Pore scale study of water adsorption and subsequent methane transport in clay in the presence of wettability heterogeneity

• https://doi.org/10.1029/2020WR027568
• https://doi.org/10.1029/2020WR027568

Abstract — Clay minerals are abundant in shale, characterized by a lamellar structure and dimensions smaller than a micron, giving rise to nanometer-scale pore sizes and large specific surface area. When the total organic content of shale is low and the organic nanopores do not form a connected flow pathway, shale gas will have to flow through the inorganic matrix, the major component of which is clay. Clay minerals are commonly associated with immobile water. However, the spatial distribution of the unsaturated water in clay is not very well understood, which significantly affects the subsequent shale gas flow capacity. Wettability heterogeneity in the presence of hydrocarbons further complicates the water distribution in clay. In this study, we used a 3D lattice Boltzmann model to study water adsorption and condensation in a reconstructed clay microstructure based on broad-ion-beam scanning electron microscopy images analysis. Three wettability conditions are considered including a water-wet case, a water-repellent case, and a mixed-wet case. The distribution of the condensed liquid water phase is visualized and the size and connectivity of the remaining open pore spaces are characterized. The water-wet case results in increasingly larger pore size as water saturation increases, indicating the preference of water to condense in smaller pores, while the water-repellent case reports the opposite trend. The mixed-wet case results in bimodal pore size distributions and connectivity of the pore space is bounded by the former two cases. We then studied the subsequent methane flow capacity with the presence of water for these three cases, and the calculated methane relative permeability curves show a percolation threshold for the water-wet case, which is in good agreement with available experimental data in the literature measured on geosynthetic clay liners.

Abstract — The transport of methane in shale is complicated by the heterogeneous pore structure, inhomogeneous surface properties arising from complex mineralogy, and the presence of adsorbed/bound water. With such complexities, the contribution of advection and diffusion to the transport of methane under reservoir conditions is not very well understood. In this work, we conduct pore scale study of methane advection and diffusion in shale and compare their relative contribution. We reconstruct the 3D grain structure of organic matter (OM) and clay-rich regions of a shale sample respectively based on broad-ion-beam (BIB) scanning electron microscopy (SEM) image analysis. The distribution of water on water-wet clay surfaces is modeled via water sorption simulation using a D3Q19 lattice Boltzmann model (LBM) with consideration of real-gas effect and surface forces. OM is assumed to be free of water since the surface is usually water-repellent. Methane advection simulation is conducted using a D3Q19 LBM incorporated with nanoscale flow physics including slip flow and variation of viscosity as a function of the distance to the pore wall. Methane diffusion simulation is conducted using a local diffusivity LBM where multiple diffusion mechanisms namely molecular diffusion, Knudsen diffusion and transition diffusion are involved. The advective and diffusive mass flow rates at a series of water saturation states and pressure conditions in the reconstructed clay and OM structures are calculated and compared. We found that advective flow dominates when porosity and pore sizes are large, the average pressure is high, and the pore space is well-connected. Diffusive flow, on the other hand, cannot be neglected when pore space connectivity is poor or pressure is low. This study provides insight into the fundamental mechanisms of methane transport in the inorganic and organic components of shale. While the analysis is detailed and requires multiscale SEM imaging, it is essential for understanding transport properties. The LBM model introduced here can be used to explore a wide range of thermodynamic or hydrodynamic behavior that happen at nanoscale.

• https://www.pge.utexas.edu/images/dis19xurui.pdf