CO2 adsorption/desorption behaviors and dispersion characteristics in shale reservoirs
CO2 adsorption/desorption behaviors in shale reservoirs are the theoretical base of CO2 flooding and geological storage in shale.Studying the dispersion characteristics in the process of displacement is of great significance in evaluating the mixing degree of CO2 and oil/gas,the purity of produced gas,the enhanced oil recovery and the CO2 storage effect.Taking the shale reservoirs in the Lower Silurian Longmaxi Formation of the Sichuan Basin and the Upper Triassic Yanchang Formation of the Ordos Basin as examples,this paper analyzes the relationships of CO2 adsorption capacity of shale reservoirs with temperature,pressure,and shale physical properties(TOC,mineral composition,porosity,and specific surface area),and the influence of CO2 injection rate on dispersion coefficient by using the volume and dynamic column breakthrough methods,through physical simulation experiments of CO2 isothermal adsorption and CO2 displacing CH4.The following results are obtained.First,the CO2 adsorption capacity of shale reservoirs increases gradually with pressure.When the pressure exceeds 12 MPa,the adsorption capacity gets saturated,with a saturation adsorption capacity between 1.8 cm3/g and 8.1 cm3/g.Second,the CO2 saturation adsorption capacity is negatively correlated with temperature,and positively correlated with pressure,shale TOC,specific surface area,and pore volume.Third,the dispersion process of CO2 displacing CH4 occurs in the transitional zone dominated by convection and diffusion,whose apparent dispersion coefficient is in the order of 10-7 m2/s and in a linear correlation with the CO2 injection rate.The research results reveal the dispersion patterns of CO2,and preliminarily confirm the feasibility of injecting CO2 to improve shale oil and gas recovery and carbon storage in shale reservoirs,while providing key parameters for the optimal development planning and laying a theoretical basis for related researches and engineering practices.
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