Supercritical Methane Sorption on Organic-Rich Shales over a Wide Temperature Range

Abstract

Methane sorption on organic-rich shales over wide temperature and pressure ranges (30–120 °C, up to 25 MPa) is analyzed by the pore filling/potential theory. The supercritical Dubinin–Astakhov (SDA) sorption model using a density term instead of the pseudosaturation vapor term is extended to methane sorption isotherms of shales with high accuracy. A modified adsorption potential approach is suggested to analyze the temperature dependence of supercritical methane sorption on shales. The temperature-invariant characteristic curves are obtained using the modified adsorption potential approach. A characteristic curve equation derived from the SDA model is provided to predict sorption isotherms at other temperatures using one isotherm. The physical meaning of the characteristic curve has been discussed, and it comprehensively reflects the available pore space for methane sorption and the affinity between methane molecules and organic matter. According to methane characteristic curves of shales and clay minerals, shales in the gas window show higher affinity than shales in the oil window and clay minerals, though the clay minerals may provide comparable adsorbed volume. The adsorption characteristic energy shows a parabola-like shape with a minimum of approximately R eq = 1.1%, which is related to the evolution of the porosity of the shales. This study advanced the fundamental understanding of the dynamic process of methane sorption on shales.

Review of Formation and Gas Characteristics in Shale Gas Reservoirs

“…Compared to the Langmuir adsorption theory, micropore volume filling is more helpful for understanding the gas adsorption mechanism and gas true storage forms, and for evaluating gas adsorption properties. Meanwhile, it has been reported that the D-A model provides a better fit to sorption data of coal than the Langmuir model [72].…”

“…Note that the above D-R and D-A models were not initially proposed for supercritical adsorption, but for subcritical sorption. Therefore, the concept of saturated vapor pressure in the D-R and D-A model was replaced by virtual saturated vapor pressure or supercritical adsorption limited pressure [72]. Generally, the virtual saturated vapor pressure can be calculated by the following empirical formulations or approaches:…”

“…Then, saturated vapor pressure could be calculated by Equation (20). 4The fourth is the Astakhov method [72], the calculation results of which fit the experimental data well in the interval of p s from 0.1 MPa up to critical pressure p c [71]. Meanwhile, it should be noted that this method gives satisfactory results for temperatures exceeding the critical temperature by 50-100 K. which involves a parameter k A to account for interactions in an adsorbate-adsorbent system:…”

“…However, Sakurovs et al [81] noted that this method cannot easily accommodate adsorption at conditions where both the pressure and temperature are above the critical values. Since the adsorbed gas density is greater than the free gas density in supercritical conditions, another method, which replaced the saturated vapor pressure p s by adsorbed phase gas density ρ ad , and gas pressure p by gas density ρ g , was proposed to extend the volume-filling models to a wider pressure and temperature application range [72]. Based on this idea and associating Equation (17) with Equation (25), we obtain:…”

“…where b is the van der Waals covolume, representing the volume occupied by a single gas molecule. (2) Gas density at critical point (p c , T c ): The second way to calculate adsorbed phase gas density is from the critical pressure and critical temperature [72,103]:…”