The large spatial variability of in-situ stress and initial reservoir pressure in steeply-dipping ultra-thick coalbed methane (UTCBM) reservoirs exert strong control on the initial distribution of stress-sensitive permeability. This results in significant differences in the propagation of reservoir depressurization, gas production characteristics, distribution of fluid saturation, and evolution of permeability relative to flat-lying and thin counterpart coalbed methane (CBM) reservoirs. We contrast these responses using the Fukang mining area of the Junggar Basin, Xinjiang, China, as a type-example using coupled hydro-mechanical modeling. Production response indicates: (1) Dual peaks in CBM production rate, due to the asynchronous changes in the gas production rate in each the upper and lower sections of the reservoir; (2) higher depressurization and water saturation levels in the lower section of the reservoir relative to the upper at any given distance from the production well that ameliorate with time to be similar to those of standard horizontal reservoirs; (3) the heterogeneity in effective stress is further amplified by the asymmetry of the initial pressure drawdown distribution of the reservoir to exert extreme control on the down-dip evolution of absolute permeability—with implications for production. Field drainage data and simulation results obtained in this study more accurately reflect the drainage characteristics of the steeply-dipping UTCBM reservoirs. For ultra-thick low-rank coal seams, permeability anisotropy plays an important role in determining the utility of horizontal wells and hydraulic fracturing to maximize rates and yields CBM production, and requiring further study.

Water sensitivity (WS) and salinity sensitivity (SS) are key issues to be investigated for instructing coalbed methane (CBM) production. This work studied the influences of minerals and pores on WS and SS of medium-volatile bituminous coal (MVBC) and highly volatile bituminous coal (HVBC) deposited in northwestern China by detecting and observing minerals using the TESCAN Integrated Mineral Analyzer, simulating WS and SS, and characterizing pore structural complexities using rate-controlled mercury penetration. The results show that (1) kaolinite is mainly distributed as irregular particles or fragile aggregates attaching on the bedding surface or filling in meso-pores or transition pores, showing a high potential for detachment; (2) MVBC and HVBC in this study are characterized as medium to weak WS and weak SS, respectively; (3) for HVBC during the WS or SS process, kaolinite distributed in meso-pores or transition pores first detaches and then migrates to the narrow throat of macro-pores and super macro-pores, leading to volume decreases of macro-pores and super macro-pores and loss of permeability; and (4) kaolinite filling in macro-pores of MVBC detaches, then migrates, and finally deposits in super macro-pores after WS and SS, leading to losses of super macro-pore volume and permeability. Results of this study can enhance the scientific knowledge on WS and SS of coal during CBM development.