Viscosifiers such as bentonite, attapulgite, organoclay, xanthan gum and guar gum give a drilling fluid the viscosity and gel structure that lift cuttings and suspend barite. API Spec 13A specifies six viscosifier materials, three of them bentonite grades, and leaves every other viscosifier to the product data sheet.
Viscosity carries cuttings up the annulus while the pumps run, and gel structure holds barite and drilled solids when circulation stops. Every drilling fluid needs both, so the practical decision is the viscosifier that suits the base fluid, salinity and temperature.
Ten materials and classes are compared below, with published temperature data from xanthan gum degrading above 248 degF (120 degC) to attapulgite tested at 428 degF (220 degC); no US discharge limit names a viscosifier.
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 a Viscosifier in a Drilling Fluid?#
A viscosifier is a drilling fluid additive that raises viscosity and gel structure so the fluid lifts cuttings out of the hole and keeps barite and drilled solids suspended when circulation stops. The class spans clays such as bentonite and attapulgite, biopolymers and cellulose ethers such as xanthan gum and hydroxyethyl cellulose (HEC), and synthetic liquid rheology modifiers for oil-based muds.
Additives are named by function, so one material can serve two classes. Bentonite builds viscosity and also forms a thin filter cake, which places it among the drilling fluid additives that control filtration as well as among the viscosifiers.
Fluid-loss and shale polymers that also viscosify#
Two polymer families raise viscosity as a side effect of their main class, listed below.
- Cellulose fluid-loss polymer: regular and high-viscosity grades of polyanionic cellulose (PAC) also viscosify a water-based mud, while low-viscosity grades add little viscosity.
- Shale encapsulator: partially hydrolyzed polyacrylamide (PHPA) extends bentonite in low-solids muds and viscosifies clear-water and coring fluids.
Neither family is sold as a viscosifier, and PHPA carries no API or ISO specification.
How Do Viscosifiers Build Viscosity and Gel Structure?#
Viscosifiers build viscosity in five ways: platelet hydration (bentonite), needle entanglement (attapulgite and sepiolite), swelling in oil (organoclay), charge bridging (mixed metal hydroxide) and polymer networks (xanthan gum and other polymers). Bentonite platelets hydrate in fresh water and associate edge to face, so salt and hardness defeat them. Attapulgite and sepiolite needles entangle without hydrating, so salt leaves them unaffected.
Mixed metal hydroxide (MMH) is cationic: it bridges bentonite platelets into a shear-thinning network with very high, fragile gels, a high yield point and a low plastic viscosity. Xanthan gum gives a high low-shear-rate viscosity and strong shear thinning, scleroglucan forms a rigid nonionic triple helix, and HEC gives viscosity with essentially no gel strength.
Gel holds solids at rest, and viscosity carries them while the mud is pumped; both are read on the rotational viscometer as part of drilling fluid rheology.
Viscosifier Materials Compared: Chemistry, Base Fluid, Dose, Temperature and Specification Status#
Ten viscosifiers are in drilling use: five clays and clay-like minerals, four biopolymers and cellulose ethers, and one synthetic class, the polymeric rheology modifiers of invert-emulsion muds. The table below compares them by chemistry, base fluid, systems, dose, temperature data and API Spec 13A status.
