Abstract
To understand the effects of thermal shock on microcrack propagation and permeability in coal, thermal shock tests were conducted on coal specimens by using a constant temperature drying oven (105 °C) and a SLX program controlled cryogenic tank. The growth and propagation of microcracks were measured with computer tomography (CT) scanning and scanning electron microscope (SEM) tests. Results showed that thermal shocks improved the permeability of coal significantly. Notably, the permeability of coal after thermal shocks increased from 211.31% to 368.99% and was positively correlated with temperature difference. CT scanning images revealed that thermal shocks increased the crack number, crack volume and crack width as well as smoothened and widened the gas flow paths, thereby enhancing coal permeability. Moreover, SEM images showed that heating-cooling shocks created more new microcracks, forming more complex crack propagation paths and better connectivity among microcracks in coal compared to cooling shocks. We proposed a crack propagation criterion for coal to explain the mechanism of crack failure and propagation during thermal shocks. Our experiment results and theoretical analysis indicate that the heating-cooling shock is more effective in damaging and breaking coal than the cooling shock. Thus, it can be used as an alternative approach to enhance coal permeability in the production of coalbed methane (CBM).
Highlights
Coalbed methane (CBM) is an important resource of energy in China
To clearly observe the propagation and growth of microcracks, scanning electron microscope (SEM) tests were conducted in laboratory under two kinds of thermal shocks
Exposure of coal specimens to thermal shocks increased the permeability of coal at various temperatures
Summary
Coalbed methane (CBM) is an important resource of energy in China. It is viewed an environmentally friendly source of energy. The key to improve the production of CBM is increasing coal permeability [1, 2]. Many macro-, meso-, and micro-pores coexist in coal, where micropores are the main points of methane adsorption. CBM is very difficult to be drained without external disturbances because methane molecules can tightly adsorb on the micro-pore surface. Many studies have focused on waterless thermal-
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