Abstract
Gas transport through porous coal contains gas laminar flow in the cleat network and gas adsorption/diffusion in the matrix block. Since permeable capacity of the cleat is greater than that of the matrix, change of the matrix pressure readily lags behind change in the cleat pressure. Such unsynchronized pressure changes can result in a complex compatible deformation of a cleat-matrix system, significantly affecting the coal permeability. In this paper, we investigated the cleat-matrix interaction on coal permeability by using a modified pressure pulse decay method integrated with numerical analysis. The experimental results indicate that the bulk volume of the coal sample rapidly expanded at the beginning of gas injection, and then the volume expansion rate of the coal sample slowed down as the downstream pressure of the coal sample gradually equilibrated with the upstream pressure. During this process, the coal permeability was observed to gradually decrease with time. Numerical analysis results indicate that gas transport from the cleat to the matrix can attenuate the differential pressure between the cleat and the matrix. A smaller ratio of initial matrix permeability to initial cleat permeability can prolong decay duration of the differential pressure inside the cleat-matrix system. Although the coal sample is subjected to a stress-controlled condition, the coal permeability response to gas diffusion is closer to the case using a constant volume boundary. The dynamic change of coal permeability is significantly affected by the cleat-matrix interaction, in cases where the short-term change is mainly attributable to the cleat network and the long-term change is controlled by matrix swelling/shrinkage.
Highlights
Coal bed methane is an unconventional gas whose storage/transport in a coal reservoir is different from that in a carbonate reservoir
Given that coal permeability is predominated by cleats, it is commonly assumed that the Darcy flow is a result of flow in the cleat system and that the contribution of flow in the coal matrix to the Darcy flow can be neglected
According to considerable laboratory measurements and field observations, coal permeability is controlled by two factors: the pore pressure change can alter effective stress and increase or decrease the bulk volume of the coal [1,2,3,4,5,6,7]; gas adsorption/desorption can swell or shrink the coal matrix, which can compress or close the cleat aperture
Summary
Coal bed methane is an unconventional gas whose storage/transport in a coal reservoir is different from that in a carbonate reservoir. The gas-bearing coal matrix provides a mass source term for the gas flow equation [11] via gas adsorption/desorption This assumption ignores the gas supply hysteresis effect from the matrix blocks and underestimates coal permeability (ii) The nonequilibrium and pseudosteady state assumption is that a time-dependent gas transfer is driven by pore pressure gradient between the coal matrix and the cleat [12]. In this paper, we modify the pulse decay method [40, 41] by intermittently measuring coal permeability under two geomechanical conditions: constant confining stress and constant differential pressure, respectively It is aimed at revealing the impact of dual-pore structure deformation on gas permeability change, due to gas transport from unsteady-state to steady-state flow. The present work gives insight into how the evolution of coal permeability is associated with the dual-pore pressure system
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
