Amplitude-Frequency Characteristics of the Oscillations of Methane Gas Bubbles in Oil

Amplitude-Frequency Characteristics of the Oscillations of Methane Gas Bubbles in Oil

Faig Bakhman Ogli Naghiyev (Institute of Mathematics and Mechanics, Baku, Azerbaijan)
DOI: 10.4018/IJCCE.2018070102
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The author presents an analytical solution to the problem of acoustic-wave-induced oscillations of methane gas bubbles in an oil reservoir at bottom-hole conditions. The amplitude-frequency characteristics are computed for realistic reservoir temperature, pressure, and dissolved gas parameters that make the oil compressibility, density, and viscosity quite different from those of the degassed oil at surface conditions. The author compares the damping decrement of these ultrasonic oscillations to those at normal atmospheric-pressure surface conditions. The solution allowed the author to estimate the ultrasonic vibration frequency range that is optimal and most effective for improving the flow characteristics in a producing reservoir in the near-well zone at a realistic bottom-hole regime.
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Ultrasonic excitation of petroleum fluids with the purpose of altering their properties has been efficiently employed in the industry (e.g., Khmelev et al., 2009; Magsumova et al., 2005; Khmelev et al., 2009).

However, the potentially high efficiency of ultrasonic cavitation treatment of non-Newtonian fluids has not been fully realized to date. This is due, in particular, to the lack of satisfactory and systematic theoretical and experimental studies that would take into account the viscosity of the liquid, the solubility of the gas, and the thermal effects in order to find specific resonance conditions for the propagation of ultrasonic waves in such media.

Establishing the relationship between these parameters and the behavior of the gas coming out of solution in oil would allow one to determine optimal regimes and conditions for the ultrasonic treatment of various non-Newtonian fluids. This may also help create specialized equipment capable of performing these functions with maximum efficiency (Golykh et al., 2010; Khmelev et al., 2011).

The effects of elastic vibrations on the processes in productive hydrocarbon layers has been discussed in, e.g., (Barskaya et al., 2012; Gadiev et al., 1981; Dyblenko, 2008; Dyblenko, Kamalov, Sharifullin, & Tufanov, 2000; Kuznetsov & Efimova, 1983; Sokolov & Simkin, 1981.

However, these studies do not fully discuss or suggest the technical means to study the influence of vibration or acoustic effects in-situ.

The unique feature of ultrasonic excitation of fluids is the formation of bubbles, which accumulate energy during their expansion and produce a large number of various physical effects during their collapse. These effects lead to changes in the structure and properties of the medium, increase the interfacial interaction surface, and also accelerate the processes of mass and heat transfer.

The phenomenon of gas release from reservoir fluids during elastic oscillations can, depending on specific conditions of the experiment, affect the wellbore zone and its filtration characteristics in various ways. Field applications of the method have shown a number of positive effects on oil production associated with the phenomenon of oil degassing.

An ultrasonic wave passing through a liquid creates compression and dilation zones. As a result, cavitation nuclei oscillate radially with small amplitudes.

With an increase in the amplitude of sound pressure above a certain critical value, cavitation bubbles reach critical sizes at which they become stable into “long-lived” (Khmelev et al., 2007).

For a large number of periods, such bubbles oscillate around their maximum sizes (more than 100–1000 μm).

Elastic waves, acting on the metastable zone of the reservoir, leading to the movement of previously stable parts of the fluid and, as a result, improve oil recovery. Outside this zone, an acoustic wave will still propagate without producing dramatic effects in the fluid.

The energy of shock waves created by a single bubble is uniquely related to the time dependence of the radius of the bubble. This dependence is determined by the modes of ultrasonic treatment, namely frequency and amplitude of wave-induced pressure and the physical properties of the medium, such as density, surface tension and rheological properties (viscosity).

Besides the damaging effects of cavitation shock waves during bubble collapse, the physical effect of diffusion of the dissolved gas into the cavitation opening upon expansion of the bubble is of no less practical importance.

In real processes of ultrasonic treatment of heterogeneous media with a liquid phase, the efficiency of the cavitation effect, determined by the total power of the shock waves of the cavitation bubbles, depends on the behavior of a single bubble.

In turn, oscillating bubbles make a significant contribution to the change in the macroscopic properties of a cavitating medium (density, viscosity, acoustic-wave velocity, energy absorption coefficient, wave resistance, and others). This, in turn, affects the intensity of ultrasonic vibration propagation in the treated fluid and the total volume of the area where cavitation occurs.

Hence, a comprehensive study of the formation of the cavitation region should include the study of the behavior of an individual bubble.

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