A Comparison of Gas and Water Permeability in Clay‐Bearing Fault and Reservoir Rocks: Experimental Results and Evolution Mechanisms

Abstract

To better understand the discrepancy between gas and water permeability and the mechanism of permeability in response to the presence of water in clay‐bearing rocks, we performed transport property experiments on synthetic quartz‐clay mixtures and natural fault gouge, as well as clay‐bearing sandstones for comparison purpose. The experiments were performed on a fluid flow apparatus with effective pressures (Pe) cycling between 5 and 105 MPa. Each sample was subjected to nine Pe cycles, along which permeability and porosity of either the gas‐saturated (nitrogen) or water‐saturated samples were measured. The results show that the permeability of all samples investigated decreases with increasing Pe. Gas permeability exhibits strong pore pressure dependence, which can be explained by the slippage (Klinkenberg) effect. Permeability results measured by water are commonly lower than the gas results. The decrease in permeability after the addition of water can be generally explained by the evolution of porosity, as can be determined from the bulk volume and solid mineral volume measurement results. The loss of permeability is associated with the hydration expansion of clay minerals that causes the reduction of porosity. A conceptual model for the evolution of porosity that incorporates clay expansion and grain rearrangement is proposed to describe the mechanism for water permeability in clay‐bearing rocks. Based on these results, we discuss the effect of clay hydration on fluid transport and mechanical behaviors in clay‐bearing fault zones and reservoir formations following fluid injection.

Quantification of fault leakage dynamics based on leakage magnitude and dip angle

“…For faults within layers of different lithologies (carbonates vs. sandstone), faults with different mineral compositions (e.g., clay content), or faults at different stages of their earthquake cycle, slip-induced alteration of the fault permeability may be different from the model used here (Equation 12) and may require analysis of lab or field data. [83][84][85]…”

A novel method for determining water saturation of porous media in relative permeability measurement using DPT

“…In addition, some clay minerals (3.32% illite and 1.36% montmorillonite) are present in tight sandstone. The hydration expansion of clay minerals will reduce the effective pore throat and porosity upon contact with water, which causes lower permeability when measured by water rather than by gas [26,27]. Therefore, the choice of absolute permeability causes a small relative permeability for both gas and water.…”

Aquifer Permeability Decreases Before Local Earthquakes Inferred From Water Level Response to Period Loading

“…Field observations have also documented similar behavior (e.g., Elkhoury et al., 2006; Xue et al., 2013). Laboratory experiments, however, suggest that permeability reduction is also possible under dynamic stress effects (Duan et al., 2020; Liu & Manga, 2009; J. Zhou et al., 2019), and this understanding has recently been supported by various field observations (Rutter et al., 2016; Shi et al., 2018; Sun & Xiang, 2019; Z. L. Zhou et al., 2020).…”