Abstract
Understanding the adsorption mechanisms of CO2 and N2 in illite, one of the main components of clay in shale, is important to improve the precision of the shale gas exploration and development. We investigated the adsorption mechanisms of CO2 and N2 in K-illite with varying pore sizes at the temperature of 333, 363 and 393 K over a broad range of pressures up to 30 MPa using the grand canonical Monte Carlo (GCMC) simulation method. The simulation system is proved to be reasonable and suitable through the discussion of the impact of cation dynamics and pore wall thickness. The simulation results of the excess adsorption amount, expressed per unit surface area of illite, is in general consistency with published experimental results. It is found that the sorption potential overlaps in micropores, leading to a decreasing excess adsorption amount with the increase of pore size at low pressure, and a reverse trend at high pressure. The excess adsorption amount increases with increasing pressure to a maximum and then decreases with further increase in the pressure, and the decreasing amount is found to increase with the increasing pore size. For pores with size greater larger than 2 nm, the overlap effect disappears.
Highlights
Because of the continuous growing of the global energy demands and the insufficient conventional oil and gas resources, more and more attentions are being focused on the unconventional energy resources[1]
As the cation dynamics and the pore wall thickness are the critical factors that influence the suitability of simulation system, we discuss the impact of these two factors before the mechanism investigation
The grand canonical Monte Carlo (GCMC) simulations with rigid potassium cations will not be able to reflect exactly the adsorption behavior in natural illite mineral
Summary
Because of the continuous growing of the global energy demands and the insufficient conventional oil and gas resources, more and more attentions are being focused on the unconventional energy resources[1]. The mechanisms of the overlap effect of adsorption behavior in micropores and its corresponding phenomenon on isotherms have not been discussed systematically To solve these problems, the grand canonical Monte Carlo (GCMC) simulations on CO2 and N2 in K-illite with varying pore sizes are performed at the temperature of 333, 363 and 393 K over a range of pressures up to 30 MPa. Before the investigation of adsorption mechanisms, the impact of cation dynamics in the simulation system and the pore wall thickness of the simulation cell is discussed to prove the suitability of the simulation system. The overlap effect of the adsorption behavior is discussed in detail by comparing the gas density profiles, the gas-clay interaction energies and the excess adsorption isotherms for pores with varying sizes, and the mechanisms of overlap effect in micropores are revealed
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