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| Material | Chemistry and base fluid | Systems | Dose (basis, source) | Temperature data (test condition or limit, source) | API 13A status |
|---|---|---|---|---|---|
| Bentonite | Sodium montmorillonite-rich smectite clay, CAS 1302-78-9; fresh water | Freshwater water-based mud, spud mud, water wells, HDD, foundations; prehydrated for salt muds | Product directions | Gels at high temperature (2025 Stanford Geothermal Workshop paper) | Three grades: bentonite, non-treated bentonite, OCMA-grade bentonite |
| Attapulgite | Fibrous magnesium-aluminum silicate (palygorskite), CAS 12174-11-7; fresh or salt water | Salt-water and saturated-salt mud; low-bentonite geothermal mud | Product directions | Suspensions of 2 to 8 % by mass tested to 428 degF (220 degC); viscosity there 72 to 92 % below the 104 degF (40 degC) value (Scientific Reports 2025) | API 13A grade |
| Sepiolite | Fibrous hydrated magnesium silicate, Mg4(OH)2(Si2O5)3·6H2O (idealized), CAS 63800-37-3 | Salt-water and high-temperature clay-free water-based mud | Product directions | Clay-free lab fluid aged 15 days at 428 degF (220 degC), filtrate 9 mL (Wang et al. 2023, Polymers) | API 13A grade |
| Organoclay | Bentonite, hectorite, attapulgite or sepiolite treated with a quaternary amine; CAS depends on amine and clay (68953-58-2 for the bentonite salt); oil | Oil-based, synthetic-based and HPHT invert-emulsion muds | 7 lb/bbl (20.0 kg/m3) in a 15 lb/gal (1.80 sg; 1,797 kg/m3) diesel invert-emulsion lab mud (Mahmoud et al., Scientific Reports 2024) | Lab mud aged and tested at 275 degF (135 degC) (Mahmoud et al. 2024); CTAB-modified montmorillonite below 302 degF (150 degC) and organo-sepiolite oil fluids losing rheology above 356 degF (180 degC) (2023 review) | No API 13A grade |
| Mixed metal hydroxide (MMH) | Cationic mixed metal hydroxide; used with bentonite in fresh water | Bentonite-MMH freshwater and HDD fluids | Product directions | Product-specific | No API 13A grade |
| Xanthan gum | Anionic microbial polysaccharide from Xanthomonas campestris, CAS 11138-66-2; fresh water, seawater, monovalent brine | Low-solids and clay-free water-based mud, drill-in fluids | Product directions | Degrades above 248 degF (120 degC) (2025 review) | API 13A grade: drilling-grade xanthan gum |
| Scleroglucan | Nonionic fungal polysaccharide (Sclerotium species), triple helix, CAS 39464-87-4 | Water-based mud and drill-in fluids | Product directions | Viscosified best of diutan, xanthan gum and scleroglucan at 212 to 284 degF (100 to 140 degC) in solution tests (Gao et al. 2023, Polymers) | No API 13A grade |
| Guar gum | Nonionic plant galactomannan, CAS 9000-30-0; fresh water | Freshwater polymer fluids for water wells, core and seismic holes | Product directions | Native guar tolerates 248 degF (120 degC) and above poorly (Gels 2025) | No API 13A grade |
| Hydroxyethyl cellulose (HEC) | Nonionic cellulose ether, CAS 9004-62-0; brines | Brine and drill-in fluids | Product directions | Product-specific | No API 13A grade |
| Polymeric rheology modifiers | Proprietary liquid polymers; oil | Flat-rheology oil-based and synthetic-based muds for HPHT wells and deepwater drilling fluids | Product-specific | Product-specific | No API or ISO specification |
Temperature data are test conditions or study findings with their source, not product ratings.
Each material earns its place through one property, listed below.
- Bentonite: the freshwater viscosifier and filter-cake former of water-based muds, prehydrated in fresh water for salt muds, never in seawater.
- Attapulgite: the salt-water gel that gives lift and barite suspension in salt and saturated-salt muds.
- Sepiolite: a needle clay for salt-water and high-temperature water-based muds.
- Organoclay: an amine-treated clay that viscosifies and gels oil-based and synthetic-based muds.
- Mixed metal hydroxide: a cationic gellant used with bentonite for gravel and cobble.
- Xanthan gum: the biopolymer of low-solids, seawater, brine and drill-in fluids.
- Scleroglucan: a fungal biopolymer with better temperature and salinity tolerance than xanthan gum.
- Guar gum: a self-degrading viscosifier for top-hole, water-well, core and seismic shot-hole drilling.
- Hydroxyethyl cellulose: a clay-free brine and drill-in viscosifier, seldom used in drilling fluids.
- Polymeric rheology modifiers: proprietary liquid polymers that give invert-emulsion muds a flat rheology profile.
One dose in the table comes from a published paper, the organoclay lab mud. Every other dose is a product value set by the supplier's data sheet.
How to Choose a Viscosifier by Base Fluid, Salinity and Temperature#
Bentonite is the default viscosifier of freshwater mud; salt-saturated water moves the choice to attapulgite or sepiolite, clay-free brines to xanthan gum, scleroglucan or hydroxyethyl cellulose, and oil to organoclay or a polymeric rheology modifier. Seven well conditions decide the choice, listed below with the viscosifier each one favors.
- Fresh water and new mud: bentonite, the base viscosifier of water-based mud.
- Salt-saturated water: attapulgite or sepiolite, or bentonite prehydrated in fresh water, in salt muds.
- Clay-free low-solids fluids: xanthan gum in polymer drilling fluids.
- Brine drill-in fluids: xanthan gum, HEC or scleroglucan in reservoir drill-in fluids.
- Gravel and cobble in horizontal directional drilling: bentonite with MMH in HDD drilling fluids.
- Water wells that must clean up: guar gum, which breaks down by design and places water well drilling mud on the polymer side of bentonite vs polymer drilling fluids.
- Oil or synthetic base fluid: organoclay or a polymeric rheology modifier, described in the next section.
Salt divides the class: the hydrating clay fails in it, while needle clays and nonionic polymers work through it. The table below compares what each material tolerates and what limits it.
| Material | Tolerates | Limited by |
|---|---|---|
| Bentonite | Fresh water; salt muds only when prehydrated in fresh water | Salt, hardness ions, temperature, bacteria and shear (treated grades) |
| Attapulgite | Salt and saturated-salt water; needs no hydration | No filtration control |
| Sepiolite | Salt water, independent of salinity; high-temperature water-based mud | No filtration control |
| Organoclay | Oil and synthetic base fluids | Loss of the amine surfactant from the clay at high temperature (2023 review) |
| Mixed metal hydroxide | Fresh water with bentonite | Anionic deflocculants and small anionic polymers: polyphosphates, lignosulfonate, lignite |
| Xanthan gum | Salt, seawater and monovalent brines; fair hardness tolerance | Extreme hardness or pH; bacteria, so a biocide is needed |
| Scleroglucan | Salinity and temperature better than xanthan gum; viscosity varies less than 15 % at pH 3 to 10 (Gao et al. 2023, solution tests) | Limits per product data sheet |
| Guar gum | Fresh water | Salinity of 12 % NaCl by mass and above (Gels 2025); enzymes, bacteria and oxidizers, which break it deliberately |
| Hydroxyethyl cellulose | High salinity; hardness ions cause no precipitation; common bacteria do not degrade it | Essentially no gel strength and little fluid-loss control |
Filtration control beside these viscosifiers comes from fluid loss control additives.
Temperature narrows the choice further. Xanthan gum, guar gum, diutan, MMH and partially hydrolyzed polyacrylamide degrade virtually completely over extended time at 300 degF (149 degC) (GRC Transactions 40, 2016). Geothermal muds keep bentonite below 10 lb/bbl (28.5 kg/m3), because bentonite gels at high temperature, and add attapulgite or sepiolite, which lack that gelation (2025 Stanford Geothermal Workshop paper). The same degradation makes temperature data a selection criterion for HPHT drilling fluids.
What Is an OBM Viscosifier?#
An OBM viscosifier is an additive that builds viscosity and gel in the oil phase of an invert-emulsion mud, and two types are in use: organoclay (organophilic clay) and polymeric rheology modifiers. Organoclay is bentonite, hectorite, attapulgite or sepiolite treated with a quaternary fatty amine so that it disperses in oil. Plate clays build rheology and help the filter cake, while rod clays gel and shear down.
Organoclay gel depends on the amine surfactant staying on the clay: a 2023 review ties the high-temperature rheology loss of organo-sepiolite oil fluids to surfactant desorption.
Polymeric rheology modifiers are proprietary liquid polymers that raise the yield point, the 6 and 3 r/min readings and the gels with little rise in plastic viscosity. Their associative structure depends less on temperature than organoclay gel, so a flat rheology profile, with yield point, low-end readings and gels relatively constant over a wide range of temperature and pressure, needs less clay. No API or ISO specification covers them, and their dose in oil-based mud is product-specific.
Which Viscosifiers Have an API Specification?#
API Spec 13A specifies six viscosifier materials: bentonite, non-treated bentonite, OCMA-grade bentonite, attapulgite, sepiolite and drilling-grade xanthan gum. Organoclay, mixed metal hydroxide, guar gum, HEC, scleroglucan and polymeric rheology modifiers have no API 13A grade.
The specification in force is 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. ISO 13500:2008 with Cor 1:2009 and Amd 1:2010 is the ISO counterpart.
In the 18th edition (2010) text, bentonite, non-treated bentonite, OCMA-grade bentonite, attapulgite and sepiolite are Sections 9 to 13, and drilling-grade xanthan gum is Section 19. In the 19th edition, OCMA-grade bentonite is Section 19 (Addendum 1, 2020) and drilling-grade xanthan gum is Section 18 (Errata 1, 2024). OCMA-grade bentonite and API standard evaluation base clay are different materials, although both appear in API Spec 13A.
How Viscosifier Performance Is Checked in the Mud#
Viscosifier performance is checked on the rotational viscometer at 600, 300, 6 and 3 r/min and by the 10-second and 10-minute gel strengths. Testing follows API RP 13B-1 (6th edition, 2026) for water-based fluids and API RP 13B-2 (6th edition, 2023) for nonaqueous fluids, and API RP 13D (8th edition, 2026) covers rheology and hydraulics. The Marsh funnel adds a quick field check of funnel viscosity.
Rheology is read at 120 degF (48.9 degC), or 150 degF (65.6 degC) for high-temperature wells (IADC Drilling Manual). Plastic viscosity and yield point come from the 600 and 300 r/min readings, and a gel strength is the maximum 3 r/min reading after the mud rests 10 seconds or 10 minutes.
Xanthan gum and polymeric rheology modifiers move the low-end readings and gels more than plastic viscosity, so their effect shows in the gel strength of drilling fluids and the 6 and 3 r/min readings.
Offshore Discharge Rules for Viscosified Muds#
No US offshore discharge limit names a viscosifier; 40 CFR 435.13 regulates the whole water-based mud and its cuttings, whatever clay or polymer viscosifies them. On the US Outer Continental Shelf, water-based mud and cuttings may be discharged only beyond 3 miles (4.8 km) from shore, with a suspended particulate phase 96-hour LC50 of at least 3 % by volume, no free oil and no diesel oil.
Organoclay muds are non-aqueous, so the whole fluid is not discharged, and base fluid on cuttings is limited to 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). In the North-East Atlantic, OSPAR Decision 2000/3 allows no diesel-based fluids and no discharge of cuttings carrying more than 1 % oil by weight on dry cuttings, since 2001.
Organoclays are not on the PLONOR list by name (OSPAR Agreement 2013-06, Update 2026). The US offshore discharge rules under 40 CFR 435 apply to the mud and cuttings, not to the viscosifier.
Viscosifier Suppliers, Grades and Purchase Documents#
Viscosifiers are bought by grade and document: the six API 13A materials with a certificate of analysis, and organoclay, MMH, guar gum, HEC, scleroglucan and rheology modifiers by product data sheet.
A request names material and grade, quantity in sacks, t or bbl, packaging, delivery point and documents, such as the certificate of analysis, the safety data sheet and the API Monogram where relevant.
Quotes compare only on the same grade, documents and delivery point, the basis for weighing drilling fluid additive suppliers.
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Viscosifiers and the Other Drilling Fluid Additive Classes#
Viscosifiers set viscosity and gel, so every viscosified mud also depends on fluid-loss additives for the filter cake, thinners for over-gelled mud and weighting agents for density. Attapulgite and sepiolite build no filter cake, so a fluid-loss additive works beside them.
Viscosifiers of completion and workover fluids belong to completion chemistry, a separate field. Viscosity-modifying admixtures for concrete belong to concrete technology, also a separate field.
Why weighted muds need a viscosifier#
Weighted muds need a viscosifier because weighting agents are inert solids that settle unless the gel structure of the mud holds them up. Organoclay and attapulgite build the gel that suspends barite and the other weighting agents for drilling fluids in oil and salt-water muds.
Fine spherical manganese tetroxide and micronized ilmenite stay suspended at a lower gel structure, which lowers the equivalent circulating density (ECD) of the weighted mud.
Why viscosifiers and thinners work against each other#
Viscosifiers and thinners work against each other because thinners disperse the gel structure that viscosifiers build. Condensed phosphates thin bentonite support fluids for re-use and break mud cake in well development (Water Well Journal 2024).
Mixed metal hydroxide is incompatible with anionic deflocculants and small anionic polymers such as polyphosphates, lignosulfonate and lignite. MMH muds and these drilling fluid thinners and deflocculants do not mix.