Addition Region

The addition region of the mud system is downstream of the last solids removal device. It is the tank equipped with the mixing hopper, chemical barrel, mud guns, mixers-agitators, and perhaps special mixing/ shearing devices.

  1. Mixing Hopper - Mixing hoppers are used to add dry materials to build and maintain a mud system. Where large quantities of dry materials are used, bulk hoppers normally gravity-feed dry materials into a rig mud hopper. Hopper mixing effectiveness depends upon design. Venturi-discharge hoppers have a venturi on the discharge side of the nozzle which provides the shearing action needed to mix dry materials (see Figure7).
  2. Chemical Barrel - Chemical barrels are used primarily to solubilize materials in water before adding them to the active mud system. Normally, this is a make-shift barrel with a valve installed to regulate flow. However, well designed and properly constructed, chemical barrels should be avail-
  3. 6/94 5-32

6x3 Swage

Figure 7 Venturi Mixing Hopper

2" Jet Nozzle

Sack Cutter

Figure 7 Venturi Mixing Hopper

Sack Cutter

6" Connection

1-1/4" Jet Nozzle To

Venturi Gap For Maximum Efficiency

Courtesy ot TRW Mission

6" Connection

1-1/4" Jet Nozzle To

Venturi Gap For Maximum Efficiency

Courtesy ot TRW Mission able on each rig for safety reasons. Causticizing materials are often mixed through a chemical barrel.

3. Mud Guns - Mud guns are useful to keep mud mixed and sheared; but, they can significantly lower the overall efficiency of solids control equipment if plumbed incorrectly. If guns must be used in the removal region, each mud gun's suction should be taken from the same tank that it discharges into. Otherwise, it is recommended that mud guns not be used in the removal region and agitators used instead.

Mud guns are of two types:

a. high-pressure (3000 or 6000 psi) and operate from a rig pump, or b. low-pressure, operated from a centrifugal pump (see Figure 8).

Mud guns can be located either on the surface (surface mud guns) or beneath the surface (subsurface mud guns). Surface mud guns are used to break up materials that float, such as lost circulation materials; but, they are prone to entrap air in the mud. Subsurface mud guns are useful to keep barite from settling.

  1. Mixers-Agitators - Mixers-agitators should be properly sized, powered and positioned to prevent settling in corners of mud pits. They are generally recommended over mud guns for mixing. There are two basic impeller designs for these mixers-agitators. Radial flow impellers have vertical blades and are positioned close to the tank bottom. Axial flow impellers have canted blades, like propellers, and are positioned at 1/3 to 3/4 of the impeller diameter distance off bottom (see Figure 9).
  2. Mixing/Shearing Devices - There are three types of mixing/shearing devices: (1) close-tolerance rotor/ stator, (2) high-pressure choke, and (3) counter-rotating shear (see Figure 10). Each of these devices produces a different degree of mixing and/or shearing.
  1. Close-Tolerance Rotor/Stator Device - such as the Seco-Colloid Mill. This device produces high shear when fluid passes through a centrifugal pump that has been modified. Fluid is forced through a stationary perforated ring which is fitted over the impeller blades in the pump
  2. 6/94 5-34
Impeller Design Calculations
housing. This design creates a high energy environment which is especially applicable to materials requiring high, prolonged shear to obtain suitable rheological properties.
  1. High-Pressure Choke Device - Shear Master from Petroleum Coordinators, produces mixing/shearing as fluid is pumped by a positive displacement pump through a restricted opening such as a choke or nozzle. As fluid accelerates through the restricted opening, both shear and water wetting of materials occurs.
  2. Counter-Rotating Shear - Jet Shear from Flo-Trend, forces a split fluid stream to enter a mixing chamber by passing through jets in a disc on each end of the mixing chamber. The configuration of the jets on each disc produces a counter-rotational flow which hydraulically water wets materials. Water wetting is essential for proper dispersion when mixing polymers such as PAC, PHPA, etc. The counter-rotational shear device does not produce a high shear energy environment; therefore, it does not cause shear degradation of long chain, high molecular weight polymers.

Mixing/shearing devices greatly improve rigsite mixing. For hard-to-mix materials such as Aqualon HP-007, sepiolite and attapulgite clays, which require extremely strong and/or long-term shearing to viscosify, high-shear devices such as the Polygator or Shear Master should be used. Oil-based muds require shear to form a stable emulsion. They are normally mixed at the supplier's mud-mixing plant; however, any of the three mixing/shearing devices could be used to mix oil-based muds on location and thereby reduce the potential for a transporting accident. Although mixing/shearing devices greatly improve rigsite mixing capabilities, these devices should never be used in the active mud system because of their ability to break up drill solids.

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