A Study of the Dynamic Changes in Wet Coal’s Water Film and Permeability under Stressed Conditions

Abstract

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.

Apparent Permeability of Clayey Shale Gas Reservoirs: Simultaneous Effects of Stress and Slippage

“…where f is the internal expansion coefficient (0 < f < 1), a 0 is the initial matrix width (μm) (a 0 = 3w 0 /ϕ 0 ), 34,35 ϕ 0 is the initial porosity (%), and ε s is the adsorption swelling deformation. The adsorption swelling deformation ε s caused by gas adsorption satisfies the Langmuir equation; thus, ε s is defined as follows: 36 = + p p p s L L (5) where ε L is the Langmuir volume strain (%), indicating the maximum adsorption strain under a certain p, and p L is the Langmuir pressure constant (MPa).…”

“…1,2 Shale reservoirs are characterized by ultralow permeability (<10 −3 mD), resulting from their tightness and nanoscale pore structure. 3 During shale gas exploitation, the permeability is affected by stress, 4,5 gas adsorption, 6 gas slippage, etc. Under the above influences, the permeability changes dynamically and the flow state of shale gas is complex.…”

“…Shale gas has become an important energy source for many countries because of the long-term market potential of natural gas, the depletion of conventional reservoirs, and the development of new technologies. , Shale reservoirs are characterized by ultralow permeability (<10 –3 mD), resulting from their tightness and nanoscale pore structure . During shale gas exploitation, the permeability is affected by stress, , gas adsorption, gas slippage, etc. Under the above influences, the permeability changes dynamically and the flow state of shale gas is complex.…”