Characteristics and Influencing Factors of Supercritical Methane Adsorption in Deep Gas Shale: A Case Study of Marine Wufeng and Longmaxi Formations from the Dongxi Area, Southeastern Sichuan Basin (China)

Abstract

Deep shale reservoirs with burial depths of >3500 m are now the target of gas producing intervals in the Sichuan Basin, China. Due to the in situ high temperatures and pressures, methane in deep shale formations is supercritical, resulting in the inapplicability of some conventional adsorption models. To determine the methane adsorption capacity of deep shale and its influencing factors, different shale lithofacies from the Dongxi area of the southeastern Sichuan Basin are investigated and compared in this work. The nanopore structure of Wufeng (WF) and Longmaxi (LMX) gas shales are characterized using gas (nitrogen and carbon dioxide) physisorption and high-resolution scanning electron microscopy (SEM). Methane isothermal adsorption experiments are carried out on typical shale samples at different temperatures (from 30 to 90 °C) and pressures (up to 32 MPa). The excess adsorption data are fitted by the supercritical Dubinin–Radushkevich (SDR) model, and the dominant influencing factors of methane adsorption in deep marine shale are determined. Four shale lithofacies are recognized in the study area, including silica-rich argillaceous shale (CM-1), siliceous shale (S), clay-rich siliceous shale (S-3), and argillaceous–siliceous mixed shale (M-2). Nanoscale organic matter (OM) pores, often in irregular, angular and flat shapes, are the dominant pore types in the WF and LMX shale samples. Generally, the pore size spectrum of OM pores is shale lithofacies dependent, e.g., 10–160 nm for S shale and 10–120 nm for CM-1 shale. Compared to other shale lithofacies, S shale exhibits the highest methane adsorption capacity, followed by M-2 shale, while CM-1 shale has the smallest methane adsorption capacity. The adsorption capacity of methane for deep WF and LMX shales is positively correlated with the total organic carbon (TOC) content, micropore volume, and micropore specific surface area. Absolute methane adsorption capacity of deep shale increases with the increase of pressure, but it will decrease at a higher temperature due to the negative and predominant effect of temperature on methane adsorption. The higher TOC content and more abundant small-sized pores promote the S shale to have the strongest adsorption capacity for methane molecules; this indicates that S shale is the most beneficial shale lithofacies for gas adsorption.

Kinetic and Thermodynamic Characteristics of Supercritical Methane Adsorption on Organic-Rich Shale: A Case Study of Ordovician Wufeng and Silurian Longmaxi Gas Shale

“…Compared with the LM-2 sample, the WF-1 sample with higher organic matter and silica content exhibits a greater methane adsorption capacity, which is consistent with previous studies on the controlling factors of methane ability in Wufeng−Longmaxi shale. 28,46,47 The adsorbed-phase density of the two shale samples is between 0.27−0.38 g/cm 3 and 0.18−0.36 g/cm 3 at different temperatures, which is below the methane liquid density (0.421 g/ cm 3 ). After absolute adsorption and adsorption phase density (ρ ads ), these data will be used to investigate the supercritical methane adsorption kinetic and thermodynamic characteristics.…”

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

“…After experiencing high-pressure and high-temperature thermal evolution, the OM in Longmaxi formation is at the high-mature and overmature stages with the vitrinite reflectance ( R o ) ranging from 2.20% to 4.20%. In addition, the total organic carbon (TOC) content is in the range 0.46–7.13% with an average of 2.66%, and the porosity is in the range 1.2–10.8% with an average of 3.0%. − …”

Reservoir characterization of the shallow to deep Longmaxi formation in the Weiyuan Block, southwestern Sichuan Basin

“…Shale is typically split into two primary zones according to the depth at which it was buried: shallow shale (depths under 3500 m) and deep shale (depths beyond 3500 m) (Ma et al, 2021). Recent years shale gas exploration from deep shale becomes the most quickly expanding trends in onshore unconventional oil/gas exploration in South China (Cai et al, 2021; Jiao et al, 2018; Li et al, 2022; Sun et al, 2021; Wang et al, 2022a; Wang et al, 2022b). Previous workers have focused on reservoir characterization, reservoir property, pore-fracture structure, gas content and development potential of deep shale gas and achieved remarkable success in this field.…”

“…Previous workers have focused on reservoir characterization, reservoir property, pore-fracture structure, gas content and development potential of deep shale gas and achieved remarkable success in this field. Wang et al (2022b) proposed a method of estimating the supercritical methane adsorption in deep shale from the Dongxi Area, Southeastern Sichuan Basin. They carried out methane isothermal adsorption experiments at different temperatures (30–90 °C) and pressures (up to 32 MPa), and suggested that the methane adsorption capacity increases with pressure and decreases with temperature.…”