Mechanical behaviour of Longmaxi black shale saturated with different fluids: an experimental study

The fluid-shale interaction during carbon dioxide (CO2) enhanced shale gas recovery processes may have significant influences on the mechanical behaviour of shale, and it is also of great importance for the longterm safety of CO2 geological sequestration in shale formations. In this work, shale samples from the Longmaxi formation in the Sichuan basin were used to investigate the mechanical properties and the acoustic emission (AE) behaviour of shale upon saturation with different fluids (water, brine, and CO2-brine mixture with different CO2 phase states) at 45 degrees C. X-ray fluorescence (XRF) analysis results show that major element alterations occur after shale is treated with supercritical CO2-brine and subcritical CO2-brine, the element content of Ca, K and Al in supercritical CO2-brine saturated shale was decreased by 16.7%, 5.8% and 6.3%, respectively. Field emission scanning electron microscopy (FESEM) analysis results indicate that significant surface structural changes occur in shale after saturated with CO2 and brine. Furthermore, the uniaxial compressive strength (UCS) and elastic modulus (E) of shale with different fluid saturation were decreased. Supercritical CO2 cause a greater reduction of UCS and E compared to subcritical CO2 due to its higher adsorption capability and larger CO2 adsorption induced swelling. The maximum reduction in UCS and E of shale was observed in the case of CO2-brine saturated samples, and the influence of the CO2 phase on the mechanical behaviour of shale under brine conditions is not negligible. The fracture propagation pattern changes in shale after fluid saturation are also explained by the AE analysis.