Bentonite (CAS 1302-78-9) is the primary viscosifier and filtrate reducer of freshwater drilling fluids, and drilling mud takes 22 % of the bentonite used in the United States (USGS Mineral Commodity Summaries 2026). A freshwater drilling fluid relies on this clay to carry cuttings, hold solids in suspension and seal the borehole wall with a thin filter cake.
API Spec 13A defines three bentonite grades: bentonite, non-treated bentonite and OCMA-grade bentonite. The base grade needs a 600 r/min dial reading of at least 30, while commercial grades carry a yield rating, the barrels of 30-cP mud that one US ton of clay makes.
A new freshwater mud takes 13 to 22 lb/bbl (37.1 to 62.8 kg/m3) of bentonite, typical value of AQUAGEL (Baroid IDP, Halliburton) data sheet. Hardness, salt and heat set its limits, bentonite has no harmonised CLP classification, and Wyoming holds as much as 70 % of the world's known supply.
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.
The identity block below lists the names, registry numbers and grades of drilling-grade bentonite.
| Attribute | Value |
|---|---|
| Canonical name | Bentonite (drilling grade) |
| Synonyms | sodium bentonite, Wyoming bentonite, API bentonite, gel, drilling gel, sodium montmorillonite, bentonite clay |
| CAS number | 1302-78-9 |
| EC number | 215-108-5 |
| Formula | no fixed formula (natural clay, mainly montmorillonite) |
| Substance type | mineral/UVCB |
| Chemical class | clay mineral (montmorillonite-rich smectite clay) |
| API Spec 13A grades | bentonite, non-treated bentonite, OCMA-grade bentonite |
| Hazard line | No Annex VI entry; ECHA C&L notified: not classified (65.9 %) |
What Is Drilling-Grade Bentonite?#
Drilling-grade bentonite is a natural clay consisting primarily of sodium montmorillonite, ground to a powder and sold to API Spec 13A as bentonite, non-treated bentonite or OCMA-grade bentonite, or as a high-yield product rated in bbl/ton.
ECHA describes bentonite as a colloidal clay with no fixed formula, listed as a mineral of unknown or variable composition (UVCB). Packaged powder has a bulk density of 68 to 72 lb/ft3 (1,089 to 1,153 kg/m3), typical value of AQUAGEL (Baroid IDP, Halliburton) data sheet. The class page on bentonite and other drilling fluid viscosifiers sets the clay beside biopolymers and synthetic polymers.
Sodium montmorillonite, gel and the other names in a quote#
Drilling-grade bentonite appears in quotes and mud reports under seven names: gel, drilling gel, sodium bentonite, Wyoming bentonite, API bentonite, sodium montmorillonite and bentonite clay. Amine-treated bentonite is a different product, an organoclay for oil-based mud. Bentonite sold for waterproofing membranes, waterstops and grouting belongs to construction chemistry. Bentonite used as a coagulant aid belongs to water treatment.
Why Is Bentonite Used in Drilling Mud?#
Bentonite is used in drilling mud because one inexpensive clay builds viscosity, gel strength and a thin, low-permeability filter cake in fresh water, so the mud carries cuttings, suspends weighting material and limits filtrate loss.
The same clay also thickens lost-circulation pills, helps stabilize the hole and goes into salt muds as prehydrated slurry. The IADC credits adequate amounts of high-quality bentonite with the best filter cake a water-based mud can build.
How Does Bentonite Hydrate and Swell?#
Sodium montmorillonite platelets hydrate and swell in fresh water, and the dispersed platelets build viscosity and gel by edge-to-face association while lying flat in the filter cake.
Swelling and edge-to-face linking explain how bentonite shapes drilling fluid rheology: more hydrated platelets raise the viscosity and the gel structure of the mud. Hardness ions, salt, heat, bacteria and shear defeat the treated grades, so the clay performs only as well as its water allows.
