A study of gas transport mechanisms in shale's confined nanopores: Examining irregularity, adsorption effects, and stresses

Abstract

Many difficulties and challenges have been encountered during the exploration and development of shale gas, among which high flexibility of the reservoir structure and low permeability have been the most notable problems that have restricted the efficient development of shale gas. In this paper, we have developed a fractal apparent permeability model for shale based on fractal theory that has taken into account the confinement effects. Also considering the effect of pore deformation on porosity, the defining equation of pore size under the combined effect of multiple factors is obtained, which, in turn, leads to the defining equation of dynamic fractal dimension. Due to the significant confinement effect due to the development of nanopores in shale reservoirs, the Peng–Robinson equation of state is modified using the adsorption effect, and the influence of the confinement effect on the critical properties and each permeability parameter is considered. Based on this, a shale fractal apparent permeability model coupled with slip flow, Knudsen diffusion, and surface diffusion was developed, and the model was validated with experimental data. The results revealed that the developed model was in relatively better agreement with the measured data. Furthermore, the confinement effect performed a positive role in shale's apparent permeability, with the calculated values of model permeability that considered the confinement effect was greater than the calculated values of model permeability, without the confinement effects being considered.