Study on the fracture initiation mechanisms of supercritical carbon dioxide and radial branch borehole cooperative fracturing in methane-bearing coal seams

Deep methane-bearing coal seams are difficult to stimulate because of high in situ stress, strong gas adsorption, and low permeability. This study develops a theoretical–numerical framework to predict fracture initiation pressure (FIP) during supercritical carbon dioxide (SC-CO₂) fracturing assisted by radial branch boreholes. The model couples stress transformation, borehole-induced stress concentration, fluid seepage, and thermal stress, and accounts for bedding and joints through three competing failure modes: coal-matrix tensile failure, structural-plane shear failure, and structural-plane tensile opening. Validation against published true-triaxial data yielded a mean absolute relative error of 7.69% and captured the lower FIP of SC-CO₂ relative to hydraulic fracturing (HF). Under baseline conditions, SC-CO₂ reduced FIP by 3.68 MPa. FIP was controlled primarily by the three-dimensional stress state, initial gas pressure, and radial branch borehole configuration, whereas wellbore orientation had little influence. Wellbore-induced perturbations decayed rapidly, and FIP stabilized beyond a radial distance r of 0.4 m from the wellbore centerline. Branch orientation controlled both FIP and failure mode, with a minimum near 90° under the adopted conditions; increasing temperature differences reduced FIP approximately linearly. These findings provide a quantitative basis for optimizing SC-CO₂ fracturing with radial branch boreholes in methane-bearing coal seams.