Completion and workover fluids are any fluids used in the completion of a well or in a workover operation. These fluids range from low-density gases such as nitrogen to high-density muds and packer fluids. The application and requirements vary for each fluid.

Workover fluids are fluids used during the reworking of a well after its initial completion. They may be gases such as nitrogen or natural gas, brine waters, or muds.Workover fluids are used during operations such as well killing, cleaning out a well, drilling into a new production interval, and plugging back to complete a shallower interval.

Completion fluids are used during the process of establishing final contact between the productive formation and the wellbore. They may be a water-base mud, nitrogen, oil mud, solids-free brine, or acidsoluble system. The most significant requirement is that the fluid does not damage the producing formation and does not impair production performance.

Packer fluids are fluids placed in the annulus between the production tubing and casing. Packer fluids must provide the required hydrostatic pressure, must be nontoxic and noncorrosive, must not solidify or settle out of suspension over long periods of time, and must allow for minimal formation damage.

Various types of fluids may be used for completion and workover operations:

  • Oil fluids (Crude, Diesel, Mineral oil)
  • Clear water fluids (Formation salt water, Seawater, Prepared salt water such as calcium chloride, potassium chloride or sodium chloride salt and zinc, calcium, or sodium-based bromides.)
  • Conventional water-base mud
  • Oil-base or invert emulsion muds

Completion or workover fluids may be categorized as

  1. Water-base fluids containing oil-soluble organic particles.
  2. Acid-soluble and biodegradable.
  3. Water base with water-soluble solids
  4. Oil-in-water emulsions
  5. Oil-base fluids

Three types of completion or workover fluids are

  1. Clear liquids (dense salt solutions)
  2. Weighted suspensions containing calcium carbonate weighting material, a bridging agent to increase the density above that of saturated solutions.
  3. Water-in-oil emulsions made with emulsifiers for oil muds.

Clear liquids have no suspended solids and can be referred to as solidsfree fluids. Weighted suspensions are fluids with suspended solids for bridging or added density. These fluids can be referred to as solids-laden fluids.

For solids-free fluids, water may be used in conjunctionwith a defoamer, viscosifier, stabilized organic colloid, and usually a corrosion inhibitor. Solids-free completion and workover fluids have densities ranging from 7.0 to 19.2 pounds per gallon (ppg) (0.84 – 2.3 SG).

Solids-laden fluids may be composed of water, salt, a defoamer, suspension agent, stabilized organic colloid, pH stabilizer, and a weighting material/bridging agent.

Solids-Free Fluids

Brines used in completion and workover applications may be single-salt brines, two-salt brines, or brines containing three different salt compounds.

Single-Salt Brines

Single-salt brines are made with freshwater and one salt such as potassium chloride, sodium chloride, or calcium chloride. They are the simplest brines used in completion and workover fluids. Because they contain only one salt, their initial composition is easily understood. Their density is adjusted by adding either salt or water. Single-salt brines are available in densities of up to 11.6 ppg and are the least expensive brines used in completions.

Potassium chloride (KCl) brines are excellent completion fluids for water-sensitive formations when densities over (9.7 ppg)(1.16 SG) are not required. Corrosion rates are reasonably low and can be reduced even more by keeping the pH of the system between 7 and 10 and using corrosion inhibitors (1% by volume).

Sodium chloride is one of the most used single-salt brines. Advantages of sodium chloride brines are low cost and wide availability. Densities up to 10.0 ppg are achievable for this single-salt brine.

Calcium chloride (CaCl2) brines are easily mixed at densities up to 11.6 ppg 1.39 SG. Sodium bromide brines can be used when the density of a calcium chloride brine is desired, but the presence of acid gas is possible.

Sodium bromide has low corrosion rates even without the use of corrosion
inhibitors. Although these brines are more expensive than CaCl2 brines, they are useful in CO2 environments.

Two-Salt Brines

The basic ingredient of calcium chloride/calcium bromide brines. 

(CaCl2/CaBr2) is a calcium bromide solution that ranges in density from 14.1 to 14.3 ppg (1.72 SG); the pH range is 7.0 to 7.5. The density of CaBr2 brine can be increased by adding calcium chloride pellets or flakes. However, a 1.81 S.G. CaCl2/CaBr2 solution crystallizes at approximately 65◦F (18◦C). CaCl2/CaBr2 brine can be diluted by adding a CaCl2 brineweighing 11.6 ppg (1.39 SG). The corrosion rate for CaCl2/CaBr2 is nomore than 5mm per year on N-80 steel coupons at 300◦F (149◦C). If a corrosion inhibitor is desired, a corrosion inhibitor microbiostat is recommended.

The viscosity of CaCl2/CaBr2 brine can be increased by adding liquefied HEC viscosifier. Reduction in filtration may be obtained by the addition of CaCO3 weighting material/bridging agent or by increasing the viscosity with polymeric materials.