Analysis of geological conditions for risk control of UCG project

Underground coal gasification (UCG) is an inescapable choice for the development of China’s coal indus- try,and geological risk control is the primary basis for the success of a UCG project. The general goal of risk control for a drilling-style UCG project is divided into four sub-goals,i.e.,the geological feasibility of geo-reactor construction,the easiness of gasification production,the safety of gasification process and the economy of production. The causal rela- tionship between geological conditions and potential risks is discussed,and a new idea of geological analysis and evaluation of an UCG project is proposed. The analysis shows that the high-quality completion of the UCG wells is reflected in the stability of borehole wall and the controllability of borehole trajectory,in which the key geological factor of the former is the coal-body structure or coal seam strength,while the latter is mainly controlled by the thickness,structure, occurrence and stability of coal seam. The geological evaluation on the controllability of the UCG production focuses on two objectives. One is the controllability of the gasification working face’and the coal seams with moderate dip an- gles are conducive to the advancement of the face. It is difficult to promote the face of coal seams with high ignition point. The coal seam with complex structure is not conducive to gasification. Micro-structure is unfavorable to the UCG process control. Other is the easiness of channel control in the UCG layer and the easiness decreases significantly with the increase of coal seam depth. The high cohesive and swelling coal seam will seriously endanger the openness of the channel. The coal seam with higher ash melting point and thermal stability is easier to keep the channel un- blocked. The geological analysis of the UCG process safety also includes two aspects. One is the safety of the UCG pro- duction,the excessive water inflow will cause the hidden risk of production safety,and the key to ensure the safety is to cover an aquiclude with sufficient thickness and stability. Other is the environmental safety of the UCG project,the land subsidence of the UCG project is not significant on the whole,the organic pollution potential is highly related to the coal type,and the high attention is paid to the effect of harmful elements with medium to difficult volatili- ty to environment. The economy of the UCG development mainly involves three aspects,i.e.,the gasification effective- ness,syngas quality and resource reliability,which are closely related to some specific factors such as the chemical re- activity,ash yield,exploration degree and resources of coal. The joint UCG-CBM (coalbed methane) production is an exploratory way to improve the economy of UCG. In fact,most of the geological factors are related to the above four sub-objectives of risk control,and there are cross influences to various degree. Therefore,it is necessary to participate in the geological evaluation of the UCG project based on relative contribution of factors. Among them, three factors,namely,micro-fault,coal cohesiveness and expansibility,and overlying aquifer distance,have the “one vote ve- to” effect. Furthermore,the basic idea of geological evaluation and optimization for the risk control of an UCG project is proposed.