Investigation on full-scale pore characterization and its implications for gas storage and development potential of deep coal seam in the Jiaxian block, NE Ordos Basin, China

Abstract

Deep coal reservoirs (buried depth > 2000 m) represent a significant yet underexploited resource for coalbed methane (CBM) production. In these reservoirs, CBM primarily exists in adsorbed and free phase, with the pore structure playing a critical role in gas storage and migration. The Jiaxian block in the northeastern Ordos Basin, has emerged as a key area for deep CBM exploration due to its promising resource potential. However, the pore structure characteristics of the No. 8 coal seam in Jiaxian block and their implications for gas storage and production remain poorly understood. A comprehensive characterization of the No. 8 coal seam's pore structure is conducted in the study using multiple methods including high-pressure mercury injection, N₂/CO₂ adsorption experiments, and integration of measured core gas content data and production history. The study results reveal that the pores can be mainly classified as vesicles and cellular pores, and the fractures are mainly static pressure fractures. Micropores (pore diameter < 10 nm) dominate the pore system (accounting for more than 99 % of the total specific surface area), providing important adsorption sites for gas storage. Although mesopores (pore diameter of 100–1000 nm) and macropores (pore diameter > 1000 nm) account for a small proportion, they feature effective storage spaces and interconnectivity, resulting in a high proportion of free gas. Therefore, the reservoirs shows great development potential after stimulation (such as hydraulic fracturing). These findings emphasize the feasibility of large-scale and long-term development of CBM in the Jiaxian block in terms of reservoir space, gas content and production characteristics. This study serves to lay a scientific basis for its efficient exploitation.