Control of ash yield on vertical pore structure development and its impact on coalbed methane adsorption in the deep coal seams of the Ordos Basin

Abstract

The vertical heterogeneity of the pore structure in deep coal seams with varying ash yields is a key control for coalbed methane storage and producibility; however, its specific impact on gas adsorption is not clearly defined. The focus of this study is the No.8 coal seam of the Carboniferous Benxi Formation in the Central-Eastern Ordos Basin. By integrating microscopic identification, proximate analysis, gas adsorption (CO₂, N₂, and CH₄), and the multifractal theory, we quantitatively characterized the nanopore structure (micropores <2 nm and mesopores 2 nm–100 nm) of coal reservoirs with varying ash yields. The results indicate that (1) ash yield is the primary factor that controls the vertical evolution of pore structures in coal seams. In low-ash yield coal seams, the extent of thermal evolution and ash yield jointly constrain the heterogeneity of pore size distribution. In medium- to high-ash yield coal seams, the heterogeneity of pore structure and pore size distribution are predominantly constrained by ash yield. (2) As the ash yield vertically increases, the mesoporous pore volume and specific surface area initially decrease and subsequently increase, while the contribution of micropores to both pore volume and specific surface area continuously diminishes. Consequently, the total pore volume and specific surface area of the coal samples exhibit a two-stage reduction close to an ash yield threshold of approximately 20 %. (3) Further, the Langmuir volume for CH₄ adsorption sharply declines below the 20 % threshold, followed by a gradual decrease; in contrast, the Langmuir pressure initially decreases and subsequently increases. Hence, the vertical increase in ash yield constrains the development of pore systems and diminishes pore connectivity, thereby reducing methane adsorption capacity and adversely affecting coalbed methane productivity. (4) Low-ash yield coal reservoirs are characterized by a rapid gas breakthrough and high productivity, whereas medium-ash yield coal reservoirs generally require prolonged depressurization to achieve peak gas production. These findings reveal that in medium-high rank coal, ash yield—and not thermal evolution—is the main factor that controls vertical pore evolution and methane adsorption efficiency. The quantitative ash yield threshold (20 %) established in this study provides a practical criterion for evaluating reservoir quality and predicting vertical variations in gas storage potential in the Ordos Basin.