N₂ influences on CH₄ accumulation and displacement in shale by molecular dynamics

N₂ is generally employed as a displacement agent to enhance gas recovery in shale gas-bearing reservoirs. However, the primary displacement mechanism in the subsurface still needs to be clarified due to the characteristics of shale reservoirs with low porosity and abundant nanopores. This study employs the Molecular Dynamics (MD) simulation method to investigate the effects of N₂ on the CH₄ accumulation and displacement processes by adopting practical conditions in the subsurface environment. In equilibrium MD simulation processes, including the N₂ from outside and inner kerogen matrix, keeping the gas ratio of 1:3 for CH₄ and N₂, the displacement is 52.4% and 65.3%, respectively, which suggests that CH₄ cannot be entirely displaced by surrounding N₂ particles, owing to the strong interaction between CH₄ and the kerogen matrix. For the straightforward displacement process by N₂, the displacement efficiency is enhanced by 71.7% at the 1:1 gas ratio. Another case of N₂, in which generation is accompanied by displacement processes, at the ratio 1:2 for N₂:CH₄, shows a 47.1% displacement efficiency. This work evidences that the straightforward displacement process is more efficient on CH₄ displacement, which enhances CH₄ production at a pronounced scale and sheds light on the N₂ displacement process in industrial shale gas reservoir production.