高温对煤炭地下气化围岩损伤的影响

Influence of high temperature on surrounding rock damage ofunderground coal gasification

煤炭地下气化是煤炭无害化开采技术创新战略方向之一,该技术可以回收老矿井废弃煤炭资源,对传统采煤技术难以开采的煤炭资源进行原位清洁转化。气化过程中燃空区形成带来的结构应力和高温造成的热应力共同作用对岩石造成损伤。以大城勘查区深部煤层为气化对象,得出典型围岩热物性及力学参数随温度变化规律。基于连续损伤力学理论,在平滑Rankine损伤模型的基础上提出高温岩石损伤变量模型,使用COMSOL Multiphysics多物理场耦合软件对深部煤层地下气化过程围岩温度、主应力、损伤变量进行模拟研究。结果表明,5种典型岩石的比热容随温度升高整体呈上升趋势,导热系数随温度升高整体呈下降趋势,抗压强度和弹性模量随温度变化规律差别较大。围岩受温度影响范围随气化时间呈指数变化,气化10 d时,温度影响范围仅为3.27 m;气化50 d时,温度影响范围达到5.73 m;气化100 d时,温度影响范围为8.21 m;气化400 d时,温度影响范围达到18.20 m。结合地下气化过程中普遍采用的控制注气点后退气化法,岩石处于高温区的时间在40 d左右,温度场对围岩的影响范围约为4.7 m。燃空区上方及两端均出现损伤区,且相互连接形成“凹”字型,燃空区长度90 m时,损伤区高度为72.2 m;燃空区长度170 m时,损伤区高度为114.1 m;燃空区长度250 m时,损伤区高度为148.8 m;燃空区长度330 m时,损伤区高度为162.6 m;随燃空区长度增加,损伤区高度与燃空区长度之比降低。高温会对直接顶造成显著影响,但随着燃空区长度的增长,温度对直接顶损伤变量的影响逐渐降低,因此在气化初期高温会明显降低直接顶的损伤程度,使其更加难以发生垮落。

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.