Mud Handling Equipment

The drilling mud is handled by a variety of equipment during (he normal drilling process. Each item must be evaluated to determine if it will meet the job requirements for the upcoming well. The handling equipment includes the following:

* pits

  • solids control equipment
  • gas control

* piping

  • chemical treatment systems
  • instrumentation

Many drilling operations are impaired from an efficiency standpoint due to mud handling problems.

Pits. The pit system and the arrangement of the mud handling equipment on the pits is a design consideration for the drilling engineer. This fact is particularly pertinent in land drilling operations if rental solids control equipment is used. Offshore rigs are usually limited with respect to space availability, and in many cases the pit system has a dedicated position for the special equipment.

Drilling operations during the early days of the industry used earthen pits to contain the mud. The fluid from the well was discharged in one end of a pit, and the inud pump suction line was located on the other end of the pit. The surface area was large enough so solids in the water muds could settle to the bottom of the pit before entering Ihc pump suction line.

Earthen pits were replaced by steel pits for several reasons. The steel pits were easier to move with the rig than the effort required to build new earthen pits. And heavier, more viscous muds required special solids control equipment that could be located on. and transported with, steel pits more effectively.

Most rigs have two or three steel pits. The general dimensions are 8-12 ft wide, 20^t0 ft long, and 6-12 ft tall. The volume may range from 200-600

bbl. Offshore operations may have 1-3 additional pits for storing excess or high-density mud.

A typical arrangement for a pit system, exclusive of various types of solids control equipment, is shown in Fig. 16^12. The discharged fluid from the well enters the pits and is processed through several types of solids removal equipment, which may include the following:

  • screen shale shaker
  • sand trap
  • desilting hydrocyclone
  • desanding hydrocyclone
  • centrifuge
  • mud scrubber (cleaner)

The fluid will eventually travel to the end of the last pit and enter the suction line.

Slugging pit

Slugging pit lines

The second pit is not used on all rigs lines

Each pit should have suction

  • L centrifugal pumps
  • and discharge 1 H lines for the
  • L centrifugal pumps

Primary solids control processing pit

Solids control equipment not shown for clarity purposes

Sand trap

Well

Sand trap

Fig. 16-42 Typical pit system

The first pit contains most of the solids control equipment. The second pit was used in the early days as a settling pit for mud solids. However, a few simple calculations indicate that most mud solids will not settle in the pit in view of the relatively short amount of time required for the mud to travel through the pit. Currently, the second pit contains some of the solids control equipment if the pit is even available. The last pit contains the mud suction lines, the slugging pit for preparing heavy slugs used prior to tripping, and the chemical treatment discharge lines.

Pits contain agitation systems to stir the mud and to minimize barite or solids settling. Two common agitators are !) submerged swirling fan blades driven by independent electric motors and 2) centrifugal pump-driven jets, or mud guns, that force high-velocity streams of mud into the pits (Fig. 16-43). Either system will function effectively if planned properly.

An important operating guideline is to ensure that the agitators do not cause air to be entrained in the mud. Aerated mud reduces the effectiveness of the centrifugal pumps used to drive the solids control equipment and reduces the volumetric output of the mud pumps.

Solids Control Equipment. Removal of undesirable solids from the drilling mud is receiving more interest and effort. Industry personnel have demonstrated that control over these solids can reduce drilling costs in the following manners:

  • increased drilling rates by enhancing cuttings removal
  • better bit hydraulics due to lower fluid viscosities
  • less wear on surface equipment such as the mud pumps

In addition, formation damage is reduced since the mud contains fewer drilled solids that invade and block the permeability llow channels {see Chapter 6).

Solids are removed from the mud system by settling, screening, centri-fuging, or dilution. Settling is ineffective in most cases due to weighted mud systems or lack of time for gravity segregation. Dilution is expensive with heavy mud systems. The principles employed for most commonly used solids removal equipment are screening, centrifuging, or a combination.

The solids control equipment must be designed to remove very small particles. Mud system particles arc 1—50 |i. in diameter. The most damaging particic with respect to solids control is less than I p,. (A micron is one-millionth of a meter.) Since commercially available solids removal equipment cannot control solids less than about 15 p. large solids must be removed prior to their regrinding and recirculation back downhole. Table 16-3 provides various items indicating the size of particles that must be removed.

Sand Traps. A sand trap is a small, compartmented section of the first pit immediately adjacent to the shale shaker. The effluent from the shale shaker flows into the sand trap. Its purpose is to allow settling of heavy particles such as sand or shale cuttings that escaped removal by the shaker screen. It is often

Sand Trap Shale Shaker
Fig. 16-43 Mud pit agitators (Courtesy Sweco Inc.)

Table 16-3 Solids Size Relationships

Classification Cuttings Sand Salt Clay

Particle Class ifica tion Coarse Intermediate Medium Fine

Ultra-fine Colloidal

Common Items for Size Comparison Cement dust (portland) Talcum powder Red blood corpuscles Fingertip sensitivity Human sight Human hair One inch

Particle Size (Diameter, ft) Larger than 500 74-500 2-74 Smaller than 2

Larger than 2,000 250-2.000 74-250 44-74

25-40 30-200 25,400

used effectively when sand screens develop tears 01 when high-viscosity muds must bypass the shaker for a short time, [t should not be considered as a primary solids control device in standard operations.

Shale Shakers. The most important member of the solids control equipment group is the shale shaker. It removes the major portion of all drilled solids circulated out of the well. In addition, it removes the large cuttings that would plug the other equipment.

Although many manufacturers produce shale shakers, the designs are reasonably similar {Fig. 16-44), Mud flows from the well flow line into the possum belly, or mud box. The possum belly has a lower bypass gate (not shown) so that mud can flow directly into the sand trap and pits in case the screens plug with viscous mud or solids. The mud and cuttings How over the scrcen(s). Depending on the openings in the screen mesh, the mud and small-diameter particles fall through the screen and exit at the discharge chute. The vibrating, or rotating, assembly on the screen causes the larger particles to move along to the end and off the screen (Fig. 16-45).

Various screen arrangements are available on common shakers (Fig. 16—4-5). Multiple-screen shakers are widely used and can offer better solids removal. The particle separation with a multiple-deck screen is determined by the finest mesh screen, which is usually the bottom screcn. The size separation with the parallel arrangement is determined by the coarsest screen size.

Screening surfaces used in solids-control equipment are generally made of woven wire screen cloth in many different sizes and shapes. Characteristics of

Mud box (possum belly)

Well Control Equipment

Basket assembly

Mud box (possum belly)

Basket assembly

Coarse solids discharge

Fig. 16-44 Rig shaker components (Courtesy Swcco Inc.)

the screen cloth that must be considered by the drilling engineer are mesh size as well as the shape and size of openings. Screen sizing will ultimately affect the mud and drilling costs.

Mesh is defined as the number of openings per linear inch. Mesh can be measured by starting at the center of one wire and counting the number of openings per inch (Fig. 16-46). Table 16-4 provides an indication of common screen sizes that are available commercially. Mesh sizes above 100X 100 are not used frequently since they may plug with heavy, viscous muds or high cuttings content.

Fig, 16-45 Fine screen shaker vibrator positions (Courtesy Sweco Inc.)

Sweco Screen Chart
Fig. 16—46 Eight-mcsh screen (Courtesy Sweco Inc.)
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Responses

  • Osman
    How many exits are required on a mud pit?
    5 years ago
  • Matta
    What is possum belly mud pit?
    2 years ago

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