NANOSCALE PORE FEATURES AND FLUID BEHAVIOR IN SHALE

Geological formations that host unconventional oil and gas are extraordinarily heterogeneous and exhibit a wide range of physical and chemical features that can vary over many orders of magnitude in length scale. The size, distribution and connectivity of these confined geometries, the chemistry of the solid, the chemistry of the fluids and their physical properties collectively dictate how fluids migrate into and through these micro- and nano-environments, wet and ultimately react with the solid surfaces. In particular, the behavior of C-O-H fluids at mineral surfaces or in confined geometries (pores, fractures) typically differs from their bulk behavior in many ways due to the effects of large internal surfaces and geometrical confinement. Phase transitions (i.e., freezing and capillary condensation), sorption and wetting, and dynamical properties, including diffusion, relaxation and chemical reactivity, may be modified, with the strongest changes observed for pores or fracture apertures ranging in size from <2 to 50 nm typical of most shale. We will first provide a brief overview is provided that highlights the use of advanced electron microscopy and neutrons scattering methods to quantify the nature of the nanopore system that hosts hydrocarbons in representative gas shale formations such as the Utica and Marcellus. Second, results will be presented that leverage the application of state-of-the-art experimental, analytical and computational tools to assess key features of the fluid-matrix interaction relevant to shale settings. The multidisciplinary approaches highlighted will include neutron scattering, thermodynamic measurements and molecular-level simulations to quantitatively assess molecular properties of C-O-H fluids confined to well-characterized porous media, subjected to temperatures and pressures relevant to subsurface energy systems. These studies are beginning to provide a fundamental understanding at the molecular level of how intrinsically different hydrocarbon-bearing fluids behave in confined geometries compared to bulk systems, and shed light on key geochemical processes such as fluid wetting, competitive sorption and the onset of mineral dissolution and precipitation.