Shida Chen

Coal reservoirs commonly contain both adsorbed and free methane, but their depth-dependent partitioning and controlling mechanisms remain difficult to quantify. Taking the Daning–Jixian block as example, the coupled storage of adsorbed and free gas was evaluated considering coal rank, pore-size distribution, reservoir pressure, effective stress, and temperature. The results show that coal rank controls nanopore development and methane adsorption capacity. With increasing R_o, the CO₂-derived micropore specific surface area and micropore volume increase, whereas the N₂-derived mesopore–macropore parameters generally decrease. Langmuir volume increases with coal rank but decreases with increasing temperature. Porosity generally increases with R_o but decreases exponentially with increasing confining pressure. By integrating stress-dependent porosity, reservoir pressure, temperature, gas compressibility factor, and coal rank, depth-dependent predictive models for adsorbed and free gas contents were established. The calculated theoretical maximum adsorbed gas capacity increases from 7.61 m³/t at 500 m to 19.95 m³/t at 3000 m, with a gradually decreasing growth rate. Free gas content increases from 0.44 to 4.42 m³/t over the same depth interval, and its contribution to total gas content increases from 5.5% to 18.1%. Comparison with measured gas content indicates that coal seams shallower than 1500 m are commonly adsorption-undersaturated, whereas deeper intervals locally exceed the adsorbed gas storage capacity, implying the accumulation of free gas. These results provide a quantitative framework for evaluating adsorbed-free gas partitioning and deep coalbed methane enrichment.

The southern Junggar Basin, Xinjiang, has abundant coalbed methane (CBM) resources. Currently, the Baiyang River development pilot test area (BYR block for short) in the Fukang east block has achieved large-scale CBM development, but the productivity characteristics and its controlling factor are still unclear. Based on the field production data of the BYR block and experimental tests, this paper summarizes the gas and water production characteristics and presents the analysis results of the geological and geochemical responses to the productivity of CBM wells. The productivity of CBM wells in the BYR block was generally characterized as medium-to-low yield. The productivity was jointly controlled by the burial depth, structure condition, thickness and number of co-production coal seams, and hydrogeological conditions. The gas production first increased and then decreased with the increase in the burial depth of the coal seam, and a burial depth between 750 and 1000 m was the most beneficial to increasing the gas production due to the good gas preservation conditions and suitable permeability and stress conditions. The total thickness of the co-production coal seams had a positive effect on the productivity of gas wells, but the productivity was also affected by the number of co-production coal seams and interlayer interference. In the BYR block, the co-production of the nos. 41 and 42 coal seams was the most favorable combination form for CBM drainage. The productivity of CBM wells had a good response to the Na⁺, K⁺ and HCO₃⁻ concentrations but a poor response to δD-H₂O and δ18O-H₂O. Based on the concentrations of the main ions and TDSs of the coal seam water, a productivity response index δ* was established, and there was a good positive correlation between the productivity and δ*.