Research on Swirl Desanding Mechanism and Development of Swirl Desander

1 Introduction to the structure of the cyclone sand remover and the principle of sand removal

Working principle of cyclone sand remover

Figure 1: Working principle of cyclone sand remover

Cyclone sand remover is a kind of mechanical separation equipment that uses centrifugal sedimentation principle to separate phases with different densities in heterogeneous mixture. The basic structure of the cyclone sand remover is a separation cavity, an inlet and two outlets (see Figure 1). The separation cavity is mostly column-conical. The entrance is generally a tangential entrance. The outlets are mostly axial outlets, which are distributed at both ends of the cyclone sand remover. The overflow port is near the feed end, and the underflow port (sand discharge port) is far away from the feed end. The drilling fluid is injected into the cyclone sand remover from the tangential inlet through a mud pump, and rotates at high speed in the cyclone sand remover cavity, generating a centrifugal force field several thousand times the gravity field. Under the action of centrifugal force, the dense solid phase is thrown around, and moves downward along the wall, and is discharged from the underflow as an underflow; the drilling fluid with a low density is brought to the middle and moves upward, and finally overflows as overflow Outlet. In this way, the purpose of purifying drilling fluid is achieved by using a cyclone sand remover.

2 Internal mud flow pattern and secondary sand removal capacity

  It can be known from the principle of sand removal that the mud performs spiral movement in the cyclone cavity. Under the influence of the structure of the sand remover, in the downward movement process, the movement direction is gradually changed, and finally an upward spiral around the center line is formed. The internal rotation fluid is discharged from the overflow port.  Therefore, in the cone cavity of the sand remover, there are 2 reverse spiral flows (see Figure 2), one is an external swirl swirling downwardly near the cyclone wall; the other is located upward in the center of the cone Swirl inside swirl.  The two swirling currents flow in opposite directions, causing a cyclic flow (closed loop flow) at the junction.  The circulating flow entrains the mud flow near its outer side (near the inner wall of the cone) and pushes them into the inner swirl in the form of a cover flow above its inner side.  Part of the mud entrained in the inner swirling flow exits the overflow port with the inner swirling fluid, and the other part enters the outer swirling flow through the circulation flow under the cover flow.  In fact, the circulating flow acts as an intermediary exchange field between the outer and inner swirl.  In this interaction process, the inner swirling flow continuously throws finer sand particles into the lower cover slurry flow in the swirling rise, and continues to complete the sand removal function until it enters the slurry outlet and is discharged, so the inner swirling flow Play the role of secondary sand removal. 
Movement of solid phase in sand remover

Figure 2: Movement of the solid phase in the sand remover

3 Disadvantages of traditional cyclone sanders

3.1 Vortex-shaped air column of swirling sand remover

A vortex-shaped gas column usually appears near the axis of the swirling sand remover (Figure 2), which is caused by the pressure drop near the axis of the swirling sand remover and the air flows in from the underflow port. The diameter of the air column is approximately 0.6 times the overflow port. Due to the appearance of the air column, a liquid phase region and a gas phase region appeared in the cavity of the swirling sand remover. In the entire liquid phase region, the tangential velocity of the fluid is a quasi-free vortex. After entering the gas phase region, the gas motion is a forced vortex, as shown in Figure 3 (a). Axial fluid is downward near the wall of the device, turning at a certain radius and continuing up to the axis. Zero velocity points appear at the place where the fluid direction changes. These zero velocity points form the axial zero velocity envelope surface. See Figure 3 (b). As for the radial velocity, in the liquid region, it gradually increases from the wall to the inside, and then decreases to zero near the gas-liquid boundary. In the gas phase region, there is no radial flow, as shown in Figure 3 ( c).
From the above analysis, it can be seen that, under ideal conditions, the air column is at the center position of the internal swirling fluid, and there is no radial motion in itself, and its motion is only reflected by the spin-up motion. If the solid phase in the mesofluid enters the gas phase zone under the influence of the radial velocity of the drilling fluid, then it is impossible to enter the circulating flow for secondary purification. However, due to the uneven flow velocity at the inlet, the gas column and the internal swirling fluid will generate synchronous lateral oscillations. In actual work, the air column will produce a large drift, and its axis is not on the axis of the cone (see Figure 2), but in a cluster of curved curved beams. With the drift of the mesogenetic fluid, the circulating flow also drifts. At this time, under the action of the external swirling flow of the mud stream entering the sand remover, the solid phase content of the mud stream is unevenly distributed in the conical cavity. The solid phase content of the horizontal mud flow is also different. From the center to the inner wall of the cone, the solid content of the large particles in the mud flow is getting higher and higher. As the circulating flow moves towards the inner wall of the cone, the solid phase content of its mud flow is getting higher and higher, causing more rock powder particles to be discharged from the overflow port as the circulating flow is drawn into the inner swirling flow.
As the circulating flow moves towards the inner wall of the cone, the farther the outer swirl particles near the outside of the cone are from the axis of the outer swirl, the smaller the centrifugal force is, which makes it easier for the circulating flow to capture large rock powder particles and send them Into the swirl.

