The Influence of Supercritical CO₂ Displacement at Different Temperatures on Porosity and Permeability Evolution in Marine Unconsolidated Strata

CO₂ geological sequestration in marine sediment is one of the important ways to lower carbon emissions. To study the influence of CO₂ sequestration on the permeability and porosity of unconsolidated strata, this paper conducted overpressure permeability, isothermal adsorption and CO₂ displacement experiments. Through nuclear magnetic resonance (NMR) technology, the effects of supercritical CO₂ (SCO₂) at different temperatures on the permeability evolution and pore size variation of unstratified strata in marine hydrate reservoirs were studied. The experimental results show that: (1) When the pressure changed from 0 to 17.5 MPa, the permeability and porosity of the soil samples decreased sharply. The porosity dropped from 36.83% to 16.07%, and the permeability also decreased from 48.53 mD to 1.18 mD. (2) During the adsorption tests, the fitted absolute adsorption capacity of CO₂ and CH₄ gradually increased with pressure growth. The maximum fitted absolute adsorption capacity of CO₂ was 2.45 times that of CH₄. (3) Through displacement experiments, the porosity and permeability increments during SCO₂ displacement were much greater than those during non-SCO₂ displacement. From 30 °C to 70 °C, the increments of porosity and permeability all increased. After SCO₂ displacement, the pores’ proportions (>0.1 μm) increased for all samples, with the largest growth rate reaching 34.37%. Above all, these results indicate that environmental pressure significantly affects the permeability of soil samples, and that SCO₂ displacement can effectively enhance the proportion of large-sized pores, thereby further improving the permeability of unconsolidated strata.