure. The results show that the breakdown pressure of supercritical CO2 fracturing, between 0.55 and 0.84 times of minimum horizontal stress, decreases linearly with temperature. The propagation pressure is also lower than minimum horizontal stress. The thermo-elastic stress induced by cold CO2 injection and the change of CO2 phase can promote the branched or crossing fractures to form a fracture network, especially when the experimental temperature is over 85 degrees C. With the temperature increasing, SC-CO2 becomes less viscous and can easily permeate into the microcrack tip then to promote fracture propagation. In conclusion, the reduced effective stresses induced by both thermal tensile stress and pore pressure are the main underlying mechanism for lowered breakdown pressure and a fracture network in supercritical CO2 fracturing. A temperature difference between the reservoir and injected CO2 during fracturing affects fracture growth significantly since temperature plays multiple roles in CO2 fracturing.