1567 using artificial coal seams under laboratory scale, in-situ coal seam gasification remains considerably more complex due to geological heterogeneity and hydrogeological conditions. Consequently, practical implementation of UCG has not yet been achieved in Japan. In this study, an in-situ UCG field experiment was conducted targeting a coal seam exposed at the final face of an open-pit coal mine. The objective was to obtain technical insights necessary for practical implementation of UCG, including ignition methods for insitu coal, control of combustion and gasification, regulation of product gas composition, and monitoring techniques for the expansion of the combustion cavity. This paper reports the results of the field-scale in-situ UCG experiment and discusses its implications for future application. 2. FIELD OVERVIEW The field-scale experiment in this study was conducted targeting an exposed coal seam remaining on the final face of an open-pit coal mine. The coal seam thickness is approximately 2 m with numerous interbedded sandstone and shale. The upper layer of the target coal seam is a sandstone formation. Figure 1 shows the arrangement diagram of the field experiment. As shown in Figure 1, a coaxial well (Well-1) was drilled on the left side and an auxiliary production well (Well-2) on the right side, both directed toward the coal seam surface. Both wells were drilled from the mid-thickness of the coal seam along the seam dip (inclined approximately 5-10° downward toward well bottom) with a diameter of 76 mm and depth of 20 m. The bottom 5 m section of each well was left uncased (open hole), while the remaining upper section was cased with 80A steel pipe and fixed using cement slurry. The horizontal distance between the inlet of Well1 and Well-2 is approximately 7 m. In addition, a monitoring well (Well-3) was drilled with a diameter of 66 mm as an open hole in the sandstone layer located 3 m above the coaxial well, where the rock mass was considered relatively stable. Figure 2 shows the drilling cores obtained from the 5 m section at the well bottom. In Well-1, the core from 17-18.5 m was partially removed for samples of proximate analysis. In both Well-1 and Well-2, the section between 17-18 m consists of relatively continuous coal with fewer interbeds, however, sandstone and shale interbeds were confirmed in other sections. Figure 3 shows the borehole water remaining in Well-1 and Well-2. The figure indicates that borehole water, which had not been discharged after drilling, was observed at approximately 19 m depth in both wells. In Well-3, it was confirmed that the upper sandstone layer consists of intact rock with few fractures. Table 1 lists the results of proximate and ultimate analysis of coal samples collected from the drilling cores of the co-axial well. The calorific value is 17.98 MJ/kg (4,300 kcal/kg), and the carbon content is relatively low at 45.8%. This is attributed to the high ash content of 38.2%, reflecting the frequent presence of interbeds within the coal seam.
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