Reservoir properties of Chinese tectonic coal: A review

Tectonic coal, formed after the intact coal being subjected to long-term intense squeezing, shearing and deformation, is characterised by brittle or ductile damaged coal body, with the characteristics of low cohesion, low strength and low permeability. Most of the outburst accidents in China occurred in tectonic coal seams due to the difficulties in gas drainage. In this review article, reservoir properties, including pore structure, adsorption, diffusion, permeability and geomechanical properties of the tectonic coal are reviewed in detail and compared with those of the intact coal, as these properties are important for gas drainage. It was found that tectonic coal in general shows larger total pore volume and specific surface area than intact coal for larger pores due to tectonism, however, no significant difference is observed in smaller pores due to the combined opposing effects of metamorphism and tectonism. Diffusion coefficient of tectonic coal is generally higher than that of intact coal, and tectonic coal typical has higher adsorption capacity than intact coal. Compressive strength and elasticity modulus are smaller for tectonic coal than intact coal. Field permeability of tectonic coal is obviously lower than that of intact coal, which is on the contrary to the experimental results from laboratory. It was found that using reconstituted samples for tectonic coal in the laboratory is the main cause for this discrepancy between field and laboratory observations. It is suggested that more work is required on tectonic coal and a few research areas are proposed for future research.