Selective Dissolution and Pore-Structure Modification of Dolostone Under Paired Temperature-Pressure Conditions: Insights from Continuous-Flow Acetic-Acid Experiments

Deep to ultra-deep dolostone reservoirs may be substantially modified by organic-acid-bearing fluids, but the relationship between fluid-phase chemical signals and residual pore-system modification under paired temperature–pressure (T–P) conditions remains insufficiently constrained. To address this gap, continuous-flow acetic-acid dissolution experiments were conducted using Cambrian Longwangmiao Formation dolostone from the Sichuan Basin, China, as the experimental material. A 3.85 mL L−1 acetic acid solution was applied under five paired T–P conditions ranging from 20 °C/0.1 MPa to 200 °C/70 MPa to simulate coupled subsurface environments. Coordinate-guided SEM observations, quasi-steady-state effluent Ca2+ and Mg2+ concentrations, gravimetric mass loss, low-temperature N2 adsorption, and FHH fractal analysis were integrated to characterize the chemical and pore-structure responses. The results show that dissolution was strongly selective and concentrated at surface defects, cleavage-related features, crystal margins, intercrystalline contacts, and pre-existing pores or microfractures. Mean quasi-steady-state Ca2+ + Mg2+ concentrations varied non-monotonically, reaching a maximum of 31.86 × 10−3 mol L−1 at 60 °C/15 MPa and a minimum of 18.92 × 10−3 mol L−1 at 140 °C/65 MPa. At 60 °C/15 MPa and 100 °C/30 MPa, SEM showed local pore widening, whereas bulk N2-accessible pore volume and BET surface area changed little. At 200 °C/70 MPa, mass loss reached 4.97%, BET surface area increased from 0.1761 to 0.5192 m2 g−1, total N2-accessible pore volume increased from 0.001176 to 0.001940 cm3 g−1, and average pore diameter decreased from 26.04 to 12.99 nm. Taken together, these results demonstrate a partial decoupling among the chemical, gravimetric, and residual pore-structure responses across the paired T–P conditions. This finding indicates that effluent chemistry alone does not fully characterize solid-phase pore modification in an open continuous-flow system.