332 Figure 3 – Borehole water remaining in Well-1 and Well-2 Table 1 – Results of proximate and ultimate analysis of coal Calorific value (MJ/kg) Proximate analysis (wt%) Ultimate analysis (wt%) Moisture Ash Volatiles Fixed carbon C H N S O 17.98 4.1 38.2 27.9 29.8 45.8 3.64 0.88 0.17 9.61 3. FIELD-SCALE UCG TRIAL 3.1 Experimental Setup The experiment was initiated by igniting the coal section in the co-axial well using LP gas. After confirming ignition, combustion and gasification of the coal were sustained through continuous injection of oxygen-enriched air with an oxygen concentration of 4050%. The oxygen-enriched air was injected after blending oxygen supplied from a compressed oxygen cylinder and air supplied by a compressor (5.5 kW) at a predetermined mixing ratio using a gas metering and mixing device. The detailed injection conditions are presented in Figure 4. To progressively advance the combustion and gasification into the unreacted coal zone, the tip of the injection pipe (ignition position) was sequentially moved toward the inlet side by approximately 100 mm every four hours. A three-axis borehole geophone (Borehole Pick 3315, Oyo Inc.) was installed in the monitoring well (Well-3) at a depth of 17.5 m from the inlet in order to detect acoustic emission (AE) events associated with coal micro-fracturing activity. Figure 5 shows the geophone arrangement. The sensor orientation was defined such that the vertical direction corresponded to the x-component, the horizontal direction to the y-component, and the borehole axial direction to the z-component. The installation location was positioned above the ignition point in the co-axial well. Although the borehole pick is primarily designed as a downhole receiver for PS logging to determine in-situ P-wave and S-wave velocities, it is also applicable for detecting micro-fracturing events and estimating their occurrence frequency and source locations. The instrument is coupled to the borehole wall by means of an inflatable packer expanded with compressed air or pressurized water, allowing for effortless installation and removal. Power supply to the geophone, signal reception, and amplification were performed using a Geode exploration seismograph (Geode NZ SmartSeis ES 3000, Geometrics Inc.). The vibration waveforms obtained by Geode were transmitted to a laptop, enabling real-time monitoring and data logging. In this study, waveform acquisition was conducted with a sampling interval of 250 µs. Product gas during the experiment was sampled every hour and analyzed using a micro gas chromatography (Micro GC 3000A, Inficon Inc.) after removal of moisture and tar components. The analyzed gases are concentrations of oxygen (O2), nitrogen (N2), carbon
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