Effects of supercritical CO2, water, and supercritical CO2-based water on the mechanical properties and fracturability of shale

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The study of the effects of supercritical CO 2 (ScCO 2 ) under high temperature and high pressure on the mechanical properties and fracturing potential of shale holds significant implications for advancing our understanding of enhanced shale gas extraction and reservoir exploration and development. This study examines the influence of three fluids, i.e. ScCO 2 , deionized water (DW), and ScCO 2 +DW, on the mechanical properties and fracturability of shale at immersion pressures of 15 MPa and 45 MPa, with a constant temperature of 100 °C. The key findings are as follows: (1) Uniaxial compressive strength (UCS) of shale decreased by 10.72%, 11.95%, and 23.67% at 15 MPa, and by 42.40%, 46.84%, and 51.65% at 45 MPa after immersion in ScCO 2 , DW, and ScCO 2 +DW, respectively, with the most pronounced effect observed in ScCO 2 +DW; (2) Microstructural analysis revealed that while ScCO 2 and DW do not significantly alter the microstructure, immersion in ScCO 2 +DW results in a more complex surface morphology; (3) Acoustic emission (AE) analysis indicates a reduction in stress for crack damage, with a decreased fractal dimension of AE signals in different fluids. AE energy is primarily generated during the unstable crack propagation stage; (4) A quantitative method employing a multi-factor approach combined with the brittleness index ( BI ) effectively characterizes shale fracturability. Evaluation results show that ScCO 2 +DW has a more significant effect on shale fracturability, with fracturability indices of 0.833% and 1.180% following soaking at 15 MPa and 45 MPa, respectively. Higher immersion pressure correlates positively with increased shale fracturability.