Nanopore Confinement and Pore Connectivity Considerations in Modeling Unconventional Resources

Abstract

Gas and liquid production from nanopore shale resources substantially increased during the past decade due to the advances in horizontal drilling and multistage hydraulic fracturing. Transport properties and mechanisms deviate from their bulk behavior when the pore sizes in unconventional formations are in the order of nanoscale. This is due to the dominant molecule–pore wall interaction effects comparing to molecule–molecule interactions in nanopores. Thus, the physics of multiphase flow in current commercial simulators should be changed to include the effect of pore size on both transport mechanisms and fluid properties. In this study, we analyze the effect of fluid confinement on phase behavior, fluid properties, and condensate banking around the hydraulic fracture where nanopores perform as the dominate storage region and dispersed with pores with bulk behavior. We modified critical properties of the fluid components for different pore sizes in the phase behavior calculations. Using experimental results, we developed a new correlation for estimating mean pore size as a function of permeability and porosity. Moreover, we considered pore size distribution of a shale sample to divide the reservoir into different regions. For each region, a specific permeability is assigned using the new developed correlation. Three different types of connectivity are considered between pores and its impact on production mechanisms is analyzed. Results of this study indicated that neglecting nanopore confinement effect on phase behavior results in an underestimation of the production while neglecting permeability change with pore size results in an overestimation of hydrocarbon production. The connectivity of different pore sizes has a significant impact on reservoir performance and determines the dominant factor.