Abstract

Cross-borehole, radio-frequency geotomographs were made across two different-sized, operating, experimental, underground, in-situ, oil-shale retorts. The tomographs taken of the smaller retort were of a plane 16.7 m wide by 18.0 m high bisecting the retort. The measurements were taken at a frequency of 25 MHz and showed excellent correlation of high signal attenuation with the high-temperature zones. Measured attenuation coefficients (inverse skin depth) at 25 MHz ranged from 0.40-0.45 m (super -1) across the cool, unrubbled, sill-pillar, and between 0.60-0.70 m (super -1) for the high-temperature 370-700 degrees C zone. Image resolution was approximately 1 m. The signal attenuation across the lower retort region was also found to correlate well with the movement and concentration of condensed water. The conventional algebraic deconvolution method (ART) was modified for limited perspective and finite beam width, and gave results which were in good agreement with thermocouple data.The measurements made on the larger retort were taken over a period of 33 days of retort burn and mapped the attenuation coefficients in a vertical plane 90 m wide by 48 m high at a frequency of 1.5 MHz. At this frequency, attenuation coefficients in the cool, dry retort regions were between 0.12 and 0.15 m (super -1) , while regions containing a high moisture content had coefficients of from 0.15 to 0.19 m (super -1) . In the regions encompassing the retorting and combustion zones, attenuation coefficients were between 0.15 and 0.25 m (super -1) Some additional effects on the attenuation measurements were observed due to nearby thermocouple piping. The movement of the contours of attenuation coefficient with time followed temperature changes, though the paucity of thermocouples in the tomographic plane only allowed a marginal correlation to be made.Overall results suggest that radio-frequency geotomography can be a useful tool for mapping in-situ moisture concentrations and temperature fronts in an operating in-situ oil-shale retort.

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