Evaluation and modeling of water vapor sorption and transport in nanoporous shale

Abstract

Understanding vapor transport of water in nanoporous shale is challenging due to the coexistence of multiple water phases within a multi-mineral aggregate with complex multiscale pore architecture. We explore this response through dynamic vapor sorption experiments and modeling on two shale samples with differing fractions of hydrophilic clays and contrasting pore architecture. Measured diffusion coefficients of water vapor in the two shales are of the order of magnitude of 10−12 - 10−10 m2/s, increasing with relative humidity (Rh) except at high Rh during adsorption process. The drop in diffusivity at high Rh during adsorption results from the impeding effect of capillary-occluding air bubbles and flattening of the pore-entry menisci. We propose a model for water vapor transport accommodating surface flow of adsorbed water and viscous flow of capillary water – with active mechanisms operational in different pore size populations. Actual pore size distributions (PSDs) are characterized by low pressure nitrogen adsorption. Predictions from the proposed transport model, utilizing these measured PSDs, are consistent with the measured diffusion behavior during desorption, also replicating water uptake behavior across the full spectrum of 0 < Rh < 1. Observations and modeling illustrate that phase type and pore size significantly influence water vapor sorption and transport behaviors. Storage of the adsorbed phase dominates the total water uptake at low Rh (< 0.6) while the condensed phase dominates at Rh > 0.6. Surface flow of the adsorbed phase contributes predominantly to the total flux over a wide range of Rh (< 0.96) while viscous flow of capillary water dominates only at very high Rh values (> 0.98). In terms of pore size effects, macropores (> 50 nm) contribute little to the total water adsorption but comprise more than of the 68% total water flux. Conversely, micropores (< 2 nm), contribute moderately to water adsorption (7%–40%) but insignificantly to the total flux. Intermediate-sized mesopores (2–50 nm) play an important role in both total water adsorption and transport over the entire range of Rh. Sensitivity analysis of temperature (30–90 °C) indicates that diffusion coefficient of water vapor can be enhanced at higher temperature due to a lower viscous resistance of water to flow.