Confinement effect in nanopores of shale and coal reservoirs: A review on experimental characterization methods

The confinement effect in nanopores significantly impacts the phase behavior of fluids in unconventional reservoirs and controls the occurrence and transport of fluids. As a result, the phase behavior, thermodynamic properties and phase equilibrium of bulk gases, in particular, could significantly change due to the surface-molecules interactions. On the one hand, due to the heterogeneous and anisotropic nature of unconventional reservoirs, experimental analysis of the confinement effects is utterly challenging. On the other hand, molecular simulation techniques for characterizing confinement effects are inaccurate since most of them are not verified with experimental data. In this work, we have reviewed the experimental results in the literature focusing on the confinement effect in nanopores. The behavior of confined gases in shale and coal reservoirs can be obtained by analyzing the small-angle neutron/X-ray scattering (SANS/SAXS) data using in-situ measurement and contrast matching small-angle neutron scattering (CM-SANS) methodologies. We reviewed the phase transition of confined carbon dioxide and methane in nanopores where the densities of both would differ from their bulk densities and are known to be pressure- or pore-size-sensitive. The current work investigates the application and future perspectives of utilizing the confinement effect in shale and coal reservoirs for enhancing gas/oil recovery and carbon dioxide sequestration. Additionally, given the recent global interest in hydrogen storage in such reservoirs, our literature review concludes that shale and coal reservoirs might be considered as repositories for hydrogen if the confinement effect of fluids is well understood.