Solids control equipment should be arranged so that each piece removes successively finer solids as demonstrated in Figure 7-2. Although all the equipment in the following list may not be needed, the most common arrangements are:
Returns from Well
Treated Fluid to Well
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Desilter or )esander Mud Cleaner eritrîfuge
To Trip Tank
Desilter or )esander Mud Cleaner
To Trip Tank
Gumbo is formed in the annulus from the adherence of sticky particles to each other. It is usually a wet, sticky mass of clay, but finely ground limestone can also act as gumbo. Enough gumbo can arrive at the surface to lift a rotary bushing from the rotary table. This sticky mass is difficult to screen and, in areas where gumbo is prevalent, it is sometimes removed before it reaches shale shakers.
Most gumbo removal devices are fabricated at the rig site, have many different shapes, and are usually slides. Because gumbo does not stick to stainless steel, one effective device is a series of f- to 1-inch diameter stainless steel rods arranged to slope downward from the end of the flow line. The rods are separated by one to two inches and are about four to six feet long. Gumbo leaving the flow line slides down the rods and is sent to disposal waste. Drilling fluid easily passes through the bars and is sent to the scalping or the main shakers.
Devices and machines designed specifically to remove gumbo are available from several manufacturers. One of these machines uses a series of steel bars formed into an endless belt. The bars are separated by a space of 1-2 inches and are disposed perpendicular to the fluid flow. The unit moves gumbo to the discharge end of the machine. Another machine uses a 5 or 10 mesh synthetic belt run at an uphill pitch to convey gumbo from a pool of drilling fluid. A counter rotating brush is used to clean gumbo from the underside of the belt.
Some shale shakers are also used as gumbo removal devices. Shakers are now available that combine gumbo removal (for example, a scalping shaker) and a main shaker all on one skid.
After the drilling fluid passes through the main shaker, it enters the mud pit system. When 80-mesh screens and coarser were routinely used, the sand trap performed a very useful function. Large, sand-size particles would settle and could be dumped overboard.
The bottom of a sand trap should be sloped at about 45° to facilitate quick dumping. The trap should not be agitated and should overflow into the next compartment. Linear and balanced elliptical motion shale shakers have all but eliminated this technique. Small drilled solids generally do not have sufficient residence time to settle. When inexpensive drilling fluid was used, sand traps were dumped once or twice per hour. Today, with the use of fine-mesh screens and expensive waste disposal, such dumping is cost prohibitive.
Drilling fluids usually encounter hydrocarbon gasses. At the bottom of a borehole, these gasses may partially dissolve in the drilling fluid or compress to occupy very small volumes. As the fluid rises to the surface, the hydrostatic pressure is reduced and these gasses expand and evolve from the drilling fluid. They must be removed from the surface mud system or pump operations will become erratic.
Shale shakers are not effective mechanisms to separate gas bubbles from a viscous drilling fluid. Degassers should be installed immediately downstream of the shale shakers, and gas should be removed before drilling fluid enters centrifugal pumps. Hydrocyclone performance requires a continuous fluid volume and head generated by the centrifugal pumps. A gaseous drilling fluid reduces centrifugal pump performance and may even "vapor-lock" the pump so that it prevents the movement of fluid. Even positive displacement rig pumps are affected by gaseous drilling fluid.
Two types of degassers are available: atmospheric and vacuum-type. Atmospheric degassers have a submerged centrifugal pump integral with the unit. It is placed into a spray chamber through a disc valve where it strikes the inside wall of the chamber. The thin spray, combined with the impact of the fluid on the wall of the chamber, separates the gas from the fluid.
Vacuum-type degassers separate gas from drilling fluids by spreading the gas-cut fluid into thin layers in a reduced atmosphere. The fluid usually flows over a series of baffles, or plates. Degassed drilling fluid is pumped through an eductor to remove drilling fluid from the vacuum chamber.
Equalization between degasser suction and discharge compartments is through a high weir at the top of the tanks. Degasser suctions should be located at the bottom of the compartments.
Hydrocyclone is a general term used to describe a device where liquid swirls inside of a cone. The centrifugal force of the swirling liquid moves the solids to the outside wall. In drilling operations, hydrocyclones use these centrifugal forces to separate solids in the 15- to 80-micron range from the drilling fluid. This solids-laden fluid is discharged from Lhe lower apex of the cone, and the cleaned drilling fluid is discharged from the overflow discharge.
Hydrocyclones consist of an upper cylindrical section fitted with a tangential feed section, and a lower conical section that is open at its lower apex allowing for solids discharge (Figure 7-3). The closed, upper cylindrical section has a downward protruding vortex finder pipe extending below the tangential feed location.
Fluid from a centrifugal pump enters the hydrocyclone tangentially, at high velocity, through a feed nozzle on the side of the top cylinder. As drilling fluid enters the hydrocyclone, centrifugal force on the swirling slurry accelerates the solids to the cone wall.
