Work

Depth-Dependent Adsorbed–Free Methane Partitioning and Deep CBM Enrichment in the Daning–Jixian Block, Ordos Basin

Year: 2026

Type: article

Abstract: 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 Ro, 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 Ro 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.

Source: Energies

DOI: 10.3390/en19143385