Supercritical CO2–Brine–Shale Interaction under Contrasting Depressurization Protocols: Pore-Structure and Micromechanical Responses

Supercritical CO2 (ScCO2) huff-n-puff can enhance shale oil mobility and support CO2 utilization and storage, yet conventional static-soaking tests do not capture transient pressure drawdown. We compared ScCO2–brine–shale interaction at 60 °C and 15 MPa under two laboratory protocols. The staged dynamic protocol included depressurization, reactor opening, characterization, and repressurization at 3, 7, and 15 d; the static 15 d protocol used uninterrupted exposure followed by one slow terminal depressurization. The staged sequence combines pressure release and CO2 exsolution/redissolution with repeated cycling and sample handling, so the two complete operating histories form the basis of comparison. Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD) characterize local normalized elemental fractions and fitted relative crystalline-phase proportions; low-field nuclear magnetic resonance (NMR) and N2 adsorption characterize pore-fluid relaxation and adsorption-accessible pore structure; contact-angle and nanoindentation measurements quantify wettability and local micromechanical response. After 15 d under the staged protocol, the N2 desorption pore volume is 0.0113 cm3/g compared with 0.0092 cm3/g for the Initial aliquot, while the NMR spectrum retains an extension toward longer T2 relative to the initial state. The water contact angle decreases to 15.85°, and the mean reduced modulus across all 36 indentations decreases by 32.5%, including a 14.27 GPa decrease from 7 to 15 d. In the present experiments, measurements obtained under the staged sequence showed larger baseline-referenced end point changes in adsorption-accessible pore volume, water wettability, and local reduced modulus than measurements obtained under the static 15 d protocol. Together, the study provides a multi-domain descriptive comparison of the two complete operating histories; beyond this end point contrast, the staged intermediate measurements provide a time-resolved cross-domain record of the tested responses. Independent specimen-level replication is required to establish general protocol effects.