Monitoring of coal fracturing in underground coal gasification by acoustic emission techniques

During the underground coal gasification (UCG) process, fracturing and cracks occur inside the gasification zone and surrounding rocks as the underground coal cavity evolves. Although fracturing activity and crack extension directly affect gasification efficiency and have environmental impacts, little research to date has focused on their effects. This study discusses the application of acoustic emission (AE) analysis for the evaluation of distinctly designed UCG models and operational parameters and describes the gasification process based on its results. We studied the cavity growth, fracturing mechanism, and the effects of various design and operational variables, such as linking-hole type, gas feed rate, and gasification agent. We found that the AE activity was closely related to the temperature change occurring inside the coal, with AE generation apparently resulting from crack initiation and extension around the coal gasification area, which occurs as a result of thermal stress. UCG modeling showed that the location of AE sources reflects the size of the gasification area and the cavity growth. In addition, the quantitative information on the located AE sources can be obtained. The introduction of a process control system into UCG modeling along with AE monitoring allowed for the real-time monitoring of the fracturing and cavity evolution inside a combustion reactor. Together, these processes have the potential to significantly reduce field risk in UCG by enabling the timely adjustment of operational parameters. Thus, AE monitoring is useful for maintaining a safe and efficient UCG process. (C) 2016 Elsevier Ltd. All rights reserved.