HFEM monitoring of coal gasification, Rawlins, Wyoming

This report covers the high frequency electromagnetic monitoring (HFEM) of an underground coal gasification test located in the North Knobs area about eight miles west of Rawlins, Wyoming in Section II T21N, R89W. At the test site, the gasified coal seam is steeply dipping (63/sup 0/) with a seam width of about 23 feet. The coal is subbituminous. The purpose of the HFEM is to monitor the burn or cavity progress between the injection well and the production well. Figure 1 shows a sectional view of the coal seam with some of the HFEM boreholes. Figure 2 is a plan view of the HFEM boreholes and their relationship to the injection and production holes. All of the HFEM boreholes are cased with PVC pipe and grouted in place, terminating in steel pipe and wellheads with ball valves, pressure gauges and small relief valves. The steel pipe, PVC pipe junction, was made above the coal seam with the PVC extending through the coal seam. Unfortunately, all the boreholes did not extend far enough through the coal seam to allow viewing of the bottom portion of the coal seam. Two key boreholes failed prior to initiation of the burn and became unusable.more » Because of these two borehole failures, we had to change our test plan and depend heavily on boreholes H-20, H-22, and H-15 which were deicated hydrology holes, but could be used for HFEM on a cooperative basis. These holes were not suitable for HFEM as they penetrated only halfway through the coal seam and the steel casing ended at the top of the coal seam, severely limiting our vertical range. Our transmission ranges were now doubled forcing us to reduce our operating frequency with a consequent loss in resolution. We were unable to observe the reverse burn linkage because of the lower operating frequencies. The results of monitoring with the available boreholes are given.« less