Acoustic velocity logging
ACOUSTIC VELOCITY LOGGING (EN: acoustic velocity logging; DE: akustische Bohrlochmessungen, akustische Karottage; FR: carottage acoustique; ES: perfilaje ecoico; RU: акустический каротаж) is the method of the geophysical surveys in the drilled wells, which is based on the study of the acoustic properties (the velocities of the propagation and the damping of the elastic waves) of the rocks, which are intersected by the drilled well.
It is used during the search and exploration of the deposits, during the monitoring of the technical condition of the drilled wells, during the interpretation of the data of the seismic exploration, as well as during the solving of the engineering geological problems. The first samples of the apparatuses of the acoustic velocity logging have been created in 1950s in the USSR and the USA; the industrial usage has begun since 1960. They use during the acoustic velocity logging the sonic (0.5-15 kHz) and the ultrasonic (20-50 kHz, 0.3-2.0 MHz) frequency ranges. They perform the acoustic velocity logging with the help of the downhole probe, which is connected by the logging wireline with the superficial measuring and registering instruments. The major elements of the downhole instrument are the emitters and the receivers of the elastic waves, and also the acoustic insulators, which are preventing the propagation of the elastic waves through the casing of the downhole instrument. There are serving as the emitters the magnetostrictive transducers, which are changing the radius of the metallic (permendur, nickel) cylinder under the action of the alternating magnetic field, or the piezoelectric transducers of the barium titanate, lead zirconate, which are creating the oscillations as the result of the impact of the alternating electric field. The receivers are the piezoelectric elements, which are converting the mechanical energy of the elastic waves into the electrical impulses.
During the performance of the acoustic velocity logging, the electrical impulses are coming from the block of the synchronization and control into the emitters, where they are converted into the impulses of the elastic oscillations with the duration of 5-10 microseconds; the predominant energy of these impulses is concentrated in the band of the frequencies of 10-15 kHz. They measure the times of the travel of the main types of the waves and the coefficient of the damping. According to the results of the measurements, they construct the geoacoustic models of the cross sections of the drilled wells for the interpretation of the data of the seismic exploration, perform the evaluation of the porosity of the productive layers, determine the elastic moduli of the rocks (the moduli of Young, shear, volume expansion), uncover the zones of the increased fracturedness and cavernosity. The combined usage of the data of the acoustic, electrical and radioactive logging permits to perform the lithological fragmentation of the cross sections, to uncover the collectors of the oil and gas, to determine the coefficient of the saturation, to monitor the development of the fields of the oil and gas.
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