A calculation model of free gas saturation in deep coalbed methane reservoirs and its application

In recent years, breakthroughs have been achieved in the exploration of deep coalbed methane (CBM) in China based on fine-scale geological research and techniques such as multistage hydraulic fracturing using proppants in a horizontal well. As a result, some wells yielded daily gas production of up to 100000 m³, inspiring confidence again in the CBM industry. However, since deep coal reservoirs occur in a complex geological environment characterized by high in-situ stress, high geotemperature, high pore pressure, and low permeability, there is an urgent need to reveal the typical parameters of coal reservoirs at different depths and the distribution of CBM in varying occurrence forms in the reservoirs, as well as their effects on CBM reserves and production. Based on Langmuir equation of isothermal adsorption, Henry's law, and the material balance principle, this study established a calculation model of free gas saturation of deep CBM reservoirs by considering the effects of adsorption layers and dissolved gas. With the deep CBM reservoirs in the Daning-Jixian block in the Ordos Basin in China as a case study, this study investigated the occurrence forms and distribution of deep CBM at varying depths and assessed the effects of free gas saturation on the reserves, production, and rational production allocation of deep CBM. Key findings include: (1) Free gas appears only when the coal seams’ burial depth exceeds the depth corresponding to the dissolution saturation of CBM reservoirs, with the free gas saturation increasing rapidly initially and then slowly as the burial depth increases. In the Daning-Jixian block, free gas emerges at a burial depth of 1875 m, and the free gas saturation reaches 90% at a burial depth of 2800 m, where the free gas accounts for up to 17.3%. (2) The free gas saturation has significant effects on the reserves calculation, gas production characteristics, and reasonable production allocation of deep CBM. With an increase in the free gas saturation, the CBM reserves linearly increase, and the cumulative gas production keeps rising, with the increased amplitude gradually decreasing in the late stage. Furthermore, an increase in the free gas saturation is accompanied by an increase in the optimal production allocation of a deep CBM well, an increase in the drop rate of bottomhole pressure, a decrease in the pressure difference between the stimulated reservoir volume (SRV) and the unstimulated reservoir volume (USRV), and an increase in the producing degree of the USRV. The dominant coal seams to be exploited in the target block are located at depths between 2100 and 2300 m, with free gas saturation ranging from 48% to 68% and a proportion of free gas varying between 10% and 13%. It is recommended that the rational production allocation of gas wells in the target block should be (4−10)×10⁴ m³/d. The results of this study will provide a theoretical basis and methods for the further exploitation of deep CBM.