Pore Accessibility and Connectivity of Mineral and Kerogen Phases in Shales

The main barrier to sustainable development of US shale, the pore structure of the nanopores storing and transporting hydrocarbon, has been quietly ignored. Fluid flow and mass transport in porous media is controlled by pore structure, which has both geometric and topological characteristics; these characteristics therefore affect exploration and production of hydrocarbons. Considering the composition of mineral and kerogen phases and their associated nanopores in shales, we have studied tracer penetration and distribution, and its association with mineral and organic kerogen phases. We examined imbibition behavior, and imbibed tracer distribution, for fluids (API brine or n-decane) imbibed into initially dry Barnett shale samples. Pore connection was also probed by injecting molten Wood's metal alloy at pressures as high as 6,000 bars, followed by imaging and elemental mapping. The extremely sensitive detection of Wood's metal component elements by laser ablation-ICP-MS mapping reveals that only about 1/1000th of the edge-accessible porosity is connected in the sample interior, which is consistent with other experimental approaches and theoretical interpretation of pore connectivity based on percolation theory. This sparse pore space connection within the shale matrix limits fracture-matrix exchange in fractured shale, resulting in steep initial production decline and overall low recovery.