Bentonite by fluid system#
Bentonite is the base clay of freshwater, spud and dispersed muds, is kept low in polymer muds and is avoided in potassium muds because of its affinity for potassium.
The table below compares the role of bentonite by fluid system.
| Fluid system | Bentonite role | Basis or note |
|---|---|---|
| Freshwater water-based mud | base viscosifier and filter cake | hydrates in soda-ash-treated water |
| Onshore spud mud | bentonite flocculated with lime | top-hole drilling |
| Low-solids polymer drilling fluids | kept low, extended by polymer | polymer carries part of the viscosity |
| Lignosulfonate dispersed muds | base clay, deflocculated | stable at 350 degF (177 degC), ferrochrome lignosulfonate system |
| Seawater and salt muds | prehydrated bentonite only | dry clay does not hydrate in salt water |
| KCl and other potassium muds | avoided | xanthan gum and PHPA carry the rheology |
| Weighted mud with barite | gel suspends the barite | barite adds density, not viscosity |
| Bentonite slurry as support fluids for bored piles | stabilizes the excavation | bored piles and diaphragm walls |
What Do the API Bentonite Grades Require?#
API Spec 13A sets four requirements for bentonite (the 600 r/min dial reading, the YP/PV ratio, the filtrate and the residue above 75 µm), each tested on 22.50 g of clay in 350 mL of deionized water.
The table below compares the requirements of the three bentonite grades with their test precision.
| Grade (clause) | Requirement | Limit | Repeatability / reproducibility |
|---|---|---|---|
| Bentonite (18th edition (2010) text, Section 9, Table 8) | 600 r/min dial reading | 30 min | 2.9 / 6.3 |
| Bentonite | YP/PV ratio | 3 max | 0.16 / 0.25 |
| Bentonite | Filtrate | 15.0 mL max | 0.8 / 1.3 mL |
| Bentonite | Residue above 75 µm | 4.0 % by mass max | 0.7 / 1.3 % |
| Non-treated bentonite (18th edition (2010) text, Section 10, Table 9) | YP/PV ratio | 1.5 max | 0.15 / 0.25 |
| Non-treated bentonite | Dispersed plastic viscosity | 10 cP (mPa.s) min | 2.1 / 3.1 cP |
| Non-treated bentonite | Dispersed filtrate | 12.5 mL max | 0.9 / 1.5 mL |
| OCMA-grade bentonite (19th edition Addendum 1 (2020), Section 19, Table 18) | 600 r/min dial reading | 30 min | 4.9 / 9.3 |
| OCMA-grade bentonite | YP/PV ratio | 6 max | 1.15 / 1.16 |
| OCMA-grade bentonite | Filtrate | 16.0 mL max | 0.8 / 1.6 mL |
| OCMA-grade bentonite | Residue above 75 µm | 2.5 % by mass max | 0.5 / 1.1 % |
Precision from Addendum 2 (2022), Tables B.3, B.4 and B.5. Suspensions are read at 77 degF (25 +/- 1 degC); bentonite is aged "up to 16 h" (18th edition, 9.3.4) and OCMA-grade bentonite "at least 16 h" (Addendum 1).
The governing text 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. API Spec 13A is a specification for manufacturers covering 13 materials; its ISO counterpart is ISO 13500:2008 with Cor 1:2009 and Amd 1:2010. The bentonite and non-treated bentonite limits carry the 18th-edition section numbers.
Bentonite, non-treated bentonite and OCMA-grade bentonite#
Non-treated bentonite adds dispersed tests to the base grade with a tighter YP/PV ratio, and OCMA-grade bentonite allows a higher ratio and filtrate but a lower coarse residue.
The non-treated test uses 25 g of clay in 350 mL of water, and the dispersed tests use sodium hexametaphosphate; the 25 g dose never applies to treated bentonite. One non-treated product shows pH 8.5 to 9.0, swell of 1,400 to 1,800 % and moisture of 7 to 12 % by mass, typical value of PRIME GEL (DMI) data sheet.
