Written and maintained by CASRAI Editorial Board
Last updated
Drilling fluid, also called drilling mud, is the circulating fluid used in rotary drilling of wells to clean and condition the hole and to counterbalance formation pressure. It is classed by its continuous phase: water-based, or non-aqueous (oil-based, enhanced mineral oil-based or synthetic-based), with air and foam as lighter alternatives. This guide is for researchers, students and laboratory staff who need the terminology, the functions, the additive families and the regulatory test methods that surround drilling fluids.
Short answer
A drilling fluid is the liquid (or, for air and foam systems, gas-based) medium pumped through a well during drilling. U.S. EPA defines it in 40 CFR 435.11(l) as “the circulating fluid (mud) used in the rotary drilling of wells to clean and condition the hole and to counterbalance formation pressure.” OSHA’s oil and gas drilling eTool adds that it cools and lubricates the bit, carries drill cuttings to the surface and keeps the uncased wellbore from caving in.
Definition and scope
The term covers the whole circulating system: the base fluid (the continuous phase), the solids suspended in it and the additives mixed in at the surface. EPA’s definition of base fluid is “the continuous phase or suspending medium of a drilling fluid formulation.” The same regulation defines drill cuttings as the particles generated by drilling into subsurface geologic formations and carried out of the wellbore with the drilling fluid, ranging from fine silt to gravel.
The subject sits within the wider field described in what is petroleum engineering, where drilling and well construction is one of the main research areas.
How drilling fluid is used in practice
According to OSHA’s Oil and Gas Well Drilling and Servicing eTool (drilling fluid page), a fluid is required in the wellbore to:
- cool and lubricate the drill bit;
- remove rock fragments (drill cuttings) from the drilling area and transport them to the surface;
- counterbalance formation pressure so formation fluids such as oil, gas and water do not enter the well prematurely, which can lead to a blowout; and
- prevent the open (uncased) wellbore from caving in.
The eTool also describes how weighting materials raise mud density “to equilibrate the pressure between the wellbore and formation when drilling through particularly pressurized zones.” OSHA notes that its eTools are guidance and do not create new requirements.
Types of drilling fluid
Two sources classify drilling fluids in slightly different ways. OSHA groups them by what the fluid is made from; EPA groups them by the continuous phase, because that determines how discharges are regulated offshore.
| Type | How the source describes it | Source |
|---|---|---|
| Water-based | Continuous phase and suspending medium for solids is a water-miscible fluid, regardless of the presence of oil. OSHA: the most frequently used muds; the base may be fresh water or salt water. | 40 CFR 435.11(l)(1); OSHA eTool |
| Non-aqueous | Continuous phase is a water-immiscible fluid, such as mineral oil, enhanced mineral oil, paraffinic oil, C16-C18 internal olefins and C8-C16 fatty acid/2-ethylhexyl esters. Divided into the three rows below. | 40 CFR 435.11(l)(2) |
| Oil-based (non-aqueous) | Continuous phase is diesel oil, mineral oil or some other oil, but contains no synthetic material or enhanced mineral oil. | 40 CFR 435.11(l)(2)(i) |
| Enhanced mineral oil-based (non-aqueous) | Continuous phase is enhanced mineral oil: a highly purified petroleum distillate with a lower polycyclic aromatic hydrocarbon (PAH) content than diesel or conventional mineral oil. | 40 CFR 435.11(l)(2)(ii), (m) |
| Synthetic-based (non-aqueous) | Continuous phase is a synthetic material or combination of synthetic materials. OSHA lists vegetable esters, poly alpha olefins, internal olefins, linear alpha olefins, synthetic paraffins, ethers and linear alkylbenzenes, among others. | 40 CFR 435.11(l)(2)(iii); OSHA eTool |
| Air and foam | Fluids that are less dense than drilling muds and typically contain few additives, because the additives are liquid or solid and will not mix with air and foam fluids. | OSHA eTool |
Additive families and what each does
OSHA states that drilling muds typically have several additives. The families it lists, with the function it gives for each:
| Additive family | Examples named by OSHA | Stated function |
|---|---|---|
| Weighting materials | Barite (barium sulfate); hematite (Fe2O3) sometimes in oil-based muds | Increase mud density to equilibrate pressure between wellbore and formation |
| Corrosion inhibitors | Iron oxide, aluminum bisulfate, zinc carbonate, zinc chromate | Protect pipes and metallic components from acidic compounds in the formation |
| Dispersants | Iron lignosulfonates | Break up solid clusters into small particles so the fluid can carry them |
| Flocculants | Primarily acrylic polymers | Group suspended particles so they can be removed at the surface |
| Surfactants | Fatty acids and soaps | Defoam and emulsify the mud |
| Biocides | Typically organic amines, chlorophenols or formaldehydes | Kill bacteria and reduce souring of the mud |
| Fluid loss reducers | Starch and organic polymers | Limit loss of mud to under-pressurized or high-permeability formations |
Where it matters for laboratories and compliance
Drilling fluids reach laboratory work in two main ways, both visible in the federal text of 40 CFR 435.11:
- Effluent test methods. The regulation ties offshore discharge limits for drilling fluids and cuttings to named EPA methods: EPA Method 1674 (base fluid retained on cuttings, derived from API Recommended Practice 13B-2, using a retort chamber), EPA Methods 1646 and 1647 (sediment preparation and closed-bottle biodegradation of non-aqueous base fluids) and EPA Method 1644 (a sediment toxicity test with Leptocheirus plumulosus). The methods are published in “Analytic Methods for the Oil and Gas Extraction Point Source Category,” EPA-821-R-11-004.
- Chemical handling. Because additive families include biocides (typically organic amines, chlorophenols or formaldehydes), weighting solids and corrosion inhibitors, anyone handling mud samples or additives works from each product’s safety data sheet. See how to read a safety data sheet and the Hazard Communication Standard in laboratories. For hydrocarbon analysis of oily samples, see total petroleum hydrocarbons test methods.
Common misunderstandings
- “Mud” means water and clay only. The regulatory definition allows a water-based fluid to contain oil, and the non-aqueous classes use oil or synthetic liquids as the continuous phase.
- “Synthetic-based” and “oil-based” are the same. Under 40 CFR 435.11, oil-based fluids contain no synthetic material or enhanced mineral oil; synthetic-based fluids use a synthetic continuous phase. They are separate classes.
- The fluid only removes cuttings. Carrying cuttings is one of several functions; pressure control and wellbore stability are equally central in the OSHA list.
- Air and foam carry the same additives. OSHA says they typically contain few additives.
Frequently asked questions
What is the difference between drilling fluid and drilling mud?
EPA uses both as one term: its definition of drilling fluid reads “the circulating fluid (mud).” “Mud” is the common name for liquid drilling fluids.
What are the main types of drilling fluid?
EPA recognises water-based and non-aqueous fluids, with the non-aqueous class divided into oil-based, enhanced mineral oil-based and synthetic-based. OSHA also lists air and foam fluids.
Why is barite added to drilling mud?
OSHA identifies barite (barium sulfate) as the primary weighting material, used to raise mud density so the pressure in the wellbore balances the formation pressure through highly pressurized zones.
Sources
- U.S. EPA / Office of the Federal Register, 40 CFR 435.11, Specialized definitions (Oil and Gas Extraction Point Source Category, Subpart A), read via Cornell LII, 8 November 2026.
- OSHA, Oil and Gas Well Drilling and Servicing eTool, “Drilling – Drilling Ahead – Drilling Fluid,” read from the UNT Web Archive capture of 24 January 2017, 8 November 2026.
