Acoustic and Vibrational Enhanced Oil Recovery. George V. Chilingar

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Acoustic and Vibrational Enhanced Oil Recovery - George V. Chilingar

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target="_blank" rel="nofollow" href="#fb3_img_img_d2591caa-66be-5b51-a152-df4c69ba5dc3.png" alt="image"/> we have cos θ ≈ 1, and the waves irradiated by the cylinder spread perpendicular to its surface. With the k decline, the θ angle is increasing, and at k = kz, the wave is spreading only tangentially to the cylinder’s surface. As soon as the axial flexural wave length becomes shorter than the sound wave length in the enclosing medium, appropriately k becomes smaller than kz, and the cylinder-well is not radiating in the enclosing medium at all. Irradiation in a case image is studied in great detail, the results are practically applied in geophysical acoustic studies in wells and at high-frequency thermoacoustic action on the reservoir’s bottomhole zone.

      Based on a solution of the dispersion equation for Rayleigh waves in the reservoir, a case has been reviewed when the irradiation was totally disappeared. The well acoustic impedance in this case becomes equal to zero and the well becomes absolutely “soft”. The total energy flow through the well’s walls at that becomes equal to zero, although in a near-surface layer quite significant densities are created of the kinetic and potential energy. At θ ˃ θR, the acoustic energy may enter the reservoir from the well only in a case of normal fading of waves in the liquid. This moment corresponds with a case of an infinitely long cylinder surface-wave mode of which is excited by a low-frequency action within the well image Therefore, at the wave radiation from a well in the reservoir, there are critical frequencies, below which the irradiation in the conventional sense is absent.

      Wave excitement form well considering reservoir elastoporosity was studied by Rakhmanov et al. (1985) [19]. The boundary conditions at the surface of a liquid-filled cylindrical well were assigned in the form of a tension in the solid phase and pressure in the liquid generated by moving at a constant speed load charge as follows:

image

      Restricted solutions of the Bio equations satisfying the irradiation conditions allows one to derive equations for calculating offsets and tensions in the solid phase as well as the pressure in a fluid-porous medium.

      In order to increase the efficiency of energy introduction into the reservoir at low frequency, it is necessary to find a possibility to increase substantially the irradiation from a well into the reservoir by changing loading parameters and optimally utilize action of resonance properties of the well and reservoir systems. The existence of resonance regimes of vibration present in a well, which are associated with parameters of the enclosing medium and emerging in a pre-critical frequency area, is experimentally confirmed. For instance, if to run in a fluid-filled well a receiver of sound vibrations and to measure the energy spectrum of the noise, then in a fluid-saturated reservoir the resonance frequency may be identified.

      The resonance excitation in a well may be achieved at the regime of high-frequency radial resonance of the fluid layer as well as a lengthwise resonance of the fluid column at low frequency. The radial resonance is determined by the well radius and emerges at high frequency on the order of dozen kilohertz and higher. Thus, coordination of the operation regime of the vibration source and the reservoir achieved, so that practically the entire power of the vibration source is transferred in the reservoir. By varying the frequency and load distribution on the surface of the vibration source, it is possible to control field energy structure in the reservoir. However, practical utilization of the radial resonance is substantially complicated for the following reasons. The frequency even of the first radial resonances at the existing well radiuses are too high both for favorable manifestation of elastic vibration action mechanisms on the reservoir and for the frequency coordination with the reservoir excitation resonance regimes. Besides, high-frequency elastic waves experience a strong fading in the reservoir. As the motions of porous medium at such resonances are radial, the generation of high-pressure amplitudes in wells is restricted by the acceptable radial displacement of the casing and cement column.

      That is why various submersible resonance devices are used, in which a set frequency vibration source is rigidly tied with acoustic reflecting filters [17]. These devices include a generator and a resonator enclosed in the body. At that, the fluid column filling the cavity of a resonator chamber forms a vibration contour in which a standing wave is generated. The resonance excitation of vibrations is achieved if the reflector length L and generator operating frequency νo are tied as follows:

image

      where c is the wave propagation phase velocity.

      Using of similar devices with fixed parameters in practice is not sufficiently efficient because the elastic wave phase velocity in a well fluid depends on the reservoir elastic properties and immersion depth and besides may change from one well to the next [4]. If the wave running in a productive interval of a well with perforation holes in the casing, then its phase velocity also depends on the reservoir porosity, permeability, compressibility, and well fluid viscosity [23]. All this results in a substantial instability of the existing devices resonance regime because these devices have been designed according to simplified rules without considering the aforementioned factors. Besides, these devices cannot be too long, and this in restricting the frequency regime of vibration excitation. As

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