Is OCMA-grade bentonite still an API grade?#
Yes: Addendum 1 (April 2020) to the 19th edition adds OCMA-grade bentonite as Section 19, Table 18, and the 18th edition (2010) carried it as Section 11. OCMA-grade bentonite is a sales grade, not the API standard evaluation base clay (formerly OCMA base clay).
Test precision and reference bentonite#
Interlaboratory reproducibility of the bentonite 600 r/min reading is 6.3 dial units, so a lot that reads just above the minimum of 30 sits inside test precision (Addendum 2, 2022, Table B.3).
Laboratories check themselves on reference bentonite lot 6-2013, whose YP/PV ratio is 0.84 (0.59 to 1.09) in Table B.12, run at least once per 40 tests (Addendum 1). Conformity rests on a composite sample of no more than one day's production.
Bentonite Yield and Commercial Grades#
Bentonite yield is the number of barrels of 30-cP mud one US ton of clay makes; good bentonite yields at least 85 bbl/ton. Yield describes the clay and has nothing to do with the yield point of the mud, which is read in lbf/100 ft2.
A higher yield means less dry clay per barrel for the same viscosity. On the printed figures, a 220-yield clay needs 41 % of the dry mass of a 90-yield clay per bbl of mud.
The table below compares yield and dry clay per barrel by grade.
| Grade | Yield, bbl/ton | Yield, m3 per short ton (907 kg) | Dry clay per bbl of mud, lb/bbl (kg/m3) |
|---|---|---|---|
| Good bentonite (definition minimum) | 85 | 13.5 | 23.5 lb/bbl (67.1 kg/m3) |
| API Spec 13A and untreated bentonite | about 90 | 14.3 | 22.2 lb/bbl (63.4 kg/m3) |
| High-yield HDD grade, low end | 150 | 23.8 | 13.3 lb/bbl (38.0 kg/m3) |
| High-yield HDD grade, high end | 220 | 35.0 | 9.1 lb/bbl (25.9 kg/m3) |
Calculated from the yield with 0.1589873 m3 per bbl and 2,000 lb per short ton; mud at the 30-cP yield viscosity.
High-yield, 90-yield and prehydrated bentonite#
High-yield HDD bentonite grades reach 150 to 220 bbl/ton against about 90 bbl/ton for API Spec 13A and untreated bentonite, and the 90-yield grade sees little use in HDD (The Driller).
Mixed metal hydroxide is the exception: it works best with straight 90-yield bentonite (The Driller, 2017). One product reaches 220 bbl/ton (typical value of MAX GEL (Wyo-Ben) data sheet), the top of the high-yield range used in HDD drilling fluids. Prehydrated bentonite is any of these grades hydrated in fresh water before it enters a salt mud.
Bentonite Dose by Fluid System#
Bentonite dose rises from a topped-up freshwater mud through a new mud and caving formations to a lost-circulation pill, each range labeled by its data sheet or standard.
The table below lists bentonite doses by system or use.
| System or use | Dose, lb/bbl (kg/m3); lb/100 gal | Source |
|---|---|---|
| New freshwater mud | 13 to 22 lb/bbl (37.1 to 62.8 kg/m3); 31 to 52 lb/100 gal | typical value of AQUAGEL (Baroid IDP, Halliburton) data sheet |
| Caving formations | 25 to 35 lb/bbl (71.3 to 99.9 kg/m3); 60 to 83 lb/100 gal | typical value of AQUAGEL (Baroid IDP, Halliburton) data sheet |
| Lost-circulation pill | 30 to 40 lb/bbl (85.6 to 114.1 kg/m3); 71 to 95 lb/100 gal | typical value of AQUAGEL (Baroid IDP, Halliburton) data sheet |
| Added to an existing freshwater mud | 4 to 10 lb/bbl (11.4 to 28.5 kg/m3); 10 to 24 lb/100 gal | typical value of AQUAGEL (Baroid IDP, Halliburton) data sheet |
| HDD, normal drilling | 6.3 to 10.5 lb/bbl (18 to 30 kg/m3); 15 to 25 lb/100 gal | typical value of MAX GEL (Wyo-Ben) data sheet |
| API test suspension (test dose, not a field dose) | 22.5 lb/bbl (64.2 kg/m3) equivalent | API Spec 13A |
| Low-bentonite geothermal mud | below 10 lb/bbl (28.5 kg/m3) | Tuttle and Tate 2025 |
The kg/m3 values and the AQUAGEL lb/100 gal values are calculated from the printed lb/bbl.
