Elastic modulus is an important parameter affecting the permeability change in the process of coalbed methane (CBM)/enhanced coalbed methane (ECBM) production, which will change with the variable gas content. Much research focuses on the constant value of elastic modulus; however, variable stiffness of coal during CO₂ injection has been considered in this work. The coupled thermo-hydro-mechanical (THM) model is established and then validated by primary production data, as well as being applied in the prediction of CO₂/N₂-ECBM recovery. The results show that the harder coal seam is beneficial to primary production, while the softer coal seam results in greater CO₂/N₂-ECBM recovery and CO₂ sequestration. N₂ and CO₂ mixture injection could be applied to balance early N₂ breakthrough and pronounced matrix swelling induced by CO₂ adsorption, and to prolong the process of effective CH₄ recovery. Besides, reduction in stiffness of coal seam during CO₂ injection would moderate the significant permeability loss induced by matrix swelling. With the increase of the weakening degree of coal seam stiffness, CO₂ cumulative storage also shows an increasing trend. Neglecting the weakening effect of CO₂ adsorption on coal seam stiffness could underestimate the injection capacity of CO₂. Injection of hot CO₂ could improve the permeability around injection well and then enhance CO₂ cumulative storage and CBM recovery. Furthermore, compared with ECBM production, injection temperature is more favorable for CO₂ storage, especially within hard coal seams. Care should be considered that significant permeability change is induced by mechanical characteristics alterations in deep burial coal seams in further study, especially for CO₂-ECBM projects.

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.

A new time function was proposed to cover the disadvantage of knothe time function in predicting surface subsidence by adding parameter k. The velocity and acceleration of the progressive surface subsidence obtained from this new time function were compared with actual subsidence data from an iron mine in China. Combining this new time function with section function of surface subsidence basin, a dynamic mode was proposed to predict surface subsidence induced by underground mining and a case study based on this model was carried out. It shows that this model accurately represents the actual subsidence pattern observed. Instruction