The fluid matters to the cost of a well and to the mineral markets behind it. Rig and drilling fluids together made up about 15 % of the total cost of a typical US onshore well in a cost study published by the US Energy Information Administration (EIA, March 2016), and more than 90 % of the barite sold in the United States goes into oil and gas drilling fluids (USGS Mineral Commodity Summaries 2026). Barite is one of the drilling fluid additives that turn water, brine or oil into an engineered mud, and the questions that follow are what is in the fluid and how it does its work.
A drilling fluid is a continuous phase of water, brine, oil, synthetic fluid or gas that carries clays, weighting solids, polymers and salts. It circulates in a closed loop from the pits to the bit and back, falls into five families by continuous phase, and takes its properties from additive classes such as weighting agents, viscosifiers and fluid-loss additives. Mud engineers check its density, rheology, filtration, solids and chemistry under API recommended practices, while US federal rules, OSPAR and Norway decide what may be discharged offshore.
Standards and regulations are checked against their issuing bodies, product values against the named data sheet and figures against the dated source named in the sentence; see our methodology and fact-checking.
What Is Drilling Fluid?#
Drilling fluid is any liquid, gas or mixture of fluids and solids that is circulated through a borehole while it is drilled. The IADC Drilling Manual sorts drilling fluids into three broad categories by their base, listed below.
- Pneumatic fluids: air, mist and foam, with gas as the circulating medium.
- Water-based fluids: water or brine as the base, abbreviated WBM for water-based mud.
- Non-aqueous fluids (NAF): oil-based mud (OBM) and synthetic-based mud (SBM).
Drilling fluid does five core tasks in a well: it controls formation pressure, removes cuttings from below the bit, cools and lubricates the bit, transmits hydraulic energy to the bit and downhole tools, and keeps the wellbore stable. A 2015 AADE presentation on the daily mud report extends that list to 11 functions, adding the suspension of cuttings and weight material, the sealing of permeable formations, protection of the reservoir, formation evaluation, corrosion control and a smaller environmental impact. The full list of functions of drilling fluid pairs each function with the property that delivers it.
Each of these functions depends on what the fluid carries. Plain water lifts few cuttings once the pumps stop and builds no filter cake, so a working mud blends a base liquid with clays, weighting solids and dissolved chemicals.
Drilling fluid or drilling mud: is there a difference?#
Drilling mud and drilling fluid mean the same thing in general use, and some engineers reserve "drilling fluid" for the more engineered systems. The two terms part ways only in pneumatic drilling: air, mist and foam are drilling fluids but not muds, because a mud is a liquid carrying solids. This reference uses both terms for liquid systems and "drilling fluid" alone for gas-based systems.
What Is Drilling Fluid Made Of?#
Drilling fluid is made of a liquid continuous phase, solids dispersed in it and, in some systems, a gas phase: water, brine, oil or a synthetic fluid carries clays, weighting solids, polymers and salts.
The continuous phase is the external liquid that touches the wellbore wall and the drill string, and it decides how the mud reacts with shale, heat and seawater. Solids in the mud split into two groups. Active solids, such as bentonite and attapulgite clays, hydrate and build viscosity, gel strength and filter cake; inert solids, such as barite, hematite, calcium carbonate and sand-sized drilled solids, add density or bridge pores without reacting with the water. The six components of a drilling fluid are listed below.
| Component | Examples | What it controls |
|---|---|---|
| Continuous (base) phase | fresh water, seawater, brine, diesel or mineral oil, synthetic fluids | shale inhibition, temperature behavior, discharge options |
| Dissolved salts | potassium chloride, sodium chloride, calcium chloride, formates | density without solids, shale inhibition, water activity |
| Active solids (clays) | bentonite, attapulgite, organoclay | viscosity, gel strength, filter cake |
| Inert added solids | barite, hematite, calcium carbonate | density, bridging of pores |
| Drilled solids | cuttings fines (low-gravity solids) | unwanted viscosity and density, kept low by solids control |
| Polymers and treatment chemicals | polyanionic cellulose, xanthan gum, starch, lignosulfonate, caustic soda | fluid loss, viscosity, dispersion, pH |
Pneumatic systems add a gas phase, such as air, nitrogen, mist or foam, to the liquid or in place of it.
Two of these components dominate the mineral markets they come from. More than 90 % of the barite sold in the United States goes into oil and gas drilling fluids, and drilling mud takes 22 % of US bentonite use (2025, estimated), both according to the USGS Mineral Commodity Summaries 2026. Drilled solids are the component the mud engineer works to remove: a 2015 AADE presentation defines low-solids water-based muds as muds carrying less than 6 vol % low-gravity solids.
