Asymmetric competitive adsorption of CO2/CH4 binary mixture in shale matrix with heterogeneous surfaces

Abstract

• Fixed mole fractions of CO 2 and CH 4 in the total pores were proposed as a parameter. • A graphene-MMT heterogeneous surface was proposed as a pore model. • Absolute and relative adsorption selectivity were proposed and compared. • 6 nm is the critical pore size near graphene surface caused by confinement effect. • The average pore size around 12 nm is optimal during cracking process. The CO 2 sequestration and enhanced gas recovery (CS-EGR) technology provides a very effective way to alleviate the greenhouse effect and energy crisis. The characteristics and mechanisms of CO 2 /CH 4 competitive adsorption in shale matrix as well as the effects of reservoir parameters play a vital role in enhancing shale gas production and CO 2 storage. However, it has not been fully understood especially in pores with heterogeneous surfaces composed of organic matter and inorganic compounds. A graphene-MMT heterogeneous surface was proposed as a pore model and molecular dynamics (MD) simulations have been applied. Absolute adsorption selectivity based on the fixed mole fractions of CO 2 and CH 4 in the total shale pores has been proposed to evaluate the competitive adsorption features with relative adsorption selectivity. A strong asymmetric competitive adsorption behavior of CO 2 and CH 4 has been observed. The competitive adsorption behavior near the MMT surface lags behind that near the graphene surface, especially in 3 ~ 8 nm pores. The influence mechanism of pore size on competitive adsorption is believed to be the confinement effect of the pore leads to the competition for the gas source in the pore between the heterogeneous surfaces. The confinement effect near the graphene surface is obvious when the pore size is smaller than a critical pore size of 6 nm. In addition, the confinement effect of CO 2 in pores with heterogeneous surfaces is more obvious than that of CH 4 . Based on absolute and relative adsorption selectivity, it’s reasonable to control the average pore size around 12 nm during cracking process, and the pressure may be chosen as low as possible within the proper range obtained based on the absolute adsorption selectivity.