Fracture mechanism and multi-field interaction effects of supercritical CO2–water–coal rock coupling

Abstract

A novel supercritical CO₂ fracturing device developed in-house was employed to devise experimental schemes utilizing water at various temperatures as a heat-carrying fluid. In this work, we studied the influence of heat source power and the initial CO₂ pressure on the coal-breaking process and fracture evolution driven by thermal shock. The evolution law of fluids' temperature and failure fields was analyzed by establishing a multi-field coupling thermo-hydro-mechanical-damageable (THMD) numerical model. Multidimensional methods were utilized to study the mineral composition and microscopic pore-fracture structure evolution of CO₂–water coupling coal rocks. The fracturing process initiated by CO₂ predominantly featured stress-induced radial fractures stemming from the rapid release of high-pressure gases. Additionally, it included branch fractures, which were a consequence of the expansion of major radial fractures due to high-temperature, high-pressure CO₂ during phase transitions. Additionally, simulated results were consistent with experimental findings, indicating that the initial CO₂ pressure limitedly affected the fracturing of coal rocks. However, the failure scope of coal rocks was enhanced by increasing the heat source power. The number of pores increased after CO₂–water–coal coupling, accompanied by an enlarged pore size. Besides, connectivity among pores was enhanced. The pores of coal samples included adsorption pores, seepage pores, and transport pores. The total porosity of CO₂–water coupling coal samples increased. Acid corrosion enhanced the effective porosity by 2.91%, whereas that of natural coal samples was reduced.