Solids Control Equipment for an Un-weighted Drilling Fluid System

The un-weighted muds are the ones that do not contain any weighting materials such as barite or lead sulfide. This is the type of mud that is usually used for drilling shallow formations. The cuttings comprise active solids like clay that hydrate and inert solids like sand, silt, limestone, feldspar and small quantity ofAmerican Petroleum Institute (API) barite. Apart from the API barite which may be used for density control, the other solids are abrasive, and tend to increase frictional pressure loss during fluid flow, increase viscosity and form thick permeable cakes. This may lead to stuck-pipe, excessive torque and drag, lost circulation, and poor cementation. The inert solids are removed from the drilling fluids by using solids control equipment like shale shaker, desanders, desilters (hydrocyclones and decanting centrifuges) in that order. The active solids, like clays, are removed by using chemical flocculant or by diluting the fluid with water.

Size capacity and operating of solids control equipment

The mud is un-weighted, so no barite is assumed to be present in the system.

  1. The returns mud weight is 14.5ppg, and solids fraction in the fluid is 0.45. The recommended range of solids fraction for water base bentonitic mud of 10.5ppg is from 0.0814 to 0.1637.
  2. Returns mud viscosity is 22 cp. The recommended range is from 5.31 cp to 16.43 cp. Your present viscosity is more。Decrease by at least 5.56 cp.
  3. Present Yield Point is 15 lb/100 ft2. The recommended range is 3.00 to 24.70 lb/100 ft2. Decide freely on this one.
  4. Contaminant CARBON DIOXIDE and contaminating ion CARBONATB is present in 78.95 mg/L amount. To remove use: a. GYPSUM in 0.0789 lbm/bbl concentration, or b. -none in -none lbm/bbl concentration.

Since the mud type is un-weighted, the usual treatment is screening, forced settling, chemical flocculation, and dilution. For coarse sand the particles size range between 190 and 1850 microns. It is advisable to primarily use Shale shaker and Desilter. Note the following:

  1. Shale Shaker

For coarse sand use Shale shakers. Use B40 or S30 mesh, B40 or S40 mesh or B80 or S50 mesh with Shale shakers. Usually the maximum recommended flow rates for these mesh sizes for mud weight of 14.5 ppg are 1186.89 GPM, 952.50 GPM and 788.30 GPM respectively. Data from Brandt Tandem Screen Separators – Dual Unit.

  1. Sand Trap

For coarse sand, Sand Traps are extremely important. For coarse sand the settling rate of particles is roughly 5 to 35 feet per minute in 68 deg F water. Check the sand-trap for dumping accordingly. Do not dump completely however.

  1. Degasser:

There is gas in the system, so line up Degasser. The yield point of the mud is more than 10 lb/100 sq. ft., hence the role of Degasser is very important as the Shale-shaker will not be able to handle the gas alone. Degassification is essential before fluid enters hydrocyclones.

  1. Desander:

The system is un-weighted, Desander can be used freely. Since cuttings type is coarse sand you must use it. If using Desander then use 8 inch minimum 5.33 nos. or 12 inch minimum 2.28 number of Hydrocyclones for 800 GPM of the feeder pump. (Round-off number of cyclones to next higher whole number).

  1. Desilter:

The system is un-weighted, so Desilters can be used. If cuttings type is coarse sand then you may use Desilters. If using them, then use 4 inch or 6 inch Hydrocyclones and use minimum 16 nos. or 9.41 nos. respectively for 800 GPM of the feeder pump flow. (Round-off nos. to next whole number). Maintain the dilution water flow rate around 13.03 gallons per hour.

  1. Mud cleaner:

Since the system is un-weighted, mud cleaners may not be used.

  1. Decanting centrifuge

Mud type is un-weighted, and cutting type is coarse sand, hence use of decanting centrifuge is less important. Reclaim barite from underflow.

Note:

  • If using decanter centrifuges then line-up desanders before them to avoid overloading. Decanter centrifuges help in reclining liquids.
  • Advisable to agitate the mud in suction tanks to desilter, desander or mud cleaner. No agitation in sand trap.
  • If any answer is prefixed by “-one” then that part is not applicable for this case.

END OF CONSULTATION INPUT TO THE UN-WEIGHTED SYSTEM

  • Type of the mud is un-weighted.
  • Type of cuttings in returns coarse sand.
  • Total solids fraction in the mud 0.45
  • Original density of mud 10.5ppg
  • Density of the return mud 14.5ppg
  • Viscosity of mud 22cp
  • Yield point of mud 15 lb/100 sq. ft.
  • The return flow rate is 850 gal/min.
  • Feeder pump flow rate 800 gal/min
  • Hydrocyclone slurry ejection rate 0.79 quart/30 secs.
  • Drilled solids type carbon dioxide
  • Drilled solids major content coarse sand
  • Major contaminant in mud carbon dioxide.
  • Contaminating ion carbonate
  • Contaminating ion concentration 78.95 mg/ml.

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