Abstract: To overcome the issues of conventional acid fracturing in deep, high-temperature carbonate reservoirs with low pressure coefficients-namely overly rapid reactions, short effective penetration distances, low fracture conductivity, and difficult post-fracture flowback-this study performed soaking experiments on tight carbonates of the Majiagou Formation at 110 ℃ and 30 MPa using supercritical CO₂ (SC-CO₂) and SC-CO₂ plus formation water. Integrating X-ray diffraction (XRD), scanning electron microscopy (SEM), nanoindentation, and uniaxial compressive tests, we constructed a coupled mineral-microstructure-micro/macro-mechanics evolution framework. The results indicate that pure SC-CO₂ is characterized by de-dolomitization accompanied by secondary calcite precipitation; with the addition of formation water, the regime shifts to carbonic-acid-dominated dissolution, manifested as preferential dissolution of calcite with relative enrichment (and possible recrystallization) of dolomite. The soaking process exhibits staged evolution: early de-dolomitization with secondary precipitation; a middle stage of intensified calcite dissolution with dolomite enrichment/recrystallization; and a late stage marked by increased porosity and connectivity and the emergence of channelization (wormholing). SEM shows surface evolution from pitting to grooves and through-going pores, with calcite more reactive than dolomite. Nanoindentation reveals power-law decay in hardness and elastic modulus-pronounced early softening followed by a short-term rebound and subsequent re-softening. At the macroscale, mechanical properties degrade concurrently: compressive strength and elastic modulus decrease markedly, Poisson's ratio increases, the load-bearing framework weakens, and ductility is enhanced.

Key words: supercritical CO₂-water-carbonate rock interactions, mineralogical composition, microstructure, micromechanical properties, macromechanical properties