Mechanism and application of supercritical carbon dioxide hybrid fracturing: A case study of shale oil in the Jiyang Depression, Bohai Bay Basin

Continental shale oil represents a strategic replacement for improving reserves and well productivity in China. However, its development is facing two major challenges: the limited mobility of hydrocarbon fluids and poor reservoir fracability. To address these issues, we explore the mechanism and application of supercritical carbon dioxide (SC-CO₂) hybrid fracturing for shale oil in the Jiyang Depression, Shengli oilfield, through an array of tests, including the SC-CO₂-water-rock interactions, diffusion and displacement, true triaxial compression, and triaxial compression. The results indicate that dissolution with SC-CO₂ can increase shale porosity by 1 ~ 5 times, resulting in the predominance of mesopores and macropores. In addition, the fractures get enlarged as the reaction goes on. Specifically, the fracture width increased from 399 nm after 2-hour dissolution to 1.535 μm after 12 h, representing a fourfold increase in width, and concurrent permeability increase by 1 ~ 3 orders of magnitude. The SC-CO₂-invovled oil extraction achieved an efficiency of up to 24 %, with minimal effects observed beyond 30 h. Compared to dry shales, the tensile strength of shales subjected to SC-CO₂-water treatment decreased by 31 %. Unlike dry fracturing, SC-CO₂ hybrid fracturing can create bedding-parallel fractures by removing barriers to enhance bedding connectivity, which tend to form a complex fracture network in combination with subsequent hydraulic fractures. Grounded on theoretical research in lab, we conduct field tests of SC-CO₂ fracturing. The test results indicate that, in well Y-1, the average fracture propagation pressure for all sections decreased from 103.4 MPa before CO₂ injection to 100.5 MPa after CO₂ injection. Additionally, microseismic events recorded during fracturing in well Y-1 became more concentrated, with an average event number of 160.8 and an average effective stimulated rock volume (SRV) of 647800 m³ overall. Moreover, both the viscosity and density of reservoir fluids decreased, resulting in an improvement in their mobility.