Velocity distribution of cyclone sand remover

Figure 3: Velocity distribution of a swirling sand remover

3.2 Cyclone sand remover blocked

Generally, solid-phase impurities contained in drilling fluid have a certain adhesion ability, which may be determined by the nature of impurities such as cuttings, or it may be caused by the clay components and additives in the drilling fluid. This sticking characteristic brings great uncertainty to the operation of the rotary flow sand remover.
Adhesive cuttings can adhere to the inner wall of the cavity near the underflow opening, thereby reducing the over-flow section of the underflow opening, and may eventually cause the underflow opening to be blocked.
The solid phase impurities and the strong flocculation characteristics of some drilling fluids may cause the volume of the impurities to expand excessively, so as to approach or exceed the size of the underflow cross-section, and block the underflow.

4 Structural design of cyclone sand cleaner

Simplified structure of cyclone sand cleaner

Figure 4: Schematic diagram of the swirling sand cleaner

From the above, it can be known that the internal rotating fluid formed on the central axis part of the sand remover during the lateral swing and the centrifugal force in the central area of the internal rotating fluid is very small, which are the main reasons for the large-scale debris to enter the overflow pipe. In order to enhance the sand removal effect of the sand remover, in addition to optimizing the structure parameters of the sand remover, the air column in the sand remover must be kept stable and the sand removal capacity of the internal swirl in the sand remover should be enhanced. In order to keep the air column stable and reduce the intensity of turbulent disturbances in the upwelling region, a round rod with a diameter equal to the diameter of the air column can be installed at the central axis of the sand remover (see Figure 4). The upper end of the rod is fixed to the joint of the sand remover, and the lower end is conically placed at the sand discharge opening, and a certain size of annular gap is formed with its inner wall. Because the diameter of the rod is equal to the diameter of the air column, the rotating fluid flows against the surface of the rod, causing the rod to generate a small lateral oscillation. Such small amplitude lateral vibration can reduce the turbulence intensity of the ascending fluid, thereby improving the effect of separating rock dust from the mud. In addition, because the "central area" of the internal swirl is occupied by the round rod, the particles cannot be thrown into the air column by the circulating flow, and only the swirling movement can be performed on the periphery, and some rock dust particles are swirled by the centrifugal force in the swirling Bottom the outflow to the inner wall of the sand remover. Therefore, the installation of round rods is conducive to internal swirl sand removal.
In order to make the sand remover have the ability to automatically flush the sand discharge port, a double-sided valve can be set at the middle of the rod and a piston is set at the upper end. When the over-flow area of the sand discharge port is reduced, it will cause the pressure in the lower cavity of the piston to rise, causing the rod with the double cone valve to start to move up, and increasing the gap between the sand discharge port and the core rod. At the same time, the cross-sectional area of the cavity is reduced. At this time, the flow of the mud discharged from the cavity of the sand remover is reduced, and the flow out of the annular gap between the sand outlet and the rod is increased, so that the sand outlet is flushed, and when the sand outlet has a normal operation of the sand remover The purpose of the required overcurrent section.

5 Application effects of cyclone sand cleaner

The experiment was carried out using a clay slurry with a viscosity of 1 m3 (horse funnel) greater than 30 s and a sand content greater than 4%. The results are as follows:
(1) After the cyclone sand removal and cleaning, the actual sand content of the mud decreased from 4% to 0.2%. More than 90% of the coarse particles with a particle size> 1.0 mm are discharged, and the total sand removal effect is 91%, which is significantly higher than that of the traditional sand remover.
(2) Under the premise of ensuring the purification effect, the mud loss is reduced to less than 20%, and the anti-blocking function under continuous sand removal conditions is realized.

6 Conclusion

(1) The cyclone sand remover is based on the theory of cyclone sand remover technology.It improves the structure and optimizes the design on the basis of the traditional cyclone sand remover. It makes up for the poor solid phase component separation effect, and the sand discharge port is easy. Inadequate clogging.
(2) Practice has proven that the cyclone sand cleaner has a good sand removal effect, with a total sand removal efficiency of 91%, which is significantly higher than that of a traditional sand remover, and it can automatically remove plugs.
(3) As the sand removal effect is good, this improved cyclone sand remover can not only be used for core drilling, but also several cyclone sand removers can be connected in parallel for oil and gas drilling to reduce the cost of mud purification.

Keywords: cyclone sand cleaner; cyclone sand cleaner; mud; sand removal efficiency