Adsorption Mechanism of CO₂/CH₄ in Kaolinite Clay: Insight from Molecular Simulation

Abstract

Understanding the adsorption mechanism of CO2/CH4 in kaolinite clay is essential for the carbon dioxide geological sequestration and enhanced gas recovery in shale reservoirs. In the present work, grand canonical Monte Carlo simulations were employed to investigate the mechanism of competitive adsorption of CO2/CH4 in kaolinite clay. The effects of pore size (1–6 nm), pressure (0.1–30 MPa), temperature (298–378 K), and moisture content (0–0.122 g/cm3) on the adsorption behaviors of pure CH4 and CO2/CH4 mixture were explored in-depth. Specifically, two adsorption layers, i.e., strong and weak adsorption layers, in kaolinite slitlike micropore under high pressure condition have been observed. It was found that pore size and pressure have great effects on the gas adsorption mechanism in kaolinite. The two adsorption mechanisms including monolayer adsorption and micropore filling under high pressure or small pore size conditions were discussed. In addition, simulation results showed that CO2 has much stronger adsorption ability than CH4 in kaolinite. The adsorption capacity of CH4 was significantly suppressed in the presence of CO2, especially in the strong adsorption layer. An adsorption selectivity over 7 has been found in the strong adsorption layer. Temperature and moisture content have great influences on the adsorption capacity and adsorption selectivity. However, the influences have different scales in strong and weak adsorption layers. It is expected the obtained results could provide insights into the adsorption mechanism of CO2/CH4 and offer fundamental data for a CO2 sequestration and enhanced gas recovery (CS-EGR) project in kaolinite clay.

Molecular Investigation of CO2/CH4 Competitive Adsorption and Confinement in Realistic Shale Kerogen

“…This means that CH 4 molecules under supercritical pressure were in an adsorbed state in the slit kerogen nanopore with a pore size of 2 nm, indicating that micropore filling is the dominant adsorption mechanism. These results are consistent with previous work on adsorption in kaolinite and montmorillonite clay nanopore in shale reservoirs [41,42]. It should be mentioned that the densities of CH 4 in the free state were calculated by molecular simulations under the same conditions of pressure and temperature in an unconfined box in this work.…”

“…In this work, we extended our previous studies on CO 2 /CH 4 competitive adsorption in graphene and kaolinite clay slit nanopores of shale reservoirs [32,41]. Firstly, different kerogen molecular models developed by Ungerer et al [45] were employed to construct a bulk kerogen matrix with different maturity levels.…”

“…It indicates the preferential adsorption of CO 2 over CH 4 in kerogen pores when S CO 2 /CH 4 > 1. For more detailed information regarding GCMC simulation, one can refer to our previous work [23,32,41].…”

“…Their results indicated that the functional group has an obvious impact on the adsorption capacity and selectivity of CO 2 over CH 4 . Additionally, the adsorption behavior and confinement effects of pure CH 4 and a CO 2 /CH 4 mixture were examined in inorganic nanopores [41][42][43]. The effects of pressure and pore size on nanoconfinement and the gas adsorption mechanism were explored and discussed in their work.…”

The role of supercritical carbon dioxide for recovery of shale gas and sequestration in gas shale reservoirs

“…281 With an increase in reservoir pressure, CO 2 shows better sorption ability than CH 4 . 281,283,288 However, CO 2 enhanced CH 4 extraction exists in an optimal pressure range. In the study of Lee, et al, 289 the suggested pressure range was 7 to 9 MPa.…”