How is drilling mud made?#
Drilling mud is made by treating the make-up water first and then adding clay, polymers, salts and weighting solids until the mud reaches its target properties. Soda ash treats the hardness of the make-up water before bentonite goes in. Bentonite is never prehydrated in seawater, so seawater and salt muds take bentonite that has been prehydrated in fresh water and then blended into the system. The targets for density, rheology and filtration come from the well plan and are confirmed by the mud check described further down this page.
How Drilling Fluid Works: The Circulating Loop From Pit to Bit and Back#
Drilling fluid works as a closed loop: pumps send it from the pits down the drill string and through the bit, it rises in the annulus carrying cuttings, and solids-control equipment cleans it before it returns.
In oil and gas drilling, an oil-based or synthetic-based mud follows the same path as a water-based one, and only the continuous phase changes. While it circulates, the fluid lifts cuttings up the annulus. When the pumps stop for a connection, its gel structure holds cuttings and weighting solids in suspension so they do not settle around the bit. On permeable walls the mud leaves a thin filter cake that limits further filtrate loss into the formation.
Pressure control rests on mud weight. The hydrostatic pressure of the mud column depends on mud weight and true vertical depth, never on measured depth, so a long horizontal section adds length but no extra hydrostatic pressure. Fresh water weighs 8.34 lb/gal (1.00 sg; 999 kg/m3) and seawater about 8.55 lb/gal (1.02 sg; 1,025 kg/m3) (IADC Drilling Manual), and weighting solids raise a mud above these base values until it balances formation pressure.
The fluid changes on every circulation, so it is tested and treated continuously. Drilled solids grind into fines, formation salts and gases such as carbon dioxide and hydrogen sulfide enter the mud, and heat degrades polymers; each change shows up in the mud check and is corrected with an additive or by dilution. Solids-control equipment, such as shale shakers, desanders, desilters and centrifuges, removes part of the drilled solids before the mud returns to the suction pit.
Water-Based, Oil-Based, Synthetic-Based, Pneumatic and Brine Fluids#
Drilling fluids fall into five families by continuous phase: water-based muds, oil-based muds, synthetic-based muds, pneumatic fluids (air, mist and foam) and brines.
The continuous phase sets three properties of a fluid at once: how strongly it inhibits reactive shale, how it behaves at high temperature, and whether the fluid or its cuttings may be discharged. The US offshore oil and gas effluent guidelines define the two non-aqueous families by their base: oil-based fluid has diesel, mineral or another oil as its continuous phase and contains no synthetic material, while synthetic-based fluid has a synthetic material or a combination of synthetic materials as its continuous phase (40 CFR 435.11). The table below compares the five families by continuous phase, with one sourced fact for each.
| Family | Continuous phase | One fact with source |
|---|---|---|
| Water-based mud (WBM) | water or saltwater | seven subclasses in a 2015 AADE presentation, from spud mud through low-solids, deflocculated weighted, calcium-treated, HTHP and salt-water mud to inhibitive mud |
| Oil-based mud (OBM) | diesel, mineral or other oil | diesel-oil-based fluids prohibited in the OSPAR area since 1987-01-01 (OSPAR Decision 2000/3) |
| Synthetic-based mud (SBM) | synthetic material | internal olefin and ester base fluids set the two reference classes of the US cuttings retention limits (40 CFR 435.13) |
| Pneumatic fluids (air, mist, foam) | gas | uphole velocity about 3,000 to more than 7,500 ft/min (914 to more than 2,286 m/min) for dry air against 200 to 400 ft/min (61 to 122 m/min) for foam (Water Well Journal) |
| Brines (including formates) | dissolved salts, no added solids | saturated potassium formate 13.3 lb/gal (1.59 sg; 1,590 kg/m3) and cesium formate 19.2 lb/gal (2.30 sg; 2,300 kg/m3) at 68 degF (20 degC) (AADE-24-FTCE-047) |
The fluid systems hub compares all five families side by side, with base fluids, inhibition and temperature behavior for each system.
Fluid choice starts with the formation, the temperature and the discharge rules at the site. The three types of mud in the IADC sense are pneumatic, water-based and non-aqueous; brines belong to the water-based category, and oil-based and synthetic-based muds share the non-aqueous one. A reactive shale section, a hot hole or a ban on oily discharges each moves the choice toward a different family.
The Additive Classes and What Each One Controls#
Each additive class controls one property of the mud: weighting agents set density, viscosifiers build viscosity and gel strength, fluid-loss additives limit filtrate, and the other classes treat shale, losses, pH, contamination, friction, foam and bacteria.
