Causality between tectonic coal and coal and gas outbursts

Tectonic coal is formed after the intact coal being subjected to long-term intense squeezing,shearing and deformation It is characterized by brittle or ductile damaged coal body,with the characteristics of low cohesion,low strength and low permeability The existence of a causal link between outbursts and tectonic coal is widely accepted Recent observations have also indicated that most of the outburst cases in China and the world occurred in tectonic coal seams However,the mechanism is not yet clear In order to reveal the internal connection between tectonic coal and outburst,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 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,and the impact on macropores is the most significant In addition,the adsorption/desorption capacity and diffusion coefficient of tectonic coal are generally high,while the compressive strength and elastic modulus are generally low Further analysis shows that the release of stress energy is the key to coal crushing,and the existence of coal particles smaller than the critical size is crucial-However,to reach the critical particle size,the stress conditions required for intact coal are much higher than tectonic coal and may even far exceed the stress levels of current mining depths Hence,the development of outbursts strongly relies on tectonic coals In this case,the existence of tectonic coal is a necessary condition for outburst development,rather than easier to outburst In addition,in terms of the reservoir characteristics of tectonic coal,the permeability obtained in the laboratory is significantly higher than the results obtained in the field (up to two orders of magnitude)The reason can be attributed to the fact that using reconstituted samples for tectonic coal in the laboratory may not fully replicate the original physical properties of tectonic coal It is suggested that in addition to systematic research on the pore structure of tectonic coal,it is also of great significance to explore new methods for the reconstruction of tectonic coal samples