Abstract:

Characteristics of microscopic pore structures of shales have important impacts on the assessment of gas and associated oil/gas
potential adsorption capacity of the shales, and previous studies have mainly focused on marine shales. Here, we conduct a pilot study of continental shales based on a case study of the fifth member of the Upper Triassic Xujiahe Formation in the western depression, Sichuan Basin. The microscopic pore structures of the shales were investigated by using low-temperature nitrogen gas adsorption method. Multiple structural parameters of the shales were calculated, including the specific surface area, pore size distribution, pore volume and average pore diameter. Then, high-pressure methane adsorption analysis was conducted to obtain the parameters of Langmuir volume and Langmuir pressure. Finally, we address the effect of microscopic pore structure characteristics on the methane adsorption capacity of the shales. Results show that the average pore diameter of the shales ranges from 7.81 to 9.49 nm. Mesopores dominate in the shales, while some micro- and macro-pores also exist. Slit-like pores are the main pore types in shales, and a small amount of ink-bottle-like pores are also present. The specific surface area of the shales are much greater than that of conventional reservoir rocks; this is good for gas adsorption in shales. The mesopores with the pore diameter between 2 and 50 nm provide the dominating pore volume, which constitutes the major space for gas adsorption and storage in the shales. Under the temperature condition of 85 ℃, the Langmuir volume of methane adsorption in shales ranges from 1.21 to 4.99 m3/t, and methane adsorption capacity varies largely within different shales. There are positive correlations between Langmuir volume and specific surface area of shales, and between specific surface area and clay minerals content of shales. However, no correlation exists between specific surface area and TOC of shales. There are positive correlations between Langmuir volume and micropore and mesopore volume. Organic matter is an important control on micro- and meso-pores of shales, evidenced by the positive correlations between micropore and mesopore volume and TOC. Comparatively, clay minerals play a much more important role on the pore volume of micro-and meso-pores of shales. For continental shales, organic-matter thermal evolution is relatively low and organic pores are thus not well developed. In contrast, abundance of clay minerals is relatively high in shales, with well development of micropores and mesopores, which constitute a considerable specific surface area of shales and thus affect the methane adsorption capacity of shales.

Key words: shalegas;continentalshale;porestructure;nitrogenadsorption;specificsurfacearea;porevolume;methaneadsorptioncapacity