Radio‐frequency geotomography for remotely probing the interiors of operating mini‐ and commercial‐sized oil‐shale retorts

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 [Formula: see text] across the cool, unrubbled, sill‐pillar, and between [Formula: see text] for the high‐temperature 370–700° 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 [Formula: see text], while regions containing a high moisture content had coefficients of from 0.15 to [Formula: see text]. In the regions encompassing the retorting and combustion zones, attenuation coefficients were between 0.15 and [Formula: see text]. 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.

Characterization of Geotomographic Studies with the EMRE System

“…He calculated the attenuation constants of porphyry and sedimentary rocks at several frequencies [1]. The work was continued by Lager and Lytle [2], Lytle et al [3], and Somerstein et al [4]. Stolarczyk and Fry [5] used RIM to detect faults in the continuity of coal seams, and this can be considered as the starting point of RIM.…”

Geotechnical and groundwater geophysics

“…That is, such inversions are capable of not only detecting an anomalous zone, but also of describing it. Notabie applications of cross-berehole EM are: groundwater flow monitoring (LytIe et al, 1981), in-situ process monitoring (Dailey, 1984;Somerstein et al, 1984), soil grouting (Laine et al, 1980), tunnellocation (LytIe et al, 1979, and salt dome description (Nickel et al, 1983). Seismie applications of cross-borehole or surface-borehole tomography follow mathematical developments identical to the EM techniques just described.…”

“…The broad scope of today's engineering geophysics is evident from numerous recent publications illustrating the application of techniques largely developed within a geotechnical and groundwater context to such diverse problems as refraction staties corrections to reflection data (Gelchinsky and Shtivelman, 1983), acoustic emissions applied to mapping hydraulically induced fractures in enhanced gas recovery programs (Dobecki, 1983), and crosshole electromagnetic (EM) and seismie tomography to map the area between wells (LytJe et al, 1981;Wong, 1984;Somerstein et al, 1984). Geophysical applications to geotechnical and groundwater problems, therefore, have !eapt from being merely sensible, cost-effective substitutes for boreholes or scapegoats in difficult subsurface geology to being often the only means by which an important problem can be addressed.…”

Electromagnetic geotomography-selection of measuring frequency

“…Thus, taking into account the deficiency of the abovementioned methods, it is advisable to apply other methods to our problems, despite ART and SIRT being proposed in [1], [2], [4], [22] for geotomographic applications. We propose to apply the One Step Late (OSL) algorithm that was devised by Green [23].…”