Pore Architecture and Connectivity in Gas Shale

Abstract

The pore size distribution and architecture in gas shales were studied using a combination of small-angle neutron scattering (SANS), mercury injection capillary pressure (MICP), and helium ion microscopy (HIM). SANS analysis shows that the pore size population is not a power-law distribution across many length scales, typical of sedimentary rocks, but contains an anomalous population of pores on-the-order ∼2 nm, housed primarily in the organic matter. A model is presented showing how a “foamy porosity” with such a characteristic size is a direct result of diagenetic evolution of kerogen. Cross-linking of the kerogen combined with phase separation of gas/oil, leads to arrested coarsening with a length scale set by the cross-length density. These pore populations determined by the scattering model are directly supported by HIM images. Pore connectivity determined through pore-size-to-pore-throat analysis, suggests that interpore connections are also distinct from typical sedimentary rocks. The pore/throat ratio, unlike the simple ratios predicted from sphere packing and found for clastic rocks, is nearly constant over all pore sizes. Kerogen diagenesis is a recognized source of excess internal pressure. When this pressure causes failure of the material surrounding the kerogen to create escape pathways for the phase-separated fluid, it is likely that escape pathways will connect intergranular porosity via microfractures, producing the relatively narrow aperture size distribution.