Storage Capacity, Sorption and Mass Transfer in Shale

Abstract:

Natural gas in shale exists both as free gas and as adsorbed gas. Accordingly, estimating gas-in-place and recovery potential requires insight into shale’s petrophysical and transport properties. Despite numerous studies in the literature, it is still a challenge to characterize shale samples, including related sorption and mass transfer phenomena, and to translate observations from laboratory scale to field scale. In this work, we investigate the role of adsorbate phase density on storage capacity, as well as on the effective transport properties. Experimental observations, such as excess adsorption isotherms and sorption kinetics are interpreted via an array of adsorbate density models and observations from molecular dynamics (MD) simulations. We perform MD simulations of pure fluids and mixtures in SiO₂ slit nanopores to examine adsorption and mass transfer for various pressure conditions, mixture compositions and pore geometries. Direct comparison allows us to connect atomic-scale simulations with experimental observations, and then we extend these conclusions to explain behavior in shale samples with heterogeneous compositions.