Phase behavior of methane in shale inorganic nanopores using Monte Carlo molecular simulation

Abstract

Since methane is an important component of shale gas, its confined phase behavior is significant to understand the essential mechanism leading to the fluid occurrence and hydrocarbons production. We studied the confinement impact of shale inorganic nanopores with pore sizes of 2–20 nm, for temperatures at phase equilibrium (120–190 K) and reservoir condition (325 K) using Monte Carlo molecular simulations. Compared with the bulk, the adsorption increases the equilibrium gas density, while the capillary condensation reduces the liquid density and equilibrium pressure, and the effect of adsorption is more significant. Affected by the equilibrium phase change, the methane reaches the critical point under lower temperature and pressure in inorganic nanopores. We fitted the critical shifts and explored the impact of pore width and wall material on fluid properties (phase density, equilibrium pressure, critical parameter, and density distribution). The confinement effect is positively correlated with the interaction between mineral composition and methane while decreases with the increasing pore width. In both phase equilibrium and reservoir conditions, the phase behavior in different mineral slits is significantly different, especially in the pores less than 10 nm. The confined phase equilibrium of methane is related to the pore pressure, and is not affected by the size and wall material of other pores. This research has a better insight into the phase behavior of methane in shale inorganic materials, and provides support for the phase equilibrium calculation of single pore and the upgrade of multiple pore sizes.