Drilling Fluid

Drilling fluid is also commonly known as drilling mud or mud. Drilling fluids are used throughout the drilling of a well.

In a drilling fluid system, the drilling fluid is pumped from the surface pits through the drill string all the way down to the bit. The pump used to pump drilling fluid is called a mud pump. The drilling fluid flows out at the bit, up the annular space in the wellbore and back up to the surface. At the surface the solids are removed using a shale shakers and other mechanical processing including desilters, desanders, mud cleaners, and decating centrifuges. The remaining drilling fluid is then reused.

The main purposes of the drilling fluid:

  • Create hydrostatic pressure on the well to prevent formation fluids from escaping
  • Keep the drill bit cool and lubricated
  • Clean the drill bit by removing drill cuttings
  • Re-enforce the borehole walls when the well is being completed
  • Transmit hydraulic horsepower to the bit
  • Allow pressure signals to travel to surface for measurement while drilling (MWD)

Types of drilling fluids:

  • Water-based Fluids
  • Oil-based Fluids
  • Synthetic-based Fluids
  • Pneumatic (Gas / Air) Based Fluid Systems

Water-based Fluids

Free water, or saltwater is mixed with clays and other additives. The most common additive is Bentonite. Bentonite helps with formation fluid loss control and help remove drill cuttings. Water-based fluids are most commonly used as they are less expensive than other options.

There are two main types of Water-based Drilling Fluids:

  1. Non-dispersed Systems
  2. Dispersed Systems

Oil-based Fluids

Diesel or mineral oil or other petroleum based products are used to create the fluid system. Oil based drilling fluid have increased lucubration, shale inhibition, and is better at removing drill cuttings.

There are two types of oil-based muds:

  1. Invert Emulsion Drilling Fluids
  2. Pseudo Oil Based Drilling Fluids

Synthetic-based Fluid

Synthetic-based fluid systems are low toxicity oil-based fluids used to create the fluid system. They were developed to reduce the environmental impact. Synthetic-based fluids are most commonly used in offshore drilling operations due to the tighter / enclosed space on a drilling rig.

Pneumatic (Gas / Air) Based Fluid Systems

Pneumatic gas or air can be used in place of drilling fluids. Pneumatic gas / air are most commonly used where the formation pressures are low.

The main advantages or using pneumatic (gas or air) drilling fluids are:

  • Little or no formation damage
  • Higher circulation rate and removal of cuttings
  • Prevents lost circulation
  • Higher formation penetration rates

The main types of pneumatic drilling fluids include:

  • Compressor Air or Gas
  • Air/Water Mixture
  • Air/Polymer Mixture (Foam)


  • Oil and Gas Drilling Fluids Training Part 1 Mud Types Overview
  • Oil and Gas Drilling Fluids Training Part 2 Functions of Drilling Mud
  • Oil and Gas Drilling Fluids Training Part 3 Mud Properties and its Additives
  • Oil and Gas Drilling Fluids Training Part 4 Mud Tests at Rig

Comments regarding

1. Drilling Fluid Types. Retrieved from http://petrowiki.org/Drilling_fluid_types#Synthetic-based_drilling_fluids

2. Drilling fluid. https://en.wikipedia.org/wiki/Drilling_fluid