Numerical simulation of coupled thermal-mechanical fracturing in underground coal gasification

A combination of thermal fracturing and stress-induced fracturing, i.e. coupled thermal-mechanical fracturing, occurs under the effect of combustion-generated heat and stress in underground coal gasification. Controlling the cracking of roofs and floors and the precise positioning of the combustion zone in underground coal gasification requires full knowledge of the characteristics of the coupled thermal-mechanical fracturing of the surrounding rocks. This study considers the variation in the physical and mechanical parameters of the rock with temperature and rock heterogeneity in order to derive a mathematical model of coupled thermal-mechanical fracturing. Then, a numerical simulation is performed, from which the following conclusions can be reached. First, temperature increases expand fractures, which emerge in the highest temperature area, before extending to lower temperature areas. Second, fracture density is directly related to temperature, with higher temperature corresponding to greater fracture density. Third, the cracking rate increases linearly with time in stages. For m = 1, the growth rate of the cracking rate are 55.76%/h, 26.08%/h correspondingly during the first stage (from 0.8 h to 1.6 h) and the second stage (from 1.6h to 2h).While for m = 2, the growth rates are 18.50%/h, 40.99%/h correspondingly during the first stage (from 0.8h to 1.3h) and the second stage (from 1.3h to 2h). Finally, fracture formation speed is slower in more homogeneous conditions when other conditions are unchanged. A few fractures emerge in the sample after 1.3h for m = 2; the sample cracking rate is only 14.84%, which is far lower than for m = 1 at the same time (32.31%). Similarly, the cracking rate reaches 29.94% after 1.6 h for m = 2 (still far lower than that for m = 1 at the same time, which is 46.91%). The average growth rate of cracking rate for m = 1 (27.16%/h) is quicker than that of m = 2(21.77%/h).