Abstract:
Underground coal gasification(UCG)has become one of the strategicdirection of harmless coal mining technology innovation.The technology can recover the abandoned coal resources in old mines and make in-situ clean use of coal resource which is difficult to be mined by traditional coal mining technology.The structural stress caused by the formation of combustion cavity and the thermal stress caused by high temperature damage the rock together during the gasification process.Taking the deep coal seam in the Dacheng exploration area as the gasification object,the laws of thermal physical properties and mechanical parameters of typical surrounding rock versus temperature were obtained.Based on the theory of continuous damage mechanics and the smooth Rankine damage model,the damage variable model of high-temperature rock was proposed.The COMSOL Multiphysics multi-physics field coupling software was used to simulate surrounding rock temperature,principal stress and damage variable in the process of deep coal seam underground gasification.The results show that the specific heat capacity of five typical rocks shows an overall upward trend with the increase of temperature,while the thermal conductivity shows an overall downward trend with the increase of temperature.The compressive strength and elastic modulus vary greatly with the change in temperature.The temperature range of surrounding rock varies exponentially with gasification time.When gasification for 10 d,the influence range of temperature is only 3.27 m.When gasification for 50 d,the temperature influence range reaches 5.73 m.When gasification for 100 d,the temperature influence range is 8.21 m.When gasification for 400 d,the temperature influence range reaches 18.20 m.Combined with the Controlled Retracting Injection Point(CRIP)system commonly used in the process of underground coal gasification,the time of rock in the high-temperature zone is about 40 d,and the influence range of temperature field on surrounding rock is about 4.7 m.There are damage zones above and at both ends of the combustion cavity,which are connected in a “concave” shape.When the length of the cavity is 90 m,the height of the damage zone is 72.2 m.When the length of the cavity is 170 m,the height of the damage zone is 114.1 m.When the length of the cave-in is 250 m,the height of the damage zone is 148.8 m.When the length of the cavity is 330 m,the height of the damage zone is 162.6 m.The ratio of the height of the damaged zone to the length of the combustion cavity decreases with the increase of the length of the combustion cavity.High temperature has a significant impact on the direct roof,but with the increase of the length of the combustion cavity,the influence of temperature on the direct roof damage variable gradually decreases.Therefore,high temperature will significantly reduce the damage degree to the direct roof in the early stage of gasification,making it more difficult to collapse.