Evaluation of permeability damage for stressed coal with cyclic loading: An experimental study

Permeability and its evolution directly control the flow behaviors of the subsurface energy fluid extractions. Coalbed is known as a fractured, dual-porous media, and whose permeability is directly related to effective stress and the manner of stress loaded and unloaded. Coal has been historically treated as a transverse isotropic media and the permeability can also be influenced by the sorption induced deformations. As a cleat-rich porous medium, coal is potentially bearing various loading paths, such as fracturing, mining and natural geological tectonics activities. The permeability variation under cyclic loading and different stress paths is crucial for the engineering design for both mining and gas extractions from coal seams. The pulse-decay method was employed to estimate the permeability of cylinder coal specimen. An average permeability damage concept with a complete loading-unloading cycle is introduced to quantify the effects of maximum stress, loading path, holding time period, and gas type on permeability evolution. It was demonstrated that the permeability hysteresis occurred during the cyclic loading-unloading, and that the average permeability damage was subject to the loading-unloading paths. The average permeability damage for the coal specimen is more sensitive to maximum loading stress and the loading-unloading path compared to gas injection pore pressure and the holding period. Gas adsorption induced swelling dramatically reduced the initial permeability and average permeability damage during cyclic loading. The permeability evolution was analyzed through poroelastic theory coupled with the matrix shrinkage concept. When applied to permeability and stress relationship, Biot's coefficient should be cautiously applied for coal reservoir because it could be larger than unity when the adsorption induced non-elastic swelling is considered, and it even could be negative when the irreversible cleats closure is considered.