To recover coal gas and heat energy from an underground coal seam abandoned for either technical or economic reasons, we are developing safe and efficient Underground Coal Gasification (UCG) techniques. A typical UCG system has two wells drilled from the surface into a coal seam with some separation of injection from production. The wells are connected underground by various linking techniques. After creating a combustion reactor in an underground coal seam, air and oxygen flow through the injection well. Heat energy and gases are collected from the production well. In this process, the fracturing activity inside the coal seam serves an important role for enlargement of the gasification zone because the surface area oxidization increases continuously by coal cracking. On the other hand, excess fractures inside the coal seam and surrounding rock can induce gas leakage, underground water contamination, subsidence, etc. Therefore, monitoring and control of fracturing activity in underground areas constitute key technologies for efficient and safe UCG development.
To monitor fracturing activity, we used Acoustic Emission (AE) monitoring during UCG model experiments using artificial coal seam. Through the experiments, it was found that the combustion and gasification volume in the coal seam was possible to estimate from the AE source location data. Therefore, AE monitoring is a useful tool to evaluate combustion reactors for efficient and safe UCG systems.