The drilling fluid, a mixture of liquid and solids, rotates rapidly while spiraling downward toward the apex. The higher-mass solids move toward the cone wall. Movement progresses to the apex opening at the cone bottom. At the apex opening, the solids along the cone wall, together with a small amount of fluid, exit the cone. The discharge is restricted by the size of the apex. Fluid and smaller-mass particles, which have been concentrated away from the cone wall, are forced to reverse flow direction into an upward spiraling path at the center of the cone to exit through the vortex finder,
The vortex finder is a hollow tube that extends into the center of the cone. It diverts drilling fluid from (lowing directly to the overlow outlet, causing the drilling fluid to move downward and into the cone. The swirling liquid is forced inward and, still rotating in the same direction, reverses the downward flow and moves upward toward the center of the vortex finder. In a balanced cone, the inner cylinder of swirling fluid surrounds a cylinder of air that is pulled in through the cone apex. Solids and a small amount of liquid spray out the lower apex of the cylinder. The apex opening relative to the diameter of the vortex finder will determine the dryness of the discharged solids.
Most balanced cones are designed to provide maximum separation efficiency when the inlet head is 75 feet, Fluid will always have the same velocity within the cone if the same head is delivered to the hydrocyclone inlet. Pressure can be converted to feet of head with the equation frequently used in well-control calculations:
Pressure, in psi Head, in feet = 0,052 x Mud Weight, in ppg
The relationship between manifold guage pressure and drilling fluid weight at a constant 75 feet feed head is summarized in Table 7-2.
FIGURE 7*5. Hydrocyclone
TABLE 7-2. Pressure for 75 feet of Head for various Mud Weights
Pressure Feed Head Mud Weight
39 75 10.0
41 75 10.5
43 75 11.0
45 75 11.5
47 75 12.0
49 75 12.5
51 75 13.0
Hydrocylones separate solids according to mass, a function of both density and particle size. However, in unweighted drilling fluids, the solids density has a comparatively narrow range and size has the greatest influence on their settling. Centrifugal forces act on the suspended solids particles, so those with the largest mass (or largest size) are the first to move outward toward the wall of the hydrocyclone. Consequently, large solids with a small amount of liquid concentrate at the cone wall, and smaller particles and the majority of liquid concentrate in the inner portion.
Larger-size (higher-mass) particles, upon reaching the conical section, are exposed to the greatest centrifugal force and remain in their downward spiral path. The solids sliding down the wall of the cone, along with the bound liquid, exit through the apex orifice. This creates the underflow of the hydrocyclone.
Smaller particles are concentrated in the middle of the cone with most of the drilling fluid. As the cone narrows, the downward spiraling path of the innermost layers is restricted by the reduced cross-sectional area. A second, upward vortex forms within the hydrocylone and the center fluid layers with smaller solids particles turn toward the overflow. At the point of maximum shear, the shear stress within a 4-inch desilter is on the order and magnitude of 1,000 reciprocal seconds.
The upward moving vortex creates a low-pressure zone in the center of the hydrocyclone. In a balanced cone, air will enter the lower apex in counterflow to the solids and liquid discharged from the hydrocylone. In an unbalanced cone, a rope discharge will emerge from the cone, resulting in excessive quantities of liquid and a wide range of solids in the discard.
There are two countercurrent spiraling streams in a hydrocyclone; one spiraling downward along the cone surface, and the second spiraling upward along the cone center axis. The countercurrent directions, together with turbulent eddy currents, concomitant with extremely high velocities, result in an inefficient separation of particles. The two streams tend to co-mingle within the contact regions and particles are incorporated into the wrong streams. Hydrocyclones, therefore, do not make a sharp separation of solid sizes.
Hydrocyclone sizes are designed arbitrarily by the inside cone diameter at the inlet. By convention, desanders have a cone diameter of 6 inches and larger; desilters have internal diameters smaller than 6 inches. Normally, discharges from the apex of these cones are discarded when used on unweighted drilling fluids. Prolonged use of these cones on a weighted drilling fluid results in a significant mud weight reduction caused by the discard of weighting material. When these cones are used as part of a mud cleaner configuration, the cone underflow is presented to a shaker screen. The shaker screen returns most of the barite and liquid to the drilling fluid system, rejecting solids larger than the screen mesh. This is a common application of unbalanced cones since the cut point is determined by the shaker screen and not the cone.
Since most hydrocyclones are designed to operate with 75 feet of head at the input manifold, the flow rate through the cones is constant and predictable from the diameter of the cone (Table 7-3). Obviously, manufacturers may select different orifice sizes at the inlet of the cone. The orifice size determines the flow rate through the cone at 75 feet of head.
The D50 cut point of a solids separation device is usually defined as the particle size at which one-half of the weight of those particles go to the underflow and one-half of the weight of those particles go to the overflow. The cut point is related to the inside diameter of the hydrocyclone. For example, a 12-inch cone is capable of a Ds0 cut point around 60 to 80 microns, a 6-inch cone is capable of around 40 to 60 microns, and a 4-inch cone is capable of around 20 to 40 microns. These cut points are representative for a fluid that contains a low solids content. The cut point will vary according to the size and quantity of solids in the feed and the flow properties of the fluid. Cut point determination procedures are explained in Chapter 9.
When hydrocyclones are mounted above the liquid level in the mud tanks, a siphon breaker should be installed in the overflow manifold from the cones.
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