Underground Coal Gasification is expected to be efficient technique for coal energy recovery from deep or complex coal seam since directional drilling technique is advancing in these days. Authors have been performing small-scale UCG model tests to clear gasification and combustion process in UCG. Then, we found that radial cracks were initiated from the cavity formed in the artificial coal seam. Understanding mechanism of the crack initiation is important for clarification of the detail process of combustion and gasification and assessment for environmental risks. In this study, thermal stress analysis was performed on the small-scale UCG model tests to consider the initiation mechanism of the cracks by assuming that combustion and gasification of coal were progressing through the following three processes which are often observed in coal carbonization:
softening and melting and
It was found that tensile stress was induced in the vicinity of the cavity in the tangential direction in process C. Direction of principal stress in the coal was almost parallel to tangential or radial direction of the cavity and the magnitude of it exceeded coal tensile strength. It was also found that tensile stress zone was extended into deeper coal seam with increase in temperature and time and compressive stress zone was formed outside of the tensile stress zone. It can be considered that the radial cracks initiated at the surface of the cavity since tangential tensile stress exceeded tensile strength of coal. Then, radial cracks were arrested at the boundary of tensile stress zone and compressive stress zone after they were propagating in coal seam.
Underground Coal Gasification (UCG) is a technique to use coal energy more efficiently and cheaply. In UCG, oxidant is injected into underground through an injection well to gasify coal seam, and syngas is recovered from a production well (Fig. 1). It is expected that UCG increases available amount of coal energy because even low-grade, complex and deep coal can be used by UCG.
It is pointed out that UCG has risks of surface subsidence and groundwater pollution because cracks are likely to initiate in coal seam by combustion and gasification. Therefore, clarification of initiation and growth mechanisms of the cracks is significant for stability assessment of ground as well as assessing environmental risks.
We performed small-scale UCG model tests on massive coal and crushed coal samples to clear gasification and combustion process in UCG. It was found that radical cracks were initiated in an artificial coal seam made by massive coal as well as crushed coal (Fig. 2 (Kodama et al., 2016)). Similar radial cracks were also observed in large-scale UCG model test (NPO Underground Resources Innovation Network, 2016).