Bentonite is the clay that makes a water-based drilling mud behave like a drilling mud. Add it to water and the suspension thickens, holds cuttings when circulation stops, and lays a thin impermeable filter cake against the formation wall. Nothing else in the additive list does all three at once, which is why bentonite is the first thing in almost every water-based mud system and why the grade you buy decides how much of everything else you need.
RAHA Gilsonite Co. supplies drilling grade bentonite in three specifications, with a batch certificate of analysis on every shipment and the technical data sheet for each grade published below. This page explains what bentonite does inside the mud system, how the three grades differ in practice, what the specification figures actually mean, and how much you need per barrel.
What is bentonite?
Bentonite is a clay formed from weathered volcanic ash. Its active component is montmorillonite, a member of the smectite group, with the approximate formula Al2(Si2O5)2(OH)4·nH2O.
What makes montmorillonite unusual is its structure. The crystal is built from stacked plates held together loosely, and water can push its way between them. The plates separate, the particle expands, and a small mass of dry powder turns into a large volume of gel. Good sodium bentonite swells to many times its dry volume.
That single behaviour is the origin of every property the drilling industry buys bentonite for:
Swelling and hydration
Dry powder disperses into a stable colloidal suspension rather than settling out. This is what turns water into mud.
Viscosity and gel strength
The hydrated plates build a loose structure that carries cuttings up the annulus and holds them when the pumps stop.
Thixotropy
Gel at rest, fluid when sheared. The mud thