Mechanism of fracture propagation for SC-CO2 fracturing and phase-change process

Supercritical CO2 (SC-CO2) fracturing technology in unconventional reservoirs attracts attention for ensuring reservoir cleanliness, conserving water, and promoting CO2 sequestration. Addressing concerns over constrained fracture paths and inadequate consideration of reservoir in simulating SC-CO2 fracturing, this study integrates triaxial testing of rock parameters in HY oilfield. A compiled program for Abaqus, assigning global cohesive elements to rocks, was developed for comparing the mechanisms of SC-CO2 and hydraulic fracturing. Additionally, by calculating the TNT equivalency the pressure characteristics after the phase transition are obtained and corrected. Findings indicate that breakdown pressure of SC-CO2 is 12.3 MPa, with a longer initiation time. While fracturing fluid can penetrate into units near main fractures to activate more NF(natural fracture), the main fracture is rougher and 1.14 times longer than hydraulic fracture. In contrast, hydraulic breakdown pressure is 23.8 MPa, primarily concentrated at natural fracture tips, resulting in a 1.65 times greater width than SC-CO2; changes in dip angles of NF notably alter hydraulic fracturing direction but minimally affect SC-CO2 fracturing; the geostress difference increases from 0 to 9 MPa, and the SC-CO2 breakdown pressure increases by 43%, while the hydraulic fracture propagation pressure increases from 10–16 MPa to 13–18 MPa; following phase transition, significant rock damage near perforation is observed in SC-CO2, with the fracture width increasing 8.1 times and stimulated reservoir volume rising noticeably. Optimizing fracturing design is recommended by controlling the main fracture propagation direction through monitoring of flow pressure and velocity at the injection point, in conjunction with geological conditions, to increase the reservoir contact volume.