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Drilling fluids

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The drilling-fluid system—commonly known as the “mud system”—is the single component of the well-construction process that remains in contact with the wellbore throughout the entire drilling operation. Drilling-fluid systems are designed and formulated to perform efficiently under expected wellbore conditions. Advances in drilling-fluid technology have made it possible to implement a cost-effective, fit-for-purpose system for each interval in the well-construction process.


The active drilling-fluid system comprises a volume of fluid that is pumped with specially designed mud pumps from the surface pits, through the drillstring exiting at the bit, up the annular space in the wellbore, and back to the surface for solids removal and maintenance treatments as needed. The capacity of the surface system usually is determined by the rig size, and rig selection is determined by the well design. For example, the active drilling-fluid volume on a deepwater well might be several thousand barrels. Much of that volume is required to fill the long drilling riser that connects the rig floor to the seafloor. By contrast, a shallow well on land might only require a few hundred barrels of fluid to reach its objective.

Functions of drilling fluids

Some of the basic functions of a drilling fluid are as follows:

  • Cleans the hole by transporting drilled cuttings to the surface, where they can be mechanically removed from the fluid before it is recirculated downhole.
  • Balances or overcomes formation pressures in the wellbore to minimize the risk of well-control issues.
  • Supports and stabilizes the walls of the wellbore until casing can be set and cemented or openhole-completion equipment can be installed.
  • Prevents or minimizes damage to the producing formation(s).
  • Cools and lubricates the drillstring and bit.
  • Transmits hydraulic horsepower to the bit.
  • Allows information about the producing formation(s) to be retrieved through cuttings analysis, logging-while-drilling data, and wireline logs.

Types of drilling fluids

World Oil’s annual classification of fluid systems[1] lists nine distinct categories of drilling fluids, including:

  • Freshwater systems
  • Saltwater systems
  • Oil- or synthetic-based systems
  • Pneumatic (air, mist, foam, gas) “fluid” systems

Three key factors usually determine the type of fluid selected for a specific well:

  • Cost
  • Technical performance
  • Environmental impact

Water-based fluids (WBFs) are the most widely used systems, and are considered less expensive than oil-based fluids (OBFs) or synthetic-based fluids (SBFs) . The OBFs and SBFs—also known as invert-emulsion systems—have an oil or synthetic base fluid as the continuous(or external) phase, and brine as the internal phase. Invert-emulsion systems have a higher cost per unit than most water-based fluids, so they often are selected when well conditions call for reliable shale inhibition and/or excellent lubricity. Water-based systems and invert-emulsion systems can be formulated to tolerate relatively high downhole temperatures. Pneumatic systems most commonly are implemented in areas where formation pressures are relatively low and the risk of lost circulation or formation damage is relatively high. The use of these systems requires specialized pressure-management equipment to help prevent the development of hazardous conditions when hydrocarbons are encountered.

Drilling-fluid considerations

Drilling-fluid selection remains one of the most important components of a successful well-construction operation. Drilling-fluid service companies help operators to overcome the familiar issues (e.g., lost circulation) as well as the challenges that are brought on by drilling in ultradeep waters, extreme HP/HT formations, or remote environmentally sensitive areas by providing:

  • Analytical tools
  • Test equipment
  • Stockpoint facilities
  • Innovative materials

The ability to simulate downhole conditions and optimize fluid design will continue to help reduce nonproductive time, and real-time management of hole conditions through data feed from downhole tools allows the operator and drilling-fluid specialist to fine-tune drilling parameters.

The demand for drilling-waste-management services that are dedicated to reducing, recovering, and recycling the volume of spent fluids and drilled cuttings continues to grow rapidly. These services and the related equipment have demonstrated their worth by helping operators achieve environmental compliance, reducing disposal costs, and returning more fluid and water for reuse in multiple applications.

Drilling-fluid services of some kind are required on every well. They encompass a broad spectrum of systems, products, software, personnel specializations, and logistical support. As wells become more complex, total drilling costs can increase dramatically. Because the drilling-fluid system comes in contact with almost every aspect of the drilling operation, proper drilling-fluid selection can help the operator minimize costs throughout the well-construction process.

Drilling fluid additives

Drilling fluid additives are also called mud additives. It is used to enhance the performance and characteristics of the drilling fluid by controlling different properties of the mud such as viscosity, weight, filtration control, lubrication etc. The following are types of drilling fluid additives:

  • Weighting agents: It is used to increase the density of the drilling fluid. Example: Barite and Hematite
  • Viscosifiers: It is used to increase the viscosity of the drilling fluid and improving its ability to suspend cuttings and hole cleaning. Example: Bentonite and Xanthan gum.
  • Alkalinity control: It is used to control the degree of acidity or alkalinity of the drilling fluids. Example: Caustic Soda and Lime.
  • Filtration control or Fluid loss control: It is used to minimize the loss of drilling fluid into the formation by forming a filter cake.
  • Corrosion inhibitors: It is used to eliminate or reduce the corrosion effect. Example: Oxygen scavengers.
  • Biocides: It is used to control the growth of bacteria and other microorganisms in the drilling fluid.
  • Defoamers: It is used to reduce the foaming in drilling fluids.


  1. World Oil 2012 Drilling, Completion and Workover Fluids. 2012. World Oil 233 (6): F-1.

Note: The drilling fluids suppliers (Baker Hughes, Halliburton (Baroid), Schlumberger (M-I SWACO), Newpark, Scomi, and others maintain company specific reference materials such as Drilling Fluids Handbooks and Drilling Fluids Engineering Manuals. These are excellent technical resources and may be available from those companies.

Noteworthy papers in OnePetro

Smith, T.R., Shell Canada Ltd.; Ravi, K.M., Halliburton Services: Investigation of Drilling Fluid Properties To Maximize Cement Displacement Efficiency, 22775-MS,

Warren, B.K., Smith, T.R., Shell Canada Ltd.,; Ravi, K.M., Halliburton Services: Static and Dynamic Fluid-Loss Characteristics of Drilling Fluids in a Full-Scale Wellbore, 26069-MS,

Noteworthy books

Caenn, R., Darley, H.C.H., Gray, G.R. et al. 2011. Composition and properties of drilling and completion fluids, 6th. Amsterdam ; Boston, MA: Gulf Professional Pub. 2011293568

SPE Reprint Series No. 44: Drilling Fluids, 1997, Edited by: Tom S. Carter, ISBN:978-1-55563-069-0

External links

See also

Drilling fluid environmental considerations

Drilling fluid challenges

PEH: Drilling Fluids