GCMC simulations on the adsorption mechanisms of CH4 and CO2 in K-illite and their implications for shale gas exploration and development

Understanding the adsorption mechanism of shale gas is an essential pre-requisite for establishing models to evaluate the adsorbed gas amount under geological conditions quantitatively and to guide shale gas exploration and development. By using the Grand Canonical Monte Carlo (GCMC) method, we simulated the adsorption behavior of CH4 and CO2 in K-illite slit pores, and revealed the key gas adsorption mechanisms and discussed their implications on shale gas exploration and development by analyzing the distribution of gas mole concentration, gas-surface interaction energy, density field and etc. It is found that even with the presence of weak adsorption layers in meso and macro pores, the adsorption behavior of both CH4 and CO2 is dominated by the strong adsorption layers and thus can only be approximated but not strictly described by the classic Langmuir model. However, the micro-pore filling effect leads to the overlap of adsorption layers in micro pores, causing more deviation when using the classic Langmuir model to evaluate the adsorption behavior. Even though the adsorption behavior is not affected by pore size dimensions in meso and macro pores, the proportion of the adsorbed gas increases with the decreasing pore size. Both CH4 and CO2 are adsorbed in the center of the six-membered oxygen ring on the silicon oxygen tetrahedron surface. The CH4 molecules (with no polarity) are at the center of the ring, but the CO2 molecules (with electric quadrupole moment) are closer to the oxygen atom with polarity in the ring. The electric quadrupole moment makes the adsorption capacity of CO2 much stronger than that of CH4 in K-illite pores, providing a theoretical basis for enhancing CH4 recovery efficiency by injecting CO2 in the development of shale gas.