330 new reactive coal face, thereby sustaining the gasification process (Su, F et al., 2024). It has also been demonstrated that the migration and expansion of the UCG reaction zone can be visualized by locating the sources of acoustic emission (AE) generated during combustion and gasification using uniaxial and triaxial acceleration transducers (Iriguchi et al., 2023). In addition, in order to promote upward expansion of the combustion/gasification zone around the ignition point, a composite co-axial UCG system has been attempted, in which an auxiliary production well was installed above the co-axial well in the artificial coal seam. In this arrangement, an auxiliary production well was installed approximately 40 cm above the co-axial well, and a vertical well near the ignition point was used to connect the co-axial well and the auxiliary production well. Oxidant injection was conducted using the same double-structured injection pipe inserted into the co-axial well as in conventional co-axial well method; however, UCG gas was produced from both the co-axial well and the upper auxiliary production well. As a result, enhanced expansion of the combustion/gasification zone toward the upper portion of the artificial coal seam was confirmed (Iriguchi et al., 2023). Although substantial knowledge has been accumulated from gasification experiments 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 Well-1 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
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