The study of the effects of supercritical CO2 (ScCO2) 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. ScCO2, deionized water (DW), and ScCO2+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 ScCO2, DW, and ScCO2+DW, respectively, with the most pronounced effect observed in ScCO2+DW; (2) Microstructural analysis revealed that while ScCO2 and DW do not significantly alter the microstructure, immersion in ScCO2+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 ScCO2+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. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Despite several parameters having been identified as having an impact on the undrained monotonic response of granular soils, the impact of the overconsolidation ratio (OCR) is still a contentious issue. One of the significant reasons for the inconsistencies in the undrained behavior is the method by which the stresses are applied--specifically, the effective preconsolidation and confining pressures. To address this, two separate series of triaxial compression tests were realized in order to examine and compare the influence of the OCR (OCR = 1, 2, 4, and 8) on the mechanical response of Chlef River (Algeria) sand, considering the way the stress state was applied. During the first series, the OCR was accomplished by consolidating the specimens to an effective preconsolidation pressure (sigma p ' = 100, 200, 400, and 800 kPa) and subsequently unloading them to a constant desired effective confining pressure of 100 kPa. In the second series, all specimens were consolidated to a maximum effective preconsolidation pressure of sigma p ' = 800 kPa (constant effective preconsolidation pressure) and then unloaded to different effective confining pressures (sigma c ' = 800, 400, 200, and 100 kPa), using two different sample preparation techniques--dry funnel pluviation and moist tamping. The test results revealed a suitable increase in the shear strength with an increase in OCR in the first series, with the opposite trend observed in the pore water pressure. For the second series, an increase in the OCR parameter resulted in a minimized shear strength and pore water pressure (although the trend in pore water pressure evolution did not really reflect the behavior of the deviator stress for this series). In addition, certain parameters, such as normalized behaviors, the brittleness index, ratio of excess pore water pressure to deviator stress at the critical state, and flow potential, appear to be reliable predictors for clarifying and, consequently, explaining the studied behaviors.