Controlling effects of ash yield on the adsorption-desorption behavior and production potential of methane in deep coal reservoirs

Authors

• HUANG Daojun, National Engineering Laboratory for Low Permeability Oil & Gas Field Exploration and Development, Xi’an 710018, China; Exploration & Development Research Institute of PetroChina Changqing Oilfield Company, Xi’an 710018, ChinaFollow
• LIU Xinshe, National Engineering Laboratory for Low Permeability Oil & Gas Field Exploration and Development, Xi’an 710018, China; Department of Geophysics, PetroChina Changqing Oilfield Branch Company, Xi’an 710018, China
• CHEN Shida, School of Energy Resource, China University of Geosciences (Beijing), Beijing 100083, ChinaFollow
• SUN Wen, National Engineering Laboratory for Low Permeability Oil & Gas Field Exploration and Development, Xi’an 710018, China; Exploration & Development Research Institute of PetroChina Changqing Oilfield Company, Xi’an 710018, China
• JIAO Pengshuai, National Engineering Laboratory for Low Permeability Oil & Gas Field Exploration and Development, Xi’an 710018, China; Exploration & Development Research Institute of PetroChina Changqing Oilfield Company, Xi’an 710018, China
• WANG Huaichang, National Engineering Laboratory for Low Permeability Oil & Gas Field Exploration and Development, Xi’an 710018, China; Exploration & Development Research Institute of PetroChina Changqing Oilfield Company, Xi’an 710018, China

Abstract

Objective The lithotype and quality characteristics of coals significantly affect the reservoir permeability and production potential of coalbed methane (CBM). However, there remains a lack of clear understanding of the controlling effects of ash yield on the gas-bearing properties and methane adsorption-desorption behavior of coal seams under comparable thermal maturity. Focusing on coal samples obtained through continuous sealed coring from the No. 8 coal seam in well M172 in the east-central part of the Ordos Basin, this study aims to clarify the impacts of ash yield on the pore structure, adsorption capacity, desorption process, and resource recoverability. This effort is expected to provide a basis for identifying favorable intervals and designing horizontal well trajectories for deep CBM reservoirs.Methods Low-temperature CO2 adsorption experiments and major element analysis were employed to systematically characterize the differences in the pore structure and inorganic composition of the No. 8 coal seam. Methane isothermal adsorption experiments were integrated with the formation temperature and pressure conditions to calculate the in situ saturated adsorption capacity and desorption efficiency of coals with different ash yields and identify key desorption pressure nodes, including the starting, turning, and sensitive pressures. Accordingly, the responses of the methane adsorption-desorption capacities to the depth-ash yield coupled control were revealed. Furthermore, vertical comparisons and production capacity simulations across different coal intervals in a single well were conducted to analyze the comprehensive controlling effects of different adsorbed and free gas configurations on resource recoverability and gas production potential.Results The heterogeneity of the ash yield of the No. 8 coal seam arises from variations in the depositional environment and terrigenous clastic input during coal formation. The coal intervals with higher ash yields exhibited significantly lower volumes and specific surface areas of pores, lower Langmuir volume (V_L), and higher Langmuir pressure (p_L), resulting in downward trends in the reservoir and gas-bearing properties. Coals with different ash yields manifested distinct in situ adsorption capacities under varying formation temperature and pressure conditions. The coals with higher ash yields exhibited lower in situ saturated adsorption capacity, with the critical depth of adsorbed gas capacity shifting toward shallower parts. The starting, turning, and sensitive pressures all decreased with increasing burial depth, while the pressure drawdown from the original formation pressure to these pressure nodes increased continuously. Under identical burial depths, the coal intervals with higher ash yields required greater pressure drawdown to enter the rapid or sensitive desorption stage. Consequently, the adsorbed gas release from these intervals was severely restricted, and their economically viable production exhibited increasing dependence on free gas supply.Conclusions Under deep formations, limited efficiency is identified in the conversion of adsorbed gas resources into actual production capacity due to the low permeability, large pressure drawdown required for gas desorption, and low gas desorption efficiency, establishing free gas abundance as the key factor controlling high gas production. Free gas-rich coal intervals with low ash yields hold both the most favorable gas resource endowment and the highest resource recoverability, serving as the major targets for both perforation in hydraulic fracturing and horizontal-well geosteering. The low-ash coal intervals that are rich in adsorbed gas but depleted in free gas face challenges in achieving high production. In contrast, a high ash yield does not necessarily indicate low production potential, and high-ash coal intervals that exhibit a low adsorbed gas content locally but free gas enrichment still hold substantial development value.

Keywords

deep coalbed methane (CBM), ash yield, methane adsorption, desorption pressure node, production potential

DOI

10.12363/issn.1001-1986.26.04.0257

Recommended Citation

HUANG Daojun, LIU Xinshe, CHEN Shida, et al. (2026) "Controlling effects of ash yield on the adsorption-desorption behavior and production potential of methane in deep coal reservoirs," Coal Geology & Exploration: Vol. 54: Iss. 9, Article 7.
DOI: 10.12363/issn.1001-1986.26.04.0257
Available at: https://cge.researchcommons.org/journal/vol54/iss9/7

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