Methane storage in nano-pores: Molecular dynamics simulation and density functional theory

Abstract

Anticipated changes in the complex reservoir production business model increase the demand for reliable methods of the total reservoir storage and capacity estimation for tight formations. The task is complicated by the fact that at nano-scale phase transition and adsorption phenomena are system size dependent. Therefore, fluid phase behavior in nano-pores is sufficiently different from those in the bulk. As the result, understanding of hydrocarbon content behavior in nano-pores at different pressures is of high demand, because such knowledge is crucial for development of an optimum production plan.In the current study, we present a detailed investigation of neat methane distribution in a system of slit nano-pores formed by graphene sheets. The fluid in slit pores is considered to be in thermodynamic equilibrium with the fluid in the larger matrix pore (bulk), thus the concentration of methane in pores is governed by the pressure in the connected bulk volume. The pressure and temperature ranges considered are chosen to represent typical reservoir conditions.The main goal of this study is to suggest a method that (1) is simple enough to be routinely applied for the practical purposes within the industry, and (2) takes into account all major phenomena, which govern fluid behavior in nano-scale pores. To match both criteria, we started with precise molecular dynamics (MD) simulations (fully atomistic and coarse grained) and then utilized the obtained data to calibrate parameters for classical density functional theory approach (c-DFT). In such a way, we managed to adjust computationally cheap and robust c-DFT method to produce the same results as atomically precise MD simulation. Finally, we summarized some results in a simple analytical formula, which describes methane adsorption on flat surfaces.The developed approach enables improved gas in-place estimation, especially in tight formations, which can be extremely valuable for both production design and economics evaluation.