Additives are the minerals, polymers and treatment chemicals added to the base fluid, from sacks of barite and bentonite to liquid lubricants and biocides. The table below lists every class with the property it controls and example materials.
Send one specification to independent suppliers of drilling fluid materials. request supplier quotes
| Class | What it controls | Example materials |
|---|---|---|
| Weighting agents | density | barite, hematite, calcium carbonate |
| Viscosifiers | viscosity, gel strength, suspension | bentonite, xanthan gum, organoclay |
| Fluid loss control additives | filtrate and filter cake | polyanionic cellulose, starch, sulfonated asphalt |
| Shale inhibitors | reactive clay and shale | potassium chloride, PHPA, glycols, amines, silicates |
| Lost circulation materials | losses to fractures and vugs | nut shells, fibers, graphite, mica, sized calcium carbonate |
| Thinners and deflocculants | excess viscosity of clay muds | lignosulfonates, SAPP, sodium polyacrylates |
| Emulsifiers and wetting agents | invert emulsion stability, oil-wet solids | primary and secondary emulsifiers, oil-wetting agents |
| Lubricants | torque and drag | water-based and oil-based drilling fluid lubricants |
| pH and alkalinity control | pH, alkalinity, calcium | caustic soda, lime, soda ash, sodium bicarbonate |
| Biocides | bacterial breakdown of polymers | glutaraldehyde, THPS |
| Corrosion control | corrosion, oxygen, hydrogen sulfide | sodium sulfite, zinc carbonate |
| Defoamers and foaming agents | foam | defoamers, foamers for air and mist drilling |
| Flocculants | drilled-solids removal, clear-water drilling | polymer flocculants |
| Spotting fluids | differential sticking | spotting fluids, pipe-freeing agents |
| Surfactants | bit balling, accretion, rate of penetration | detergents, ROP enhancers |
Each class page lists its materials, doses and temperature data.
The chemicals found in drilling mud are these additive materials, and their trade names vary by supplier while the chemistry stays the same. Polyanionic cellulose (PAC) is a cellulose ether that controls filtrate in water-based muds, and on this site the abbreviation PAC always means polyanionic cellulose. API Spec 13A covers 13 of these materials, among them barite, bentonite, polyanionic cellulose and drilling-grade xanthan gum; the other additives carry no API Spec 13A grade.
What Gets Measured in the Mud and How#
A mud check measures density, rheology, filtration, solids and chemistry, following API RP 13B-1 (6th edition, 2026) for water-based fluids and API RP 13B-2 (6th edition, 2023) for nonaqueous fluids.
Rheology carries the strictest unit convention of the mud check, and yield point is never written in cP. The rotational viscometer reads at 600 and 300 r/min, and gel strengths are read after 10 seconds and 10 minutes at rest. Laboratory testing of drilling fluid materials and fluid performance follows API RP 13I, 9th edition (2020). The table below lists eight properties of the mud with the unit and the field test for each.
| Property | Reported as (oilfield (SI)) | Field test |
|---|---|---|
| Mud weight (density) | lb/gal (sg; kg/m3) | mud balance |
| Drilling fluid rheology | plastic viscosity in cP (mPa.s); yield point and gels in lbf/100 ft2 (Pa) | rotational viscometer |
| Filtration | filtrate volume and filter cake thickness | low-pressure and HTHP filter press |
| Solids, water and oil | vol % | retort |
| Sand content | vol % | sand content kit |
| Reactive clay | bentonite equivalent | methylene blue test |
| Chemistry | pH, alkalinity, chlorides, hardness | titrations and pH meter |
| Electrical stability (non-aqueous fluids) | volts | electrical stability meter |
Ranges per system are on the properties hub, and step-by-step procedures are on the testing hub.
Sand content is the volume percentage of particles larger than 75 µm, the opening of a No. 200 sieve. API filtrate is reported in mL per 30 min, from a low-pressure filter press or a high-temperature, high-pressure (HTHP) cell. The readings feed the calculations for mud weight, hydrostatic pressure, equivalent circulating density and rheology, which run in the calculators of the reference sections at the end of this page.
Downhole Problems the Fluid Prevents and Treats#
Lost circulation, differential sticking, shale instability, bit balling, contamination and poor hole cleaning are the downhole problems a drilling fluid prevents or treats.
