Coalbed methane (CBM) is a relatively common unconventional natural gas, which has great exploitation value. Coal permeability is an important parameter that affects the production and production efficiency of CBM, which is mainly controlled by the sorption expansion/contraction strain and effective stress. To study the seepage characteristics of coal in the process of CBM production, we have used CH4 and CO2 as test gases separately and conducted comparative seepage tests of different gases under constant pore pressure conditions. At the same time, the elastic modulus reduction coefficient R m has been introduced to characterize the sorption strain of coal, following which the permeability models suitable for different boundary conditions were derived according to the stress–strain relationship. Under the two gases, the new model could not only better reflect the law of coal sorption strain but also better reflect the relationship among effective stress, pore pressure, and coal permeability. Under the conditions of constant pore pressure, coal permeability was mainly controlled by effective stress; with the increase of effective stress, permeability decreased sharply initially and then gradually. Under the conditions of uniaxial strain and constant external stress, with an increase of pore pressure and R m, the matrix sorption expansion strain increased, resulting in a narrowing of the seepage channel, and R m indirectly inhibited permeability. At this point, coal permeability was mainly controlled by sorption expansion/contraction strain and effective stress. In addition, compared with other permeability models, the new permeability model possesses higher applicability both in theoretical mechanism and in data matching. The general change trend concerning coal permeability, determined by rebound pressure p rb, was consistent with the test results, which further verified the applicability of the model. It is believed that the results of this study could provide a basis for subsequent research on the stress–strain–permeability relationship and for the study of efficient development of CBM.
Coal permeability is an important influential factor in the efficient development and utilization of coalbed methane (CBM); however, coal is in an environment of combined gas and water and is exposed to continuous reservoir pressure for considerable periods, which makes permeability changes in its natural state extremely complicated and those changes difficult to evaluate. In this study, we established a dual-porosity permeability model suitable for wet coal that considered the influence of stress and gas adsorption. In the process of modeling, the shapes of the matrix pores and fractures were simplified into regular cylindrical and slit; based on the generalized Hooke’s law, the effective stress–strain relationship of the coal matrix and fracture was represented; meanwhile, the adsorption capacity decay coefficient λ was introduced to describe the influence of moisture on gas adsorption; and then changes in the pore radius and fracture width under the action of stress and gas adsorption were quantified. Moreover, the interaction between the water film and the pore wall and the influence of stress and gas adsorption in the natural reservoir environment were considered, relating to the dynamic water film calculation formula in matrix pores and fractures under the influence of stress and gas adsorption that was derived, revealing the dynamic evolution law of water film thickness under the action of stress and gas adsorption. By combining the above influential factors and based on the relationship between permeability–porosity–pore radius (fracture width), a dual-porosity permeability model that considered the effects of stress, gas adsorption, and dynamic water film combinations was established. Further, we compared the predicted results of this model with published experimental data and discuss the influence of stress and gas adsorption on water film thickness and the contribution of matrix permeability and fracture permeability to resultant permeability under different water saturations. The complex variation of wet coal permeability under stress and gas adsorption is revealed.
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