Pore‐Scale Study of Water Adsorption and Subsequent Methane Transport in Clay in the Presence of Wettability Heterogeneity

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. They are commonly associated with 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 use a 3-D lattice Boltzmann model to study water adsorption and condensation in a reconstructed clay microstructure based on broad-ion-beam scanning electron microscopy images. Three wettability conditions are considered including a water-wet case, a water-repellent case, and a mixed-wet case. The spatial distribution of the condensed liquid water phase is visualized, and the connectivity of the remaining open pore spaces is characterized. For the water-wet case, water preferentially condenses in the small and narrow pore spaces, while the water-repellent case reports the opposite trend. The mixed-wet case results in mixed behavior, and connectivity of the pore space is bound by the former two cases. We then study 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.

Determination of Apparent Pore Size Distributions of Organic Matter and Inorganic Matter in Shale Rocks Based on Water and N2 Adsorption

“…During a thermal evolution process, oxygen functional groups are gradually lost, and the surfaces of OM pores become hydrophobic. − Therefore, this approach is a relatively accurate method for a mature or postmature shale sample while it may result in remarkable deviations for an immature one. Shale is a nanoporous media, and its pores are interconnected with each other to form a complex network. During the water or low-pressure N 2 adsorption–desorption process, a condensation effect in small nanopores may lead to gas phase trapping in connected large pores, while the latter will not appear on the APSD curves. , This phenomenon can result in an underestimation of a BJH pore volume. In our proposed method, illite is assumed to be representative of IOM because (i) the main clay composition of our shale samples is illite, and (ii) most IOM nanopores smaller than 200 nm mainly exist in clay minerals. , There is no doubt that clay type will influence the accuracy of our method. In my previous study, the APSD curves of different clay minerals (i.e., smectite, illite, and kaolinite) show the same condensation phenomenon in smaller nanopores (<5 nm) .…”

“…Shale is a nanoporous media, and its pores are interconnected with each other to form a complex network. During the water or low-pressure N 2 adsorption–desorption process, a condensation effect in small nanopores may lead to gas phase trapping in connected large pores, while the latter will not appear on the APSD curves. , This phenomenon can result in an underestimation of a BJH pore volume.…”

“…During the water or low-pressure N 2 adsorption−desorption process, a condensation effect in small nanopores may lead to gas phase trapping in connected large pores, while the latter will not appear on the APSD curves. 50,51 This phenomenon can result in an underestimation of a BJH pore volume. (3) In our proposed method, illite is assumed to be representative of IOM because (i) the main clay composition of our shale samples is illite, and (ii) most IOM nanopores smaller than 200 nm mainly exist in clay minerals.…”

A comprehensive review on the flow behaviour in shale gas reservoirs: Multi‐scale, multi‐phase, and multi‐physics

“…(a) Fluid transport behaviour in shale nanopores with realistic surfaces: although the elementary compositions and molecular structures of OM and IOM can be obtained with chemical analysis, nanopores with realistic surface properties still cannot be fully structured in MD simulations, including the maturity of OM, surface charge of IOM, surface roughness, and the wettability heterogeneity of pore surfaces, which can have significant influences on gas adsorption and water transport. [265][266][267] (b) The properties of fluids in nanopores: for gas transport, multicomponent gas transport in nanopores should be paid more attention; for water transport, pure water can be extended to a saline solution or non-Newtonian fluid. [35,268,269] (c) Compared with single-phase transport, simulation of multiphase transport in shale nanopores is insufficient: the flow pattern and interfacial interactions of multiple fluids at the nanoscale are still unclear.…”

Hybrid Pore-Scale Adsorption Model for CO2 and CH4 Storage in Shale

“…Although these models vary in their complexity, their application can often be restricted to small, simple domains (single pore systems with simplified pore geometries). The development of multitechnique approaches − is particularly promising for the most realistic description of gas storage and transport in shale. While they provide useful insight into the adsorption process on a molecular level, these numerical approaches remain computationally expensive, requiring the application of parallel computing to offset this cost, as has been successfully demonstrated for lattice Boltzmann methods. , Pore-scale numerical approaches, such as the lattice density functional theory (lattice DFT), have been shown to be a practical, yet physically sound, compromise for the description of supercritical gas adsorption in porous solids. , The lattice DFT model incorporates the pore size distribution of the adsorbent to describe the adsorption behavior on a pore-by-pore basis, and it explicitly accounts for adsorbate–adsorbate and adsorbate–adsorbent interactions.…”