Each problem has a fluid-side cause and a mechanical side, and the mud engineer works on the fluid side. The fix is a property change, such as a lower filtrate or a higher low-shear viscosity, or a pill, a small volume of treated fluid placed across the trouble zone. The table below pairs eight problems with their fluid-side cause and the additive classes that treat them.
| Problem | Fluid-side cause | Classes that treat it |
|---|---|---|
| Lost circulation | fractures, vugs, high equivalent circulating density | lost circulation materials |
| Differential sticking | thick filter cake, high overbalance | fluid loss control additives, spotting fluids |
| Shale instability | water uptake by reactive clays | shale inhibitors |
| Bit balling | sticky clays on the bit and bottom-hole assembly | surfactants |
| Contamination | cement, anhydrite, carbonate, salt, carbon dioxide, hydrogen sulfide | pH and alkalinity control, corrosion control |
| Poor hole cleaning | low low-shear rheology | viscosifiers |
| Formation damage | filtrate and solids invasion of the reservoir | drill-in fluids, bridging solids |
| Gas hydrates | low temperature and high pressure | salts, glycols |
Causes, warning signs and fluid treatments for every problem are on the drilling fluid problems hub.
Problems compound one another in a well. A thick filter cake raises the risk of differential sticking, and losses lower the fluid level in the hole and with it the hydrostatic pressure. Well-control procedure is outside this reference, while mud weight, the fluid property that holds formation pressure, stays in.
Where the Fluid Is Used: Oil and Gas, HDD, Water Wells, Geothermal, Mining and Piling#
Drilling fluids are used in oil and gas wells, horizontal directional drilling, water wells, geothermal wells, mineral exploration and core drilling, and bored piles and diaphragm walls, and each application changes what the fluid must do.
A deepwater well and a shallow water well both need pressure control and cuttings transport, but they differ in depth, temperature, discharge rules and the groundwater the fluid may touch. The table below lists eight applications with the conditions that shape the fluid.
Describe the well or bore and we pass it to independent drilling-fluid service providers. find drilling fluid support
| Application | What shapes the fluid |
|---|---|
| Oil and gas, onshore | depth, pressure, reactive shale |
| Offshore and deepwater | riserless top hole, cold-temperature rheology, discharge rules |
| High pressure, high temperature (HPHT) | high pressure and temperature |
| Horizontal directional drilling (HDD drilling fluids) | bore stability, cuttings in clay, sand and gravel, inadvertent returns |
| Water wells and mud rotary | bentonite and polymer fluids, potable-water contact |
| Geothermal | high temperature, lost circulation |
| Mineral exploration and diamond core drilling | polymer fluids, core recovery |
| Bored piles and diaphragm walls | bentonite slurry, polymer support fluids |
Fluid systems and additives for each use are on drilling fluids by application.
Bentonite and polymer fluids serve shallow construction, HDD and water-well work, where the fluid must hold an open bore in unconsolidated ground and stay compatible with groundwater. Non-aqueous and specialty fluids serve offshore and HPHT wells, where shale inhibition, temperature stability and discharge rules govern the choice.
Environment and Discharge Rules#
On the US Outer Continental Shelf, water-based mud and cuttings may be discharged only beyond 3 miles (4.8 km), with a 96-hour LC50 of at least 3 % by volume and no free oil or diesel oil (40 CFR 435.13).
The LC50 is measured on the suspended particulate phase of the mud, and free oil is detected with the static sheen test. Non-aqueous fluids, US coastal waters, the North-East Atlantic and Norway each carry their own rule, and the table below lists them by instrument and jurisdiction.
| Topic | Rule (instrument) | Jurisdiction |
|---|---|---|
| Non-aqueous fluid and drill cuttings | whole fluid not discharged; base fluid on cuttings at most 6.9 g/100 g wet cuttings (C16-C18 internal olefin) or 9.4 g/100 g (esters), measured by EPA Method 1674 (40 CFR 435.13) | US Outer Continental Shelf |
| Coastal waters | no discharge of drilling fluid or cuttings, except Cook Inlet (40 CFR 435.43) | US coastal subcategory |
| Oil-contaminated cuttings | no discharge above 1 % oil by weight on dry cuttings, since 2001 (OSPAR Decision 2000/3) | North-East Atlantic (OSPAR area) |
| Cuttings with oil, Norway | no discharge above 10 g/kg dry mass of base fluid or formation oil (Activities Regulations section 68) | Norway |
| Chemical notification | PLONOR list (OSPAR Agreement 2013-06, Update 2026); HOCNF as the OSPAR-wide notification format; OCNS in the UK and the Netherlands | OSPAR area |
Toxicity, cuttings treatment and waste routes are on drilling fluids and the environment.
