High-Pressure Methane Adsorption and Desorption in Shales from the Sichuan Basin, Southwestern China

Abstract

Adsorbed methane is an important component of shale gas. Methane desorption, commonly ignored, can affect gas production. Gaining a better understanding of methane sorption in shale is important for evaluating gas reserve and predicting gas production. This paper presents the results from a study of the methane adsorption and desorption of four shale samples from the Sichuan Basin, southwestern China. Adsorption experiments were performed on dry samples at temperatures of 40−100 °C to a maximum equilibrium pressure of 30 MPa. The desorption measurements were conducted at a temperature of 60 °C. Slight sorption hysteresis was observed over the entire pressure range for every sample. Both the three-parameter Langmuir model and the supercritical Dubinin−Radushkevich (sD-R) model were applied to describe the adsorption data. Model-fitted methane maximum absolute adsorption capacities (n 0 ) ranged from 1.83 to 3.04 mg/g at 40 °C, and n 0 increased linearly with increasing total organic carbon content. The decrease in n 0 with increasing temperature can also be represented by a linear model. The isosteric heat of adsorption ranged from 16.54 to 22.03 kJ/mol. A possible mechanism for sorption hysteresis is the irreversible swelling of micropore throats caused by methane adsorption. The swelling reduces the size of the pore throats and makes them smaller than the kinetic diameter of methane molecules so that when the pressure is reduced, more energy is required for the methane molecules in the pores to escape. Thus, fewer methane molecules can desorb from the material than were previously adsorbed at the same pressure.

Absolute adsorption and adsorbed volume modeling for supercritical methane adsorption on shale

“…As illustrated in Fig. 6, the adsorbed methane density exhibits a slight reduction above 10 MPa and most of the values are lower than the liquid methane density at the boiling point (26.35 mol/L at -162 ℃), which is widely accepted as the upper limit of the adsorbed density [60,61]. The slight reduction of the adsorbed phase density might be attributable to the preferential adsorption at different pressures.…”

Effects of Supercritical CO2 Treatment Temperatures on Mineral Composition, Pore Structure and Functional Groups of Shale: Implications for CO2 Sequestration

“…The XRD device was used with Cu Kαradiation at 40 mA and 40 kV, and carried out in a 2θ range of 3 • to 45 • with a step size of 0.02 • and a scanning speed of 2 • /min. Quantification phase analysis was conducted with the value K method, following the Chinese oil and gas industry standard SY/T 5163-2018 [38][39][40], in which the relative deviation of test results is less than 10%, when mineral content is more than 40%. When the mineral standard and corundum are formulated into a two-phase mixture in a ratio of 1:1 by mass, the reference strength of the mineral i can be calculated as Equation (1).…”

Impact of the Water Adsorption Monolayer on Methane Ad-/Desorption Behavior in Gas Shale Nanopores

“…A larger specific surface increases the number of adsorption sites. Furthermore, a smaller pore diameter increases the adsorption potential. , In fact, it is of great significance to investigate the supercritical methane adsorption at high temperatures and pressures to evaluate the quantity of shale gas in a place. , The shale gas production process is more of a desorption process, rather than an adsorption process . However, research on the desorption process of methane in shale is limited .…”

“…An ad-/desorption hysteresis appears in shale samples, similar to coalbed methane. Unfortunately, the hysteresis has yet to be thoroughly researched because the degree of hysteresis of shale is lower compared to coal …”