|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Journal Paper|
|Title||Estimates of Formation Sound Speed From Ultrasonic Reflections|
|Authors||Davidson, J.A., Morriss, S.L., Podio, A.L., U. of Texas|
|Journal||SPE Formation Evaluation|
|Volume||Volume 10, Number 2||Pages||72-78|
|Copyright||1995. Society of Petroleum Engineers|
This paper presents results from experimental research concerning the ultrasonic reflectivity of rock samples. The goal of the research is to determine whether meaningful petrophysical properties can be obtained through the analysis of reflected acoustic signals of the type produced with the new generation of ultrasonic borehole televiewers. Present emphasis is on analysis of the full reflected waveform. Time domain samples of the transmitted and reflected signals are transformed into the frequency domain by means of digital Fourier methods to determine the complex reflection coefficient of the rock. The reflection coefficient estimates are converted into wave velocity estimates using a procedure based on a simplified viscoelastic wave propagation model. Sound speed estimates in reasonable agreement with those determined using standard experimental techniques have been obtained for a variety of sandstone samples. Signal processing techniques for the analysis of reflected waveforms recorded in an attenuating mud-filled borehole environment have been developed and tested in the laboratory.
The new generation of acoustic borehole televiewers utilize small diameter (2") ultrasonic transducers which rotate to scan the full circumference of the borehole. The transducers operate in a pulse-echo mode and record the reflected amplitude and travel time of a short duration pressure pulse which is transmitted normal to the borehole wall. Variations in lithology, rock texture, compaction and physical properties such as fractures and laminations cause variations in the acoustic impedance of the formation face. These variations influence the amplitude of the reflected signal. Up to 250 amplitude and travel time samples are recorded per revolution. Images are produced by assigning a grey scale or color scale to the range of the recorded amplitude values. The travel time data is used to produce a three dimensional caliper log.
Acoustic theory for porous media predicts that information concerning the porosity, permeability and other acoustic and petrophysical properties of the rock is present in the reflected signal. These properties influence the amplitude and phase of the various frequency components which make up the reflected pulse. Therefore, analysis of the full reflected waveform rather than just the maximum amplitude and travel time may provide information about the variations in porosity, permeability and other rock properties. Experiments are being conducted to determine if information about these properties can be drawn from reflected signals using current signal-processing technology.
The present investigation was undertaken to determine whether compressional velocity estimates similar to those obtained from conventional sonic logging tools could be obtained from ultrasonic reflection data. A method to make such estimates was proposed by Liben in 1965. Liben's method is based on a reflection coefficient calculated using time domain amplitude measurements and does not provide a means to account for the dispersive nature of real porous media. The method presented here differs in that frequency domain samples are used to calculate the complex reflection coefficient of the rock as a function of frequency. This data is then used to estimate the sound speed using a viscoelastic model which does account for dispersion.
|File Size||537 KB||7|