An analytical model to couple gas storage and transport capacity in organic matter with noncircular pores

Abstract

Scanning Electro Microscope (SEM) images illustrate the variety of possible pore shape in organic matter of shale reservoirs. The size of the pores with different geometries is at nanoscale (10–100 s nm), hence the ratio of wetted surface area to the volume of pores (specific surface area, SSA) is high. For the systems with high SSA the collisions between gas molecules and pore walls become significant, therefore, fluid flow is not dominantly controlled by the bulk flow, i.e., fluid-wall surface interaction becomes important. Most shale permeability models assume circular nanopores that results in poor prediction of permeability. We present a novel analytical apparent porosity and permeability model to model gas storage and permeability in shale gas reservoirs with noncircular nanopores. The SSA and the aspect ratio of height to the width of noncircular nanopores, were both used in our model to couple gas storage and transport capacity. We validated our model with permeability values calculated from pore network simulations of five shale samples from Jianghan Basin of China. The results showed that sharp edges in nanopores could dramatically affect permeability. For examples, the noncircularity deviation of gas flow in a rectangular nanopore is more than an equivalent nanopore with elliptical cross-section. The assumption of circular cross-section nanopores in estimating apparent porosity and permeability could impose up to 55% error depending on the pore geometry.