Lateral logging

The three-electrode lateral logging

LATERAL LOGGING (EN: lateral log; DE: Seitenkarottage; FR: diagraphie laterale, carottage lateral; ES: diagrafia lateral; RU: боковой каротаж) is the method for the geophysical researches within the boreholes, which is based on the study of the specific electrical resistivity of the rocks with the help of the probe, which is providing the distribution of the electrical current perpendicularly to the wall of the borehole. The lateral logging has been proposed by the American scientist G. J. Doll during the 1950; within the USSR, it has got the development since the 60-ies, and has become one of the effective types of the electrical logging.

During the lateral logging, the electrical current from the source, which is situated at the surface, is fed into the borehole instrument, arrives through the current electrodes of the probe into the borehole and into the rocks, which are surrounding this borehole (see the Figure).

The control (focusing) for the field of the probe is performed with the help of the screen electrodes, which prevent the spreading of the current from the major electrode along the borehole, and direct it into the layer, which is researched. There are measured the difference of the potentials between the electrodes (one of the screen electrodes, and the remote measurement electrode), and the magnitude of the current through the major current electrode. The apparent resistivity (the result of the division, namely, the quotient, of these values) is registered with the help of the logging station, which is situated on the surface.

Depending on the quantity of the electrodes, and on the conditions for the control of the field, they distinguish the three-electrode or multi-electrode probes for the lateral logging. The three-electrode probe is the cylinder, which is divided by the insulating intervals into the electrodes: the short major electrode, and the two screen electrodes, which are symmetrical in relation to the major electrode. There is passed through the electrodes the current, the magnitude of which is regulated in such a manner, that the potentials are equal, which ensures the distribution of the current lines along the radius. The multi-electrode probe for the lateral logging, besides the major current electrode, comprises the two pairs of the measuring electrodes, and several pairs of the screen electrodes. The electrodes of the same name are situated symmetrically at the both sides of the major electrode, and are pairwise connected one with another in the short-circuit manner. By changing the polarity of the electrodes, and the sizes for the inter-electrode distances of the multi-electrode probes for the lateral logging, it is possible to regulate the radius of the research.

They use the lateral logging for the studying of the cross sections of the boreholes (including the boreholes, which are filled with the mineralized drilling fluid), which are represented by the rocks with the high resistivity, or by the frequent alternation of the layers with different parameters. During the drilling of the petroleum and gas wells with the mineralized drilling fluid, they use the lateral logging within the complex with the lateral micro-logging; during the documenting of the cross sections of the coal boreholes, they use the lateral logging within the complex with the methods for the radioactive logging. The major advantage of the lateral logging (in comparison with the other types of the electrical logging) is the insignificant influence of the drilling fluid and of the country rocks onto the results of the lateral logging, which permits to dissect the cross section in the more detailed manner, to determine more precisely the specific resistivity of the layers within the wide range (from 1 to 10^5 ohm-metres). The development of the lateral logging is associated with the development of the new probes with the more perfect regulation of the field.