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

Abstract

CRC-MD simulations show that nanopores in shales bounded by clay minerals have a strong preference for CO ₂ relative to CH ₄.

Abstract

The interactions among fluid species such as H ₂O, CO ₂, and CH ₄ 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 CO ₂, and CH ₄ is less well understood than that of H ₂O. This paper presents the results of a computational modeling study of the partitioning of CO ₂ and CH ₄ 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 CO ₂ relative to CH ₄ suggesting that methods of enhanced oil and gas production using CO ₂ will readily displace CH ₄ from such pores. This preference for CO ₂ is due to its preferred interaction with the surfaces and extends to approximately 20 Å from them.