Permeability evolution and production characteristics of inclined coalbed methane reservoirs on the southern margin of the Junggar Basin, Xinjiang, China

• Published in: International journal of rock mechanics and mining sciences (Oxford, England : 1997) England : 1997), 2023-11, Vol.171, p.105581, Article 105581
• Main Authors: Liang, Shun, Liang, Yaowu, Elsworth, Derek, Yao, Qiangling, Fu, Xuehai, Kang, Junqiang, Hao, Yisong, Wang, Meng
• Format: Article
• Language: English
• Subjects: Asymmetric distribution; Coalbed methane; Inclined coalbed methane reservoir; Permeability; Reservoir pressure
• ISSN: 1365-1609
• DOI: 10.1016/j.ijrmms.2023.105581

The thick and steeply inclined coal seams of the Junggar Basin of Xinjiang, China, are unique with dip angles generally >50° but over the range 0°–85°. Initial and evolving permeability and pressures change drastically around wells down-dip within the steeply inclined reservoir as a result of the depth differential. Hence, the evolution of permeability and fluid pressures during drainage exhibits significant differences from those of flat-lying or even slightly inclined reservoirs. We apply a hydro-mechanical model to evaluate the interaction of two-phase flows of gas and water in the inclined system. The influence of different reservoir inclinations (15°, 30°, 45°, 60°, and 75°) on the evolution of permeability, reservoir pressure, and gas production are explored through finite element modeling of this system. The results show that: 1) Reservoir inclination induces differences in permeability, reservoir pressure, gas content and methane production between the shallower updip reservoir and deeper downdip reservoir. The difference in permeability between the updip and downdip reservoirs is amplified as the dip angle increases and as drainage proceeds in the presence of the varying stress gradient. 2) An apparent asymmetric distribution of reservoir pressures results for wells along dip. The difference in reservoir pressure between the updip and downdip reservoirs intensifies as the inclination increases but lessens with the progress of drainage. The larger the dip angle, the smaller the final reservoir pressure. 3) The pressure reduction in the updip reservoir is larger than that in the downdip reservoir, resulting in the unsynchronized desorption of methane in the updip and downdip reservoirs. Methane within the updip reservoir desorbs preferentially over that in the downdip reservoir. For reservoir dip angles 45°. The time gap in gas desorption between the updip and downdip reservoirs results in the “dual-peak” on gas production profile. 4) A larger well spacing along the dip of a more highly inclined reservoir results in more efficient water drainage and gas production. An inverted trapezoidal well pattern is recommended to facilitate the drainage and gas production of reservoirs with significant dip angles. •Impact of CBM reservoir inclination on the gas-water fluid system is investigated.•Evolution of permeability and reservoir pressure of i