The Role of Coal Mechanical Characteristics on Reservoir Permeability Evolution and Its Effects on CO[sub.2] Sequestration and Enhanced Coalbed Methane Recovery

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 CO[sub.2] 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 CO[sub.2]/N[sub.2]-ECBM recovery. The results show that the harder coal seam is beneficial to primary production, while the softer coal seam results in greater CO[sub.2]/N[sub.2]-ECBM recovery and CO[sub.2] sequestration. N[sub.2] and CO[sub.2] mixture injection could be applied to balance early N[sub.2] breakthrough and pronounced matrix swelling induced by CO[sub.2] adsorption, and to prolong the process of effective CH[sub.4] recovery. Besides, reduction in stiffness of coal seam during CO[sub.2] injection would moderate the significant permeability loss induced by matrix swelling. With the increase of the weakening degree of coal seam stiffness, CO[sub.2] cumulative storage also shows an increasing trend. Neglecting the weakening effect of CO[sub.2] adsorption on coal seam stiffness could underestimate the injection capacity of CO[sub.2]. Injection of hot CO[sub.2] could improve the permeability around injection well and then enhance CO[sub.2] cumulative storage and CBM recovery. Furthermore, compared with ECBM production, injection temperature is more favorable for CO[sub.2] 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 CO[sub.2]-ECBM projects.