CO2 hydrate formation and dissociation in carbonate porous media

Formation of CO 2 hydrate may significantly impair permeability and reduce injectivity. In carbonate formations, hydrate behavior is further complicated by pore-scale confinement, capillary effects, and transport limitations, which can substantially shift hydrate thermodynamics and kinetics relative to bulk conditions. Using outcrop and reservoir chalk samples, this study investigates the combined effects of pore-scale confinement, transport limitations, and phase distribution on hydrate thermodynamics and kinetics through a series of hydrate formation-dissociation experiments. Temperature, pressure, and electrical resistivity were monitored to capture both thermodynamic and kinetic responses. The results demonstrate systematic deviations of hydrate formation conditions from bulk equilibrium, driven by capillary confinement and salinity effects. A semi-empirical relationship between capillary-induced temperature suppression, pore-scale properties, and pressure was established, linking hydrate stability shifts to water saturation, rock quality, and operating pressure. Hydrate saturation was estimated from the thermal response associated with the exothermic hydrate formation reaction using an energy balance approach, revealing a non-monotonic dependence on water saturation. Maximum estimated hydrate saturation occurred at low water saturation, decreased at intermediate water saturation due to reduced gas accessibility, and increased again at higher water saturation, where phase continuity improves. A non-monotonic pressure dependence was also observed, with maximum hydrate formation at ∼75 bar, reflecting a balance between thermodynamic driving force and transport limitations. Additionally, hydrate hysteresis decreased with increasing pressure and exhibited strong dependence on pore-scale parameters. Repeated formation-dissociation cycles revealed a pronounced memory effect, characterized by increased formation temperature, reduced hysteresis, and diminished thermal response, suggesting a transition toward more distributed and kinetically favorable hydrate formation. These suggest that CO 2 hydrate behavior in tight carbonates is governed by a coupled interplay of thermodynamics, kinetics, and transport processes, all influenced by the pore-scale structure. The proposed relationships provide a framework for predicting hydrate formation conditions and saturation in carbonate reservoirs.