Oil Cavitation Treatment to Prevent Formation of Paraffin Deposits

 The problem of paraffin deposits in pipelines exists yet for a long time. Many techniques have been developed to eliminate paraffin accumulation; they may be divided into two groups: certain techniques are used directly to clean pipe walls from the deposits while other ones are intended to prevent their formation. The last attitude is more efficient both from engineering and environmental viewpoints.

   Recently designed, a hydrodynamic vortex cavitating device is attributed to the second group. Its intended use is to create the uniform fine gas-liquid emulsion (natural gas-oil). As a result of device operation, the flow of the uniform gas-oil mixture is created, which results in less hydraulic resistance during its transportation in a pipe as well as in paraffin deposit prevention. Two versions of the device are designed up to date: submerged and land-based one.

Inside a Well and on the Surface

   The basic objective of a submerged version is to prevent the formation of asphaltene-paraffin deposits on the walls of tubing strings (TS) in wells. This type of the device is called hydrodynamic ultrasound deparaffinization device (HUD). Fig. 1 illustrates HUD position [2] after its installation in a well. In the given example, the device is located directly above a pump [1] that transports oil to the surface. Treated oil comes to surface [4] via tubing string [3] and then is pumped overland into storage tanks [5].

   Potential energy of a fluid column (gas-oil mixture), which is located in a well over the device, is used at such HUD position. Higher pressure is exerted in such a way promoting, in turn, emission of higher energy during cavitation in HUD.

Two main problems may be solved using a HUD installed in a well at a tubing string:

   First of all, operating costs are decreased for well maintenance; and Secondly, energy consumption is optimized while raw brining to the surface.

Land-based version of the device is used in overland pipelines for pumping promotion (as the main task) and deposit formation. This device is mounted on the bypass to the main pipeline. During the device putting into operation, the dampers on both sides of the device are opened while closing the damper located in a main pipeline between the bypass inlet and outlet.

HUD specifications:

Capacity range:     5-500 cu. m/day; Maximum pressure at the device mounting level:     100 mPa (1000 kgf/cm²); Maximum pressure drop along the device:     up to 1 mPa (10 kgf/cm²); Kinematical viscosity of the product being treated:     -1 – 100 cSt.

How does it work?

   Fig. 3 illustrates the operating principle of the simplest vortex pressure wave former (hereinafter acoustic generator or oscillator). 

During fluid supply via tangential hole [2], the system containing two swirling flows is formed in vortex chamber [3] and outlet nozzle [4]. A so-called primary whirl moves along the chamber circumference area (I), which is ring-shaped with outer radius R=D/2 and internal one rm. This flow is a working fluid that is supplied into a generator. Secondary whirl (II) occupies the near-axial area of the chamber and rotates as a quasi-solid body. Primary whirl involves into motion the liquid from surrounding medium, in which the liquid flows from the generator, thus forming the secondary whirl. Basic Physical Parameters

   The experiments performed have shown that a flow motion remains stable in case of free-discharging liquid jet (for example, during its discharge into gaseous environment); and there is no pressure and speed fluctuation in a flow. If the discharge of a swirled jet is immersed (i.e. operating fluid in a vortex chamber and surrounding matter is in the same – liquid – state), regular pressure fluctuations are generated in a flow at certain dimensions of a vortex chamber as well as of an inlet and outlet nozzle. Their frequency and magnitude depend upon the medium flow rate, dimensions of a vortex generator, input and output pressure. Fig. 4 shows acoustic oscillations in audible and ultrasound parts of the spectrum with considerable intensity caused by pressure fluctuation and observed in surrounding medium.

   In case of an immersed discharge, the secondary flow deviates from the vortex chamber axis and makes circular precession motion around it. In this case, displacement amplitude ε of the secondary whirl may achieve its maximum at a certain dimension ratio. Deviation of the secondary flow causes deformation of the primary one at their interface.

   Self-oscillations in the swirled fluid flow are caused by speed and pressure oscillations in the primary whirl, which are induced by periodical deformation of its borders by the secondary whirl, which makes precession rotational motions relatively the chamber axis. Development of the secondary whirl precessions becomes possible only in the case, when the rotational component distribution in it comes near the rigid body rotation law u/r=const, where u is the tangential component of the secondary whirl speed; r is the whirl current radius. In this case, the amount of rotational energy transmitted from the primary whirl to the secondary one, becomes so significant that its part is transformed into the lateral oscillation energy.

   Mathematical simulation of a vortex flow is sufficiently complicates task; it is performed by continuity and Navier-Stokes equations using numeric procedures.

The study of a vortex emission has shown that there is the minimum length of a vortex chamber LMIN , within which the secondary whirl has n time to take rotational movement across the whole cross section. Thus, precession of the secondary whirl does not appear and, as a result, there is no sound emission. It follows from here that oscillations appear only at L>LMIN. Oscillation strength increases with the chamber length increase achieving maximum value at certain length L=LOPT and then decreases. This optimal length of a vortex chamber, from the viewpoint of intensity maximizing for acoustic oscillations, is defined by the non-dimensional parameter that depends on the basic dimensions: A=D•(D−d)/(n•d2), where n is the number of inlets. This variable is called a swirling degree. Meanwhile, LOPT directly depends upon the swirling degree of a flow.

   Relative diameter of the vortex chamber output nozzle is the important factor that defines acoustic irradiation frequency as well as acoustic output and generator efficiency: Dc*=Dn/D, where D is the chamber diameter; Dn is the nozzle diameter. Nozzle output diameter may not be the same as the vortex chamber diameter as it is shown in Fig, 1 since the outlet (nozzle) of the vortex chamber may both taper and expand at the output into a flooded space. Dn* in this case is the optimal diameter value.

   The method was developed for several parallel emitters intended to increase the magnitude of precession oscillations and to strength acoustic irradiation at the expense of hydrodynamic interaction of the flows, which come out of the acoustic generator vortex chambers and are swirled in opposite directions.

It is recommended to install several vortex generators at the flow rate of the treated fluid more than 10 cu. m/hour in order to split equally the total flow between the oscillators installed in parallel. Fig. 5 shows in cross section the layout of eight vortex oscillators, which are located symmetrically upon circumference and create the flows swirled in opposite directions (relatively the neighbor turbulizer). In such case, oscillators are mounted inside the single body of an acoustic generator. Thus, acoustic irradiation output will be mutually increased due to the interaction of the very same flows swirled in opposite directions.

Oil Cavitation Treatment

   During emitter operation, cavitation phenomena are observed together with acoustic wave generation. Sharp pressure drop in fluid resulting in cavitation may be caused only by hydrodynamic effects (for example, owing to Bernoulli’s law – hydrodynamic cavitation). For this cavitation type steam-gas bubbles may be big (up to several centimeters) while during acoustic cavitation their size is small enough – 10-3 ÷ 10-2 cm.

   These bubbles are quite unstable. Depending upon the pressure drop between a bubble and fluid, they grow in size, pulse and collapse. In this case, the pressure in the bubble center sharply increases; as a result, a spherical pressure wave generates and expands in a fluid from the center of the collapsed bubble. Besides, a bubble collapse also results in the sharp temperature increase in it. The pressure and the temperature appeared during this process may achieve several hundreds MPa and several thousand degrees correspondingly.