Accessories

Drilled cuttings, other particles from the hole and sometimes gas, have to be removed from the mud system before it is suitable to be recirculated. Mud chemicals and water have to be regularly added to maintain the physical properties of the drilling fluid at desired levels. Shale shakers, the settling pits (or sand traps), desanders, desilters, centrifuges, degassers and mud cleaners remove unwanted solids and gas from the returning mud. The mixing hopper is used to add dry chemicals to the mud.

7.6.5.1 Shale Shaker

The shale shaker is the first mud cleaning equipment that encounters the returning mud. It contains one or more sloping and vibrating screens. The screens let the mud fall through but prevent large solids from going into the mud pits. The solids travel to the edge of the screen where they are “dumped”. Some of them are caught as geological samples.

There are two types of shale shakers:

• Circular/elliptical motion shale shakers • Linear motion shale shakers

The circular/elliptical shale shaker uses elliptical rollers to generate a circular rocking motion to provide better solids removal through the screens.

The linear motion shale shaker a forward and back rocking motion to keep the fluids circulating through the screens.

Figure 55 A linear motion shaker.

7.6.5.2 Settling Pit or Sand Trap

The settling pit or sand trap is located below the shakers. It is the first pit that receives the returning mud from the shakers. The pit has a sloped bottom so that particles that passed through the shaker screens will segregate out, by gravity, and settle towards cleanout valves. These valves are opened periodically in order to dump the solids that have accumulated.

7.6.5.3 Centrifugal Separation Devices

There are two types of centrifugal separation devices namely: Decanting centrifuges

•

• Hydrocyclones 7.6.5.3.1 Decanting Centrifuges

coarse particles to the discharge port and back to the active system. The conveyor rotates in the same direction as the outer bowl but at a slightly slower speed.

Figure 56 Cross-section of a decanting centrifuge. Arrows indicate the path of solids and liquids (from EXLOG). For low-density mud systems a single centrifuge unit set for total solids discard is used. The primary function of a centrifuge is not to control total percent solids in a system, but rather to maintain acceptable and desirable flow properties in that system. Two centrifuges operating in series are recommended for the following systems:

Invert emulsion (i.e., synthetic and oil-based systems) •

• • • •

High-density, water-based systems

Water-based systems in which base fluid is expensive (i.e., brines) Closed loop

Zero discharge

The first centrifuge unit is used to separate barite and return it to the mud system. The second unit processes the liquid overflow from the first unit, discarding all solids and returning the liquid portion to the mud system.

Note: Centrifuge efficiencies are influenced by mud weight and mud viscosity.

7.6.5.3.2 Hydrocyclones

Hydrocyclones, classified as desanders or desilters, are conical solids separation devices in which hydraulic energy is converted to centrifugal force in order to separate the solids from the rotating fluid. A pump is used to feed the drilling mud through a tangential opening into the feed chamber (the large end of the cone-shaped housing). The centrifugal forces thus developed multiply the settling velocity of the heavier- phase material, forcing it toward the wall of the cone. The lighter particles move inward and upward in a spiraling vortex to the overflow opening at the top. The discharge at the top is the overflow or effluent. It is returned to the mud system. The discharge at the bottom that contains the coarse solids (which are discarded) is the underflow. The underflow should be in a fine spray with a slight suction at its center.

Figure 57 Hydroclone cross section. Arrows indicate direction of flow (from EXLOG).

Even though hydrocyclones are effective in removing solids from a drilling fluid, their use is not recommended for fluids that contain significant amounts of weighting materials or muds that have expensive fluid phases. When hydrocyclones are used with these fluids, not only will undesirable drilled solids be removed, but also the weight material along with base fluid, which can become cost-prohibitive.

Hydroclones are often used on low-weight, water-based muds to remove coarse drilled solids.

7.6.5.3.2.1 Desanders

Desanders consist of a battery of 6-inch or larger cones. Even though desanders can process large volumes of mud per single cone, the minimum size particles that can be removed are in the range of 40 microns (with 6-inch cones).

Figure 58 A desander.

Figure 59 A desilter.

7.6.5.4 Mud Cleaner

The mud cleaner is a solids separation device that combines a desilter with a screen device. The mud cleaner removes solids using a two-stage process. First, the desilter processes the drilling fluid. Second, a high-energy, fine-mesh screen shaker processes the discharge from the desilter. This method of solids removal is recommended for muds containing significant amounts of weighting materials or having expensive fluid phases.

Note: When recovering weight material with a mud cleaner, be aware that any fine solids that go through the cleaner's screen are also retained in the mud. Over time, this process can lead to a fine-solids buildup.

Figure 60 Mud cleaners.

7.6.5.5 Degasser

The degasser is a device that removes the gas from the mud before it is recirculated down the hole. Gas-cut mud is mud containing gas. It should not be recirculated down the hole because it decreases the density of the mud, which could lead to a blowout.

There are two types of degassers:

• Mud-gas separators – for high-pressure gas and mudflows during kicks • Vacuum degassers – for separating entrained gas

7.6.5.5.1 Vacuum degasser

The vacuum degasser gets mud from one of the mud pits. As the mud enters the top of the degasser and streams down an inclined plane that extends the whole length of the feed pipe, a vacuum in the vapor space causes the gas to leave the mud. The liberated gas is removed from the tank by a vacuum pump. The degassed mud returns to the pits ready to be recirculated.

Figure 61 Vacuum degasser. 7.6.5.5.2 Mud-gas separator

(See the section on Mud-gas separator under Well Control Equipment.)