Understanding methane/carbon dioxide partitioning in clay nano- and meso-pores with constant reservoir composition molecular dynamics modeling

Abstract

The interactions among fluid species such as H₂O, CO₂, and CH₄ confined in nano- and meso-pores in shales and other rocks is of central concern to understanding the chemical behavior and transport properties of these species in the earth's subsurface and is of special concern to geological C-sequestration and enhanced production of oil and natural gas. The behavior of CO₂, and CH₄ is less well understood than that of H₂O. This paper presents the results of a computational modeling study of the partitioning of CO₂ and CH₄ between bulk fluid and nano- and meso-pores bounded by the common clay mineral montmorillonite. The calculations were done at 323 K and a total fluid pressure of 124 bars using a novel approach (constant reservoir composition molecular dynamics, CRC-MD) that uses bias forces to maintain a constant composition in the fluid external to the pore. This purely MD approach overcomes the difficulties in making stochastic particle insertion–deletion moves in dense fluids encountered in grand canonical Monte Carlo and related hybrid approaches. The results show that both the basal siloxane surfaces and protonated broken edge surfaces of montmorillonite both prefer CO₂ relative to CH₄ suggesting that methods of enhanced oil and gas production using CO₂ will readily displace CH₄ from such pores. This preference for CO₂ is due to its preferred interaction with the surfaces and extends to approximately 20 Å from them.