ABSTRACT:

Understanding of the progress of the burn front and cavity growth of the gasifier is crucial for the control of underground coal gasification. The high temperature generated from coal burning produces high thermal stress to the surrounding rock mass in the roof and floor. Thermal fracturing occurs when the stress exceeds the strength of the rock mass. Microseismic monitoring of thermal fracturing thus provides a way to detect the progress of the burn front. This paper presents results from a microseismic monitoring project carried out in an underground coal gasification experiment at a coal mine in China. The microseismic system consisted of eight geophone sensors which were deployed underground 20-70m from the burning area and along the firing progress direction. During the experiment, several types of seismic events associated with thermal fracturing and roof caving were detected. The thermal events showed a higher frequency range compared to the other events and their locations change with the progress of firing. Numerical modeling results showed a strong concentration of thermal stress near the burn front could break the rocks in the roof and floor of the firing coal seam.

1. INTRODUCTION

Unlike conventional mining methods which physically mine coal, underground coal gasification (UCG) is a technology which burns coal in-situ and converts it into combustible gas [1] through controlled chemical reaction processes. As only gas is produced from coal, UCG is regarded as an environment friendly and safe mining technology. In addition, UCG allows access to more coal resources than traditional mining technologies and it has great potential for the economical extraction of coal from seams which are buried in complex geological structures, too deep, too steep, too thin or too thick. The control of underground gasification is very important to produce large amounts of quality gas and fully utilise coal resources. This requires good understanding of the underground gasifier growth. Rock damages induced by thermal stress and caving processes also need to be understood because rock fractures change the rock permeability which may have impact on the gasification. The developed rock fractures may also form channels allowing the escape of pollutants to the atmosphere and damage of local underground water systems. Several measurement techniques were tested in a number of UCG experiments to map the changes of the gasifier shape and size as a function of time. Borehole thermocouples were used to map temperature field from which the burn front was estimated [2]. Experiments were also conducted to use electrical resistivity [2], electromagnetic [3] and seismic [2, 4] methods for mapping the burn front. After the coal is burn out, a cavity is formed and its size is extended as gasification proceeds. The creation of the cavity changes the stress regimes in the rock mass near the burning area and causes the rocks to fracture and unsupported roof to collapse. The change of fracture locations can then be used to infer the growth of the cavity. Microseismic monitoring is a technique which is efficient for the detection of rock fracturing and its location.

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