Coal permeability models for enhancing performance of clean gas drainage: A review

Coal seam gas drainage has manifold benefits including ensuring mining safety and efficiency, reducing greenhouse gas emissions and generating clean gas resource. Gas drainage performance is primarily determined by coal permeability. Permeability is a result of the competitive effect of mechanical properties and matrix shrinkage. Aiming to summarize extensive investigations on coupled permeability models and provide both fresh researchers and mining engineers with references on current understanding and future directions of permeability models, coal structures (matrix, cleats and pores) are first discussed in this work, as those structures providing a physical basis for permeability-model establishment. Then coupled permeability models are categorized and reviewed from viewpoints of stress/sorption-based permeability models, stress and sorption-based permeability models, damage-based permeability models and anisotropic permeability models, including model-development theoretical basis, boundary conditions and applications. Current research gap and future work directions in permeability study are identified. Results show that theoretical assumptions of constant vertical stress and uniaxial strain are mainly adopted in permeability models. Meanwhile, the effects of gas desorption/adsorption and stress variation are always incorporated in models compared with coal anisotropy and damage. Future studies on permeability variation during underground coal mining are expected to be conducted with coupling effects of coal damage, coal anisotropy, water and coal fines, for increasing permeability-evaluation accuracy and providing better guidance for gas drainage design.