The Influence of Microporosity on the Effective Permeability of Reservoir Rocks

A number of reservoir rocks are characterized by two types of porosity: Macroporosity, a network of pore space along the edges and boundaries of the major mineral phases, and microporosity consisiting of clusters of porous fine grained aggregates distributed through the rock. Owing to the small size of micropores, they are often beyond the resolution of standard microtomographic imaging. This study investigates the effects of micro-porosity on flow in two homogenous, porous sandstones through the measurement of permeability with and without microporosity. We use micro-CT scans of the Doddington ( 18 vol% total porosity) and Knorringfjellet ( 8 vol% total porosity) formations to assess the influence of microporosity on the pore-network connetccivity and permeability. We segment the grayscale images to segregate the macroporosity, microporosity and grains. We then separate the connected porosity before extracting a pore network model and simulating absolute permeability. The analysis indicates similar porosities and permeabilities to prior lab-based results. Pore network modeling of the rocks indicate that microporosity increases connectivity and produces wider throats, particularly in the tight Knorringfjellet sandstone. We propose that this may be due to the majority of the micro-porosity occuring on grain edges rather than isolated within the matrix. Our analysis also demonstrates that the fraction of total connected pore space and total permeability are substantially influenced by the volume fraction of pores in the microporous clusters.