The API test suspension grades the clay in the laboratory and is not a field dose. A low-solids polymer mud keeps bentonite low and a potassium mud avoids it, so the fluid system sets the dose before any data sheet does.
Doses quoted per 100 gallons of water#
Water-well and HDD crews quote bentonite per 100 gal of mixing water, and one HDD product lists 15 to 25 lb/100 gal for normal drilling (typical value of MAX GEL (Wyo-Ben) data sheet).
A barrel holds 42 US gal, so 1 lb/100 gal equals 0.42 lb/bbl and 1.1983 kg/m3. Crews mixing water well drilling mud measure their water in gallons and use the per-100-gal figure, while mud engineers on oil and gas rigs work in lb/bbl.
What Limits Bentonite in a Drilling Fluid?#
Bentonite is limited by the water it hydrates in and by heat: calcium and magnesium hardness and dissolved salt cut its swelling, and high temperature flocculates bentonite muds.
Soda ash answers hardness, prehydration in fresh water answers salt, and low-bentonite formulations answer heat.
Make-up water hardness and soda ash#
Soda ash treats make-up water to pH 8.5 to 9.5 before bentonite goes in (The Driller), at about 0.5 to 1 lb/100 gal (0.21 to 0.42 lb/bbl; 0.6 to 1.2 kg/m3) (Water Well Journal).
Bentonite then goes in through the hopper and mixes until it is hydrated, and polyanionic cellulose and other polymers follow. Calcium and magnesium hardness is the reason for the order, and soda ash removes that hardness and sets the pH before the clay arrives.
Salt, seawater and prehydration#
Salt water stops dry bentonite from hydrating, so bentonite for seawater and salt muds is prehydrated in fresh water treated with caustic soda and soda ash and then blended in.
Lignosulfonate is an optional addition to the prehydration tank. Bentonite is never prehydrated in seawater, and attapulgite or sepiolite replace it where the whole mud is salt water.
Temperature and high-temperature gelation#
A ferrochrome lignosulfonate bentonite mud stays stable at 350 degF (177 degC) and decomposes and flocculates seriously at 450 degF (232 degC) (Zilch et al. 1991, as cited in an AADE paper).
Low-bentonite muds below 10 lb/bbl (28.5 kg/m3) avoid high-temperature gelation and are proven above 680 degF (360 degC) bottom-hole temperature, with attapulgite or sepiolite as secondary viscosifiers (Tuttle and Tate 2025). These formulations are the basis of geothermal drilling fluids for the hottest wells.
Which Tests Verify Bentonite Quality?#
Three instruments run the four API Spec 13A bentonite tests: a viscometer for the 600 r/min reading and the YP/PV ratio, a low-pressure filter press for filtrate and a wet screen for residue above 75 µm.
The filter press runs at 100 psi (690 kPa), and the reported filtrate is twice the volume collected between 7.5 and 30 min. Once the clay is in the mud, field tests to API RP 13B-1 (6th edition, 2026) track it: the Marsh funnel, the viscometer gels and the methylene blue test, which reports reactive clay as lb/bbl bentonite equivalent.