Drilling fluids and other exploration and production wastes from oil, gas and geothermal wells are excluded from hazardous waste under the US Resource Conservation and Recovery Act by 40 CFR 261.4(b)(5), and state rules still apply to their storage and disposal. The hazard of a given mud follows from its components and their safety data sheets, and each material page states the classification of its material.
In the North-East Atlantic, OSPAR decisions and agreements set the frame, and the national authorities of the UK, the Netherlands and Norway apply them through their own notification schemes and limits. Offshore operators there register each drilling chemical before use, and substances on the PLONOR list (pose little or no risk) need less test data.
API Specifications, Recommended Practices and Their ISO Counterparts#
API Spec 13A specifies drilling fluid materials, and the API RP 13B, 13D and 13I recommended practices set how drilling fluids are tested and modeled.
The difference between the two document types is binding force. API Spec 13A is a Specification, with requirements a manufacturer of a listed material must meet, while API RP 13B-1, 13B-2, 13C, 13D, 13I and 13K are Recommended Practices that describe how to run a test or a calculation. The table below gives the current edition and the ISO document of the same scope for the five API documents a mud check and a material purchase rely on.
| Document | Current edition (checked 2026-10-27) | Scope | ISO document of the same scope |
|---|---|---|---|
| API Spec 13A | 19th edition (2019) with Addendum 1 (2020), Addendum 2 (2022) and Errata 1 (2024), the current edition (API Standards Plan, checked 2026-10-27); a 20th edition is in development | drilling fluid materials, requirements for manufacturers | ISO 13500:2008 with Cor 1:2009 and Amd 1:2010 (ISO counterpart) |
| API RP 13B-1 | 6th edition (2026) | testing water-based drilling fluids | ISO 10414-1 |
| API RP 13B-2 | 6th edition (2023) with Addendum 1 (2025) | field testing nonaqueous drilling fluids | ISO 10414-2 |
| API RP 13D | 8th edition (2026) | rheology and hydraulics of drilling fluids | No ISO counterpart |
| API RP 13I | 9th edition (2020) with Addendum 1 (2023) | laboratory testing of drilling fluids | ISO 10416:2008 |
Every API, ISO, EPA and OSPAR document cited in this reference is listed under drilling fluid standards and rules.
The four ISO documents in the table belong, together with ISO 13501 on processing equipment evaluation, to ISO/TC 67/SC 3, the ISO subcommittee for drilling and completion fluids, well cements and treatment fluids (ISO Open Data, 2026). API revises each recommended practice on its own schedule, so a test report names the document and the edition it follows.
Materials, Comparisons, Suppliers and Calculators#
Every material in a drilling fluid has its own page with grade, function, dose and safety, and the supplier, service and calculator sections turn that reference into a quote, a service request or a calculation.
No company is ranked, listed for payment or recommended anywhere in this reference. A mud engineer, also called a drilling fluid engineer, tests the fluid at the rig, maintains and calibrates the test equipment, advises on safe handling and waste disposal, and coordinates with the solids-control crew. The table below lists the reference sections outside the fluid chapters above.
| Section | What it holds |
|---|---|
| Drilling fluid materials A to Z | grade, function, dose and safety of every material |
| Comparisons | fluid systems and additives side by side |
| Drilling fluid companies | service companies, additive suppliers and how to compare them |
| Mud engineer | duties and how mud engineering services are scoped |
| Drilling fluid calculators and quote tools | mud weight, hydrostatic pressure, equivalent circulating density, volume and rheology calculators, and the quote request |
| Drilling fluids news | standards, discharge rules and additive supply |
| About | who publishes the reference and how it is funded |
Request supplier quotes
Send one specification to independent suppliers of drilling fluid materials.
Requests are forwarded to independent suppliers and service providers. No company pays to be listed or ranked here; see the editorial policy.
Where Drilling Fluid Ends: Neighboring Fields#
This reference stops where the drilling fluid leaves the hole, and two neighboring fields share some of its materials. The two fields are listed below.
- Completion and production chemistry is a separate field.
- Well cementing is a separate field; cement contamination of the mud stays in as a mud problem.
Everything the fluid carries out of the hole stays in this reference: drill cuttings, drilling waste, discharge and notification rules, and contamination of the mud by cement, salt and gases.
How this reference checks every figure#
Every number in this reference traces to a standard, a regulation, a government agency such as the USGS or the EIA, a peer-reviewed or AADE/SPE paper, a named handbook or a named trade journal, with its edition and date. The reference textbook behind the definitions is Caenn, Darley and Gray, Composition and Properties of Drilling and Completion Fluids, 7th edition (2017). No product value appears without the product and the data sheet it comes from.