Effects of CO2-saturated brine imbibition on the mechanical and seepage characteristics of Longmaxi shale.

Supercritical carbon dioxide (SC– CO 2) serves as a promising anhydrous fracturing fluid that has the potential to simultaneously enhance shale gas recovery and accomplish CO 2 sequestration within shale reservoirs. However, the hydromechanical properties of shale are considerably affected by CO 2 -brine-shale interaction. This study conducted shale imbibition experiments involving subcritical/supercritical CO 2 saturated brine (Sub/SC-CO 2 +brine). Triaxial compressive strength tests were performed to evaluate the impact of Sub/SC-CO 2 +brine imbibition on the mechanical properties of shale, as well as the influence of SC-CO 2 +brine imbibition on shale permeability. The results indicate that, after SC-CO 2 brine imbibition, noticeable macroscopic cracks are formed parallel to the bedding plane. The imbibition of Sub/SC-CO 2 +brine results in the reduction in axial stress and Young's modulus, while simultaneously increasing the axial strain of shales. Compared to Sub-CO 2 +brine, SC-CO 2 +brine exhibits a higher imbibition pressure and a more intense geochemical reaction with shale. Due to increased compressibility sensitivity, the averaged permeability after SC-CO 2 +brine imbibition shows a reduction of nearly 70 % compared to that before imbibition. The insights from this research are expected to provide theoretical support for shale gas recovery and CO 2 geological sequestration. • The effects of Sub/SC-CO2+brine imbibition on the mechanical properties of shale were investigated. • Compared to SC-CO2 +brine imbibition, Sub-CO2+brine imbibition induced higher axial stress and lower axial strain. • The permeability reduced by more than an order of magnitude due to SC-CO2+brine imbibition. [ABSTRACT FROM AUTHOR]