A non-equilibrium molecular dynamics study of methane transport in clay nano-pores

• Molecular dynamics is used to estimate methane gas diffusivities in shale models.
• Temperature control algorithms (thermostats) influence apparent diffusivities.
• Computational cost is minimized by judicious selection of thermostat scheme.
• Knudsen model fails to predict methane diffusivity in clay nano-pores.

Accurate characterization of shale reservoir properties is vitally important for reliable gas production forecasting and reserve estimation. In this work, we use boundary-driven non-equilibrium molecular dynamics (BD-NEMD) to investigate methane transport in porous clays, which are one of the major mineral components of shale. One of the critical issues in BD-NEMD simulations is robust temperature control schemes to maintain isothermal conditions when the external force is applied to drive fluid flow. To this end, we scrutinize the performance of six temperature control schemes for BD-NEMD. We identify a number of effective temperature control schemes, and we show that careful algorithm selection can considerably reduce the number of simulations required to estimate transport diffusion coefficients. Using a robust temperature control scheme identified in our study, we examine the validity of the Knudsen model for predicting methane diffusivities in clay nano-pores. Although the Knudsen model is one of the most widely used theoretical approaches for predicting reservoir properties, we find that it fails to accurately predict methane transport diffusivities in small clay nano-pores. Our findings therefore suggest that reservoir models based on Knudsen theory may result in significant over- or under-prediction of gas production, depending on reservoir conditions. These findings are consistent with other studies, which posit that the Knudsen diffusion model breaks down in nano-confined systems because it neglects to account for the effects of adsorption on gas transport.

Figure optionsDownload high-quality image (456 K)Download as PowerPoint slide