The table below compares the tests that verify bentonite and the mud built with it.
| Test | What it measures | Method and edition |
|---|---|---|
| 600 r/min reading and YP/PV ratio, viscometer test | viscosity of the test suspension | API Spec 13A |
| Filtrate at 100 psi (690 kPa), filter press tests | mL of filtrate | API Spec 13A |
| Residue above 75 µm, API Spec 13A residue test | % by mass of coarse particles, wet screening | API Spec 13A |
| Funnel viscosity of the mud, Marsh funnel viscosity test | s/qt | API RP 13B-1, 6th edition, 2026 |
| Reactive clay, methylene blue test | lb/bbl bentonite equivalent | API RP 13B-1, 6th edition, 2026 |
| Gels of the mud, gel strength | lbf/100 ft2 after 10 s and 10 min | API RP 13B-1, 6th edition, 2026 |
Bentonite is the reference clay for filtration and fluid loss of drilling fluids, because its platelets lie flat and build a thin, low-permeability filter cake.
Is Bentonite Hazardous? Safety and Regulatory Status#
Bentonite has no harmonised CLP classification, and its hazard line reads: No Annex VI entry; ECHA C&L notified: not classified (65.9 %).
ECHA CHEM shows no REACH registration for bentonite (EC 215-108-5) (checked 2026-10-26). Drilling-grade bentonite is a fine 200-mesh powder, so mixing crews handle each product according to its own SDS.
Discharge and Water-Supply Rules for Bentonite#
No rule sets a limit for bentonite itself: US offshore rules regulate the water-based mud and cuttings it is part of, and water-supply rules regulate which additives enter a well with it.
Offshore discharge of bentonite muds#
40 CFR 435.13 (Subpart A, BAT) allows water-based mud and cuttings to be discharged only beyond 3 mi (4.8 km) from shore, with an SPP 96-h LC50 of at least 3 % by volume, no free oil and no diesel oil.
The table below compares the rules that apply to bentonite muds by jurisdiction.
| Jurisdiction | Instrument | What applies to bentonite muds |
|---|---|---|
| US Outer Continental Shelf, 40 CFR 435 | 40 CFR 435.13 (Subpart A, BAT) | whole-mud limits; Hg 1 and Cd 3 mg/kg dry weight in the stock barite of a weighted mud |
| US coastal waters | 40 CFR 435 Subpart D | no discharge except Cook Inlet |
| Minnesota, Minnesota well additive rule | Minnesota Rules ch. 4725 | drilling-fluid additives, bentonite included, meet ANSI/NSF Standard 60 |
| US EPA Region 4, mud rotary drilling | SESDGUID-101-R2 (effective 2018-01-16) | monitoring wells: potable water and pure bentonite only |
| Projects under FERC jurisdiction, FERC HDD guidance | FERC HDD guidance (October 2019) | SDS for every additive other than pure bentonite and water |
The mercury and cadmium limits cap the barite, not the clay, and section 435.13 sets no bentonite-specific value. The same section anchors all US offshore discharge rules for water-based mud, so a bentonite mud passes or fails as a whole fluid.
Bentonite in water-supply and monitoring wells#
Water-supply and monitoring wells take bentonite only as a certified product or as pure clay, under the Minnesota and US EPA Region 4 rows of the table above.
Region 4 calls mud rotary an undesirable method for monitoring wells. Certification is checked product by product, and the AQUAGEL (Baroid IDP, Halliburton) data sheet states NSF/ANSI 60 certification for that Wyoming sodium bentonite.
Bentonite Suppliers, Grades and Origin#
A bentonite quote names the grade, the quantity, the packaging, the delivery point and the documents, with the grade given as an API Spec 13A grade or as a high-yield product with its bbl/ton rating.
The five items of a bentonite specification are listed below.
- Grade: bentonite, non-treated bentonite, OCMA-grade bentonite or a high-yield product and its bbl/ton rating
- Quantity and unit: sacks, t or bbl
- Packaging: bulk, bulk bags or sacks
- Delivery point: country and port or site
- Documents: CoA, SDS, NSF/ANSI 60 certificate and API Monogram where relevant
A certificate of analysis (CoA) reports the lot against the requirement table above, and the API Monogram is voluntary licensing that needs an API Spec Q1 quality system. Buyers compare additive suppliers on these documents and on the yield rating of each high-yield grade.
Bentonite enters the United States duty-free under HTS 2508.10.00.00 (US International Trade Commission, HTS, 2026).
Where bentonite is mined: Wyoming and world production#
Wyoming holds as much as 70 % of the world's known bentonite supply (Wyoming State Geological Survey, citing the Wyoming Mining Association), and the United States led world bentonite production in 2025 (USGS MCS 2026, estimated).
The table below lists the USGS bentonite supply record.
| Item | Value | Year |
|---|---|---|
| World production | 20,000 kt (19,400 kt in 2024) | 2025e |
| United States | 4,100 kt (3,990 kt in 2024) | 2025e |
| India | 3,700 kt | 2025e |
| Turkey | 2,500 kt | 2025e |
| China | 2,100 kt | 2025e |
| Iran | 1,300 kt | 2025e |
| Greece (including crude ore) | 1,000 kt | 2025e |
| US exports (main destinations Canada, Japan and Mexico) | 640 kt | 2025e |
| US end use: drilling mud | 22 % | 2025e |
| US end use: pet waste absorbents | 48 % | 2025e |
USGS Mineral Commodity Summaries 2026; e = estimated. Analyst estimates of the drilling fluids market are a separate data set from these tonnages.
Bentonite unit values in the public record#
The USGS put the average unit value of bentonite at USD 110 per t ex-works in 2025 (estimated), a dated public-record value, not a quote for any delivery.
The table below lists USGS unit values.
| Year | USD per t, ex-works |
|---|---|
| 2021 | 100 |
| 2022 | 101 |
| 2023 | 103 |
| 2024 | 105 |
| 2025e | 110 |
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How Does Bentonite Compare with Other Viscosifiers?#
Bentonite is the reference clay against which attapulgite, sepiolite, xanthan gum, organoclay and mixed metal hydroxide are chosen, and polymers replace it at about 1/50 to 1/20 of its concentration where a clay-free fluid is needed.
The polymer ratio comes from foundation support fluids (CFMS conference paper). Uses outside drilling mud took 78 % of US bentonite in 2025, calculated from the USGS end-use share, and they are not covered here.
The full class of viscosifiers spans clays, biopolymers and synthetic polymers.
Salt-water clays and polymers as alternatives to bentonite#
Attapulgite and sepiolite replace bentonite in salt-water muds, organoclay replaces it in oil- and synthetic-based mud, and xanthan gum replaces it where a clay-free fluid needs low-shear viscosity.
The table below compares the alternatives to bentonite.
| Material | Chosen over bentonite for | API Spec 13A |
|---|---|---|
| attapulgite | salt-water and saturated-salt muds; no filtration control | 18th edition (2010) text, Section 12 |
| sepiolite | salt-water and high-temperature water-based mud | 18th edition (2010) text, Section 13 |
| xanthan gum | clay-free low-shear viscosity | drilling-grade xanthan gum |
| organoclay | oil- and synthetic-based mud | no section |
| mixed metal hydroxide | fragile high gels with bentonite, not instead of it; incompatible with polyphosphates, lignosulfonate and lignite | no section |
| polyanionic cellulose (PAC) | filtration control added to bentonite muds | PAC-LV and PAC-HV |
Bentonite builds viscosity and filter cake while barite only adds weight, as set out in barite vs bentonite in drilling mud, and whole-fluid choices are weighed in bentonite vs polymer drilling fluids.
Attapulgite and sepiolite lift and suspend cuttings by needle-shaped particles whose viscosity is independent of salinity, but neither clay controls filtration.