Numerical quantification of gas adsorption in unconventional shale rocks

Abstract

A first principle based numerical simulation technique for describing gas adsorption process in unconventional shale formations, particularly shale gas deposits, is hereby presented. The model is based on the principles of mass conservation and was developed from incompressible Navier–Stokes equations. First, Navier–Stokes equations are solved for pressure and velocity fields. The obtained velocity fields are embedded numerically in the pressure-diffusion transient equation to obtain new flow and transport models capable of resolving shale rock dynamic behaviour from pore matrix to the shale rock surface due to adsorption. The resulting mathematical models are solved using finite element and finite volume methods. Discretisation of porous synthetic and real shale geometrical samples was carried out numerically using finite element method. The adsorption behaviour in synthetic 3D samples exhibited close gas volume results under both steady-state and transient conditions. In addition, results from real samples highlight rapid decrease in gas volume within the shale matrix as pressure increases. In addition, it was observed that increased pressure gradients accelerated the transport of gas molecules from the shale pore to the shale surface through the process of adsorption. Comparisons were made with an experimental data with the aim of evaluating adsorption process. The extent to which increase in pressure gradient leads to a corresponding rise in adsorption was also investigated. This novel model provides a fundamental framework for investigating adsorption phenomena within unconventional shale formations during multiphase flow and transport which can lead to enhanced gas recovery.