Abstract
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.
Highlights
Coal seams are typified as dual-porosity systems consisting of micropores in a matrix and two sets of cleats
CO2 has a great affinity to coal than CH4 and N2; significant matrix swelling caused by CO2 adsorption may result in permeability and well injectivity loss during CO2 injection [21, 22], which has been one of the technical obstacles suffered in CO2-enhanced coalbed methane (ECBM) recovery or CO2 storage
CO2 sequestration and CO2-ECBM in deep coal seams usually face the problem of matrix swelling caused by CO2 adsorption
Summary
Coal seams are typified as dual-porosity systems consisting of micropores in a matrix and two sets of cleats. Primary coalbed methane (CBM) recovery begins with the dewatering to reduce reservoir pressure and increase gas effective permeability [3, 4]. CO2 has a great affinity to coal than CH4 and N2; significant matrix swelling caused by CO2 adsorption may result in permeability and well injectivity loss during CO2 injection [21, 22], which has been one of the technical obstacles suffered in CO2-ECBM recovery or CO2 storage. The dramatic reduction in injectivity and permeability has not been observed in field trials and laboratory tests, where pure N2 or flue gas was used [11, 24, 25] due to net matrix shrinkage caused by a much lower sorption capacity and strain of N2. Analysis and discussion are offered to link the modeling results with the field/laboratory observations
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