Deep coal reservoirs in the Ningwu Basin: Geothermal field characteristics and their effects on gas-bearing properties

Temperature acts as an important factor affecting the gas-bearing properties of deep coal reservoirs, further affecting the development performance of deep coalbed methane (CBM). Therefore, ascertaining the geothermal conditions of coal reservoirs and their effects on the gas-bearing properties of deep coal reservoirs is critical to the systemic understanding of the enrichment characteristics and production patterns of deep CBM. For coal seam No.9 in the Taiyuan Formation within the Ningwu Basin, over 90% of the area has burial depths exceeding 1000 m. In the hinterland of the basin, coal seams generally exhibit burial depths greater than 1500 m, with a maximum exceeding 2500 m. These findings suggest typical deep CBM. Based on the log data, experiments data, and well test data, this study determined the geothermal field characteristics of the coal reservoirs in the Ningwunan block and their effects on the gas-bearing properties. The results show that coal seam No.9 in the Ningwunan block exhibits reservoir temperature ranging between 15.5℃ and 40.1℃ and there is a significant positive correlation between the reservoir temperature and the burial depth. The coal reservoirs have geothermal gradients ranging from 1.27 to 1.95℃/hm, with an average of 1.52℃/hm, indicating the characteristics of low geothermal fields. These reservoirs demonstrate gas saturation ranging from 40.1% to 93.7%, with an average of 71.7%, suggesting low gas saturation of the deep coal reservoirs. With an increase in the burial depth, the coal reservoir temperature increases slowly due to the low geothermal gradients. This results in significantly reduced negative effects of temperature on the adsorption capacity of deep coal reservoirs in the study area compared to deep CBM-bearing blocks at similar burial depths, such as Daning-Jixian and Linxing. Accordingly, the depth at which adsorbed gas transitions into free gas increases for coal reservoirs in the study area, with the transition ratio between both types of gases decreasing. Low temperature serves as an important factor affecting the gas-bearing properties of deep coal reservoirs in the study area, further influencing their recoverable degree. Therefore, the geothermal conditions of coal reservoirs should be highlighted in the exploration and recovery of deep CBM.