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well logging[′wel ‚läg·iŋ]
geophysical testing of wells, used to study geologic cross sections and to detect minerals. The term “well logging,” which was first used in mining, does not completely correspond to the idea described. In addition to “logging,” the terms “geophysical well-testing methods,” “field geophysics,” and “drilling geophysics” are also used in the scientific and technical literature.
The first geophysical well tests—temperature measurements —were performed by D. V. Golubiatnikov in 1908 at the Baku oil fields. Electrical well logging by the apparent-resistivity method was proposed in 1926 by the Schlumberger brothers (France). The high effectiveness of electrical well logging has brought about its rapid adoption by the petroleum industry and provided impetus for the development of other well-testing methods. In the Soviet Union, major contributions to the theory, methods, and technology of well logging were made by L. M. APpin, M. I. BaPzamov, G. V. Gorshkov, V. N. Dakhnov, A. I. Zaborovskii, A. A. Korzhev, S. G. Komarov, B. Pontekorvo, A. S. Semenov, M. M. Sokolov, V. A. Fok, and V. A. Shpak. Important research in logging theory and methods has also been done in the USA (G. Archie, H. Guyod, J. Dewan, H. Doll, M. Martin, W. Russell, and J. Wyllie).
Geophysical well testing is done by electrical, magnetic, radioactivity (nuclear), thermal, and acoustic (ultrasonic) methods. During the performance of these tests, certain measurements that depend on a single property or on a combination of physical properties of the rock through which a well is driven are made along the well bore by means of geophysical sensors lowered into the well on a cable. The signals from the sensor are transmitted to the surface and recorded in analog form (diagrams) or in digital form by ground equipment installed in a vehicle.
Resistivity, diffusion-adsorption, and artificially induced electrochemical activity of rock are studied using methods of electrical well logging. The apparent-resistivity method, including the microprobe method, the shielded-ground resistance method (side logging), and the induction method are based on the study of electrical resistivity. The difference in the diffusion-adsorption activity of rock is used in the spontaneous polarization method, but the ability of rock to become polarized under the effect of electric current is used in the induced-polarization method. The magnetic method is used to measure the magnetic susceptibility of rock. Radioactivity (nuclear) logging methods are based on well measurements of the natural or artificially induced radioactive radiation of rock. In the latter case the neutron, gamma-gamma, induced-activity, and radioactive-isotope methods are used. The nuclear-magnetic logging method consists in observation of the change in the electromotive force that occurs in rock after it has been subjected to a polarizing magnetic field. Thermal methods are used for the study of temperature in wells. The acoustic (ultrasonic) method is based on the study of the velocity and attenuation of elastic waves in rock. Gas and luminescent bituminologic logging are geochemical well-testing methods. Methods based on the study of the mechanical properties (drill-ability) of rock during the drilling process (mechanical logging) are also sometimes used.
The task of geophysical well logging includes correlation (comparison) of well cross sections, determination of the li-thology and depth of occurrence of the rocks through which the well passes, determination and evaluation of mineral reserves (petroleum, gas, water, coal, ores, and building materials), and monitoring of the development of oil and gas fields. Logging is the principal geologic method of documentation of deep well sections.
REFERENCESKomarov, S. G. Geofizicheskie metody issledovaniia skvazhin. Moscow, 1963.
Pomerants, L. I., and V. T. Chukin. Promyslovo-geofizicheskaia apparatus i oborudovanie. Moscow, 1966.
Dakhnov, V. N. Interpretatsiia rezul’tatov geofizicheskikh issledovanii razrezov skvazhin. Moscow, 1972.
V. M. DOBRYNIN