Solids control and mud circulation and cleaning are the key operations for these systems.
by Kirk VonRuff, O'Drill MCM, Inc.
July 31, 2012

Many crucial operations occur daily on every rotary drill rig. One of these vital roles involves the mud system that acts as the circulatory system of the rig. The drilling mud provides lubrication and cooling to the drill bit. The fluid then carries the drill cuttings back to the surfaces. At the surface, the mud is cleaned and returned down the drill string in a constant, circulating flow during the drilling. This process is referred to as the solid control system.

Solid Control System Components

There are several parts of the solid control system that includes equipment, such as:

  •  Centrifugal pumps
  •  Shale shakers
  •  Desilters
  •  Desanders
  •  Centrifuges
  •  Degassers
  •  Mud agitators
  •  Mud guns
  •  Mud hoppers
  •  Mud tanks

The mud engineer also plays an immensely important role in maintaining a constant mud weight during the drilling process. This is required because any significant fluctuation in mud weight can make the drill string unstable. This can cause the drill string to buckle or be extracted from the hole, causing a major downhole fishing incident or a rig blowout.

Mud Weight

In the drilling process, mud weights tend to vary because of many factors—such as the holes’ size, depth and geology. Therefore, mud weight requirements can run between 10 pounds per gallon on smaller rigs to more than 20 pounds per gallon on large rigs. Since water weighs 8.34 pounds per gallon, the weight of the fluid per gallon is divided by the weight of water to get the specific gravity. This measurement is generally used in mud weight calculations.

Example: 17-pound-per-gallon mud has a specific gravity of 2.03 (17 divided by 8.34).

Centrifugal Pumps

Most major drilling rigs use more than a dozen centrifugal pumps to operate water and mud systems throughout the rig. In the solid control applications, centrifugal pumps are used to operate the mud mixing, desilter, desander and degasser units. In these operations  5 x 6 x 14 or 6 x 8 x 14 pumps are used to handle the flow and head requirements in each station.

Example: A required flow rate of 1,000 gallons per minute with a head requirement of 100 ft. at a 1.9 specific gravity (16 pounds per gallon of mud) would use a 6 x 8 x 14 pump with an 11-inch impeller and a 75-horsepower, 1,800-rpm motor.

Mud Cleaning Equipment

As the mud and cuttings are brought to the surface, the mud cleaning process begins.

Mud cleaner consisting of a desilter, desander and shaker combined for saving space. Photo courtesy of RCI, Inc.

Stage 1

Shale shakers are the first station in the solid control system. Shakers can be elliptical, linear motion or a hybrid. Some linear motion shakers use two vibrator motors to create up to 7.7 gravitational force (G)in vibration power.

As the mud is pumped over the screens, the larger solids vibrate off, and the mud flows through the screens.

The flow rate across the shaker is determined by the mesh size of the screen and the weight of the mud used.

The partially cleaned mud is then pumped to the next station in the mud cleaning process.

Example: 13-pound-per-gallon or 1.55-specific-gravity mud flowing over an
84-mesh screen will flow at about 500 gallons per minute.

Stage 2

Desilters, desanders and degassers are used in the next stage of mud cleaning. The desilters and desanders use hydro cyclones to remove sand and silt from the mud. Desilter cones are 4 inches and 5 inches. The number and size used depends on the flow rate required in application.
Desander cones are typically 10 inches and are configured in one or two cone designs for most applications, again based on the flow rate needed. As the mud is pumped into the cones, at a pressure between 35 to 45 psi, a centrifugal effect inside the cones forces the solids out of the fluid. This removes solids to the micron level. The remaining solids drain out of the apex, and the cleaned fluid is moved through the process.

  •  4–inch cones can flow up to 50 gallons per minute
  •  5-inch cones can flow up to 80 gallons per minute
  •  10-inch cones can flow to 500 gallons per minute

Example: For a required flow rate of 1,000 gallons per minute, a 10–inch, two-cone desander and a 4–inch, 20-cone desilter will be used.

The degasser is used to remove any gas that is entrained in the mud. Because gas is much lighter, it can cause fluctuations in the mud weight. The gas is also flammable and must be pumped to a storage tank or flared off to minimize the danger of poisonous gases and explosion.

After these processes, the mud is essentially cleaned, and is returned to the mud tanks. As the fluid flows back into the mud tank, the mud engineer will take samples to determine the weight of the fluid. He will then either add water or bentonite through a mud hopper to attain the correct mud weight needed in application.

Mud hoppers are available in 6-inch and 4-inch sizes. The mud hopper has a choke nipple and a venturi tube that cause a turbulent motion so the bentonite can be stirred into the mud mixture. The flow rate is determined by the bore of the choke nipple and the size of the venturi tube.

Example: A 6–inch mud hopper with a 2–inch bore on the choke nipple will flow about 650 gallons per minute.

Mud Agitators and Guns

Other vital equipment used in solids control is mud guns and mud agitators. Both pieces of equipment are used inside the mud tank. The mud agitator is mounted on top of the tank and stirs the mud much like a blender. The typical gear ratio is 30:1. A 1,800-rpm motor geared to 30:1 will output 60 rpm. The size of the motor and impeller diameter is determined by the capacity of the mud tank.

Mud tank showing mud agitators near and desilter, desander and shaker at the far end. Photo courtesy of RCI, Inc.

The shaft length is determined by the depth of the mud tank. The number needed is based on the mud tank’s capacity.

Mud guns are also mounted at the top of the mud tank and have a handle and swivel so the guns’ flow can be adjusted.

They are used to stir the bottom of the tank. They have nozzles at the end of the guns that move the mud at an accelerated rate at the bottom of the mud tank.

The size of the bore in the nozzle determines the flow rate for each mud gun. The required flow rate will determine the number needed in application.

Mud agitators and mud guns are important because they keep the mud moving in the mud tank so that the mud does not separate or fisheye, which would affect the flow characteristics and mud weight consistency.

Example: At about 100 feet of head, a 1–inch nozzle will flow 200 gallons per minute. For a 1,000–gallon-per-minute flow rate, five guns will be needed.

Conclusion

Solids control is an important aspect of drilling. Maintaining a constant mud weight is paramount in the performance of a drilling rig and for the safety of the crew during this process.