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Determination of rocks porosity by means of nondestructive techniques that use ultrasound generated by laserM. T. Bernal, Guillermo Capporaletti, María Aurora Rebollo, Fernando Pérez Quintián, Carlos A. Raffo
Laboratorio de Aplicaciones Opticas. Dto. de Física FIUBA
Paseo Colón 850-2do. piso-(1063) Buenos Aires-Argentina
José E. Ruzzante, Isabel López Pumarega
ENDE-Centro Atómico Constituyentes
Av. del Libertador 8250-(1429) Buenos Aires-Argentina
The purpose of this work is to determine the sound attenuation coefficient in porous rocks extracted from oil wells and in concrete samples, at different frequencies of the ultrasonic waves induced by laser incidence at the surfaces.
We generate ultrasonic waves at one of the faces of cylindrical samples of different thickness and then measure the signal at the opposite end by way of piezoelectric detectors .Using Fourier Transform techniques we obtain the frequency spectrum of said signal whereby we can correlate the signal with the propagation distance inside, and obtain the attenuation coefficient in each case. The measured coefficients in the case of rocks are then compared with the porosity as measured by classical methods.
keywords: laser ultrasonic - porosity
One important property of reservoir rocks is the porosity since its capacity to store fluids is a very important parameter. Porosity is defined as the fraction of the total bulk volume of the rock not occupied by solids . The ultrasonic propagation is related with the physical characteristic of the rocks and it is possible to think that the pores act like scatters of the ultrasonic waves. Then by analysing the transmitted ultrasound signal the porosity can be obtained. The ultrasonic laser generation method is an optical non-contact one, and allows the production of repetitive short pulses which generate a wide range of sound frequencies.
In this work the velocity of propagation is experimentally obtained, as well the frequency attenuation of the ultrasonic waves generated. These magnitudes were then correlated with the porosity of the samples as known from other sources.
Figure 1 shows the experimental set-up. Pulses from a Nd:Yag Q-switched laser impinge on the front face of a cylindrical sample. Since ultrasonic generation is more efficient as the optical wavelength is reduced  the harmonic at 0.256 mm with a repetition rate of 10 Hz and pulse length of 6 ns, was chosen. The average power in the beam was about 500 mw. A beam-splitter was used for monitoring the laser power, and the sample was acoustically isolated. The ultrasonic signal was detected with a wide range piezoelectric sensor at the rear face of the sample in alignment with the laser impact point. The detected signals were sent to an acquisition data card to be computer processed. The signal was triggered from the laser, and the record started when the laser fired.
The velocity measurements were made over samples having different porosity. Actually : 2.3; 6.6; 7.6; 13.4; 16; 17.3; 18, 19,5 in percent.
Figure 3 shows the relationship between velocity and porosity. It is possible to observe that the acoustic velocity of longitudinal waves linearly decreases with porosity.
The attenuation is produced through two different mechanisms:
In the frequency range for which l >>d , where d is the average porous diameter, the frequency dependence of the observed attenuation A can be expressed as
A = a1 f +a2 f4 
The first term represents the absorption. The second term arises from the scattering and is usually referred as the Rayleigh term, in analogy with scattering of light by small particles (pores).
The frequency-dependent attenuation coefficient A is calculated using the Fourier Transform of the windowed acoustical signals. The frequency spectrum was obtained for samples with 2.2; 13.4; 16 and 20 % porosity and 40; 30; 25; 20 and 10 mm of thickness.
Figure 4 shows the amplitude of the acoustical signal Fourier transform for a 13.4 % porosity and different thickness.
Figure 5 shows the attenuation dependence with the frequency.
These preliminary studies allows to conclude that the ultrasonic laser technique is adequate to be used in porous rocks characterisation. The reason is that the detected acoustic signals carry abundant information, among which the linear relationship found between porosity and velocity (fig.3), and the clear trend for the increase of the attenuation with frequency (fig 5).
The incident laser power was so low as 500 mW, which permits to think that it should be possible to produce good ultrasonic signals using transportable lasers for "in situ" measurements.
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