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.