Control of CO₂ Permeability Change in Different Rank Coals during Pressure Depletion: An Experimental Study

Abstract

The gas permeability of different rank coals varies because of the summative effects of increasing effective stress, gas slippage, and coal matrix shrinkage during gas pressure depletion. In this paper, the natures of CO 2 permeability change were primarily investigated in a high-volatile A bituminous coal (core D2-2), a moderate volatile bituminous coal (core S1), and an anthracite coal (core P11-2-1). Under a 4.3 MPa confining stress condition, as the gas pressure declines, the CO 2 permeability of core D2-2 gradually decreases and then has a slight increase at mean gas pressures of less than approximately 0.8 MPa, the CO 2 permeability of core S1 initially decreases but subsequently increases above a mean gas pressure of approximately 1.3 MPa, and the CO 2 permeability of core P11-2-1 continuously increases, especially at mean gas pressures of less than approximately 1.8 MPa. These pressure-depletion observations on CO 2 permeability are considered to be the result of three effects: (a) increasing effective stress decreases CO 2 permeability; (b) increased gas slippage increases CO 2 permeability exponentially, becoming significant at mean gas pressures of less than approximately 0.8 MPa for the three cores; and (c) a positive effect on CO 2 permeability from matrix shrinkage occurs at mean pressures of less than approximately 1.3 and 1.8 MPa for cores S1 and P11-2-1, respectively, whereas the CO 2 permeability of core D2-2 is negatively affected by matrix shrinkage at all tested pressures. Additionally, it is found that the three effects on the CO 2 permeability depend upon the permeability of the coal and gas pressure.

Insights on the gas permeability change in porous shale

“…The effect of effective stress on shale permeability is closely related to shale pore-fracture system (Chalmers et al, 2012). In the study by Li et al (2014), they reported that the coal with a lower permeability will have a stronger sensitivity of the permeability to the effective stress. Zhang et al (2015b) investigated the impacts of nanopore structure and elastic properties on shale permeability, and showed that (a) pore compressibility increased with a decreasing pore aspect ratio and Youngs modulus; (b) the permeability of micro-fractures in shale were more sensitive to effective stress than hydraulic fractures.…”

“…and is mathematically expressed by the Klinkenberg equation. Gas slippage causes that the apparent gas permeability is greater than liquid permeability (Klinkenberg, 1941;Li et al, 2014;Firouzi et al, 2014). The slippage effect is mainly affected by pore size and gas pressure but is not obviously influenced by the temperature (Fig.…”

“…Recently, Li et al (2013Li et al ( , 2014Li et al ( , 2015Li et al ( , 2017b analyzed the effects of effective stress, gas slippage, matrix shrinkage and Knudsen diffusion on the gas permeability change in coals. They also proposed a prediction model of gas permeability change based on the effects of effective stress, gas slippage and matrix shrinkage (Li et al, 2013).…”

Adsorption Properties of Hydrocarbons (n-Decane, Methyl Cyclohexane and Toluene) on Clay Minerals: An Experimental Study

“…The adsorption of hydrocarbons may affect hydrocarbon migration in the subsurface [1], which significantly impacts the recovery from unconventional reservoirs [2][3][4][5][6]. Clay minerals form the primary adsorbent surface for adsorbed hydrocarbons in mudstone/shale [7][8][9][10].…”

“…Then, experiments are carried out to determine the adsorption capacity of minerals (e.g., clay minerals, quartz, feldspar, calcite and dolomite) as a function of equilibrium time or hydrocarbon concentration under a certain temperature. Previous studies showed the following: (1) for shale-oil, clay minerals had the highest adsorption capacity (18.0 mg/g), quartz had an intermediate value (3.0 mg/g), and carbonates had the lowest adsorption capacity (1.8 mg/g) [39]; (2) for quartz, the adsorbed amount (2 mg/g) of asphaltene [36], which was diluted with a solution of n-heptane and toluene, is apparently lower than that of asphaltene diluted with toluene (4.5 mg/g) [45] and 6.4 mg/g [44]; (3) feldspar has a greater adsorption capacity than quartz [45]; (4) for calcite, the maximum adsorbed amount of polar compounds (resin and asphaltene) is 2.1-3.6 mg/g [42]; and (5) when compared to limestone, dolomite has a smaller adsorption capacity and a smaller adsorbed amount of asphaltene diluted in toluene [46].…”