A criterion for evaluating the effect of shale-matrix dual-continuum flow on gas production

Abstract

Because of the large range of pore size distribution and the chemical/mineralogical heterogeneity within shale matrix, gas flow in it may exhibit dual-continuum flow behavior even though rock matrix is not fractured. The relative importance of dual-continuum flow has considerable implications to the shale reservoir characterization and gas production. A matrix-dominant gas flow model is developed to evaluate the impacts of dual-continuum gas flow on gas production. The simulation results reveal three regimes with regard to the relative importance of the dual-continuum flow. When the mass transfer coefficient between the mobile and immobile continua is relatively high, the dual-continuum behavior becomes insignificant within the context of production rate prediction. When the mass transfer coefficient is very small, the dual continuum gas flow behavior also becomes insignificant because gas flow between the two continua is essentially negligible. Thus, the dual-continuum gas flow behavior becomes important to the long-term gas production rate only for the intermediate range of mass transfer coefficient that is shown to depend on porosity and gas adsorption properties. Based on the physical reasoning and numerical simulation results, we propose a criterion to determine the relative importance of the dual-continuum gas flow. The criterion is expressed with a dimensionless parameter that combines several parameters including the mass transfer coefficient, gas viscosity and densities and matrix porosity. The numerical simulation results show that when the dimensionless parameter is less than a critical value, the dual-continuum gas flow becomes important under a variety of different conditions.