337 Table 4 – Average values of product gas volume, recovered energy, and gasification efficiency obtained in this field-scale experiment and in previous artificial coal seam model experiment Product gas volume, N2, O2 free Recovered Energy Gasification efficiency /coal /carbon /coal /carbon (m3/kg) (m3/kg) (MJ/kg) (MJ/kg) (%) Field experiment (O2: 40~50%) 1.33 2.89 12.86 28.09 71.56 Model experiment (Hamanaka et al., 2021) (O2: 50%) 1.37 2.47 14.01 25.34 61.84 4.2 Acoustic Emission In this experiment, AE monitoring and AE source location analysis were conducted using a single triaxial borehole geophone, assuming future application to monitoring of a UCG gasifier and gasification zone. In general, AE source location using a triaxial sensor is performed by determining the P-wave and S-wave arrival times, calculating the source-tosensor distance from the P-wave and S-wave velocities, and estimating the arrival direction from the P-wave vibration direction (Lissajous analysis). However, in the waveform data obtained in this study, clear distinction between P-wave and S-wave onsets was difficult. Therefore, after determining the P-wave vibration direction, source location was performed under the assumption that the AE sources were generated within the central portion of the coal seam. Furthermore, the potential source area was restricted to a volume bounded by 2.5 m on either side of the co-axial well inlet, 40 m along the axial direction of the coaxial well, 6 m below the well, and 21 m above it. Detailed examination of the recorded waveforms identified 35 AE events considered to be signals rather than noise and suitable for source location analysis; however, only 10 events were successfully located within the assumed model. Figures 8(a) and 8(b) show the results of the AE source location analysis, where the red dots indicate the estimated source locations. It is difficult to quantitatively evaluate the overall fracture progressing trend due to the limited number of located events (10 data). Nevertheless, a tendency was observed in which AE events occurred beneath the geophone during the early stage of the experiment, whereas in the later stage the estimated source locations migrated toward the inlet side of the co-axial well. This result may reflect migration of the combustion/gasification zone associated with the progression of the UCG process (Iriguchi et al., 2023). For future in-situ UCG operations at deeper depths, a monitoring system capable of detecting a sufficient number of AE events will be required. If a larger dataset with reduced noise influence can be acquired, source distribution analysis method applied in this study can enable visualization of the migration of the combustion/gasification zone, thereby providing a promising tool for real-time monitoring the progression of UCG.
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