N influences on CH accumulation and displacement in shale by molecular dynamics.

N 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 N on the CH accumulation and displacement processes by adopting practical conditions in the subsurface environment. In equilibrium MD simulation processes, including the N from outside and inner kerogen matrix, keeping the gas ratio of 1:3 for CH and N, the displacement is 52.4% and 65.3%, respectively, which suggests that CH cannot be entirely displaced by surrounding N particles, owing to the strong interaction between CH and the kerogen matrix. For the straightforward displacement process by N, the displacement efficiency is enhanced by 71.7% at the 1:1 gas ratio. Another case of N, in which generation is accompanied by displacement processes, at the ratio 1:2 for N:CH, shows a 47.1% displacement efficiency. This work evidences that the straightforward displacement process is more efficient on CH displacement, which enhances CH production at a pronounced scale and sheds light on the N displacement process in industrial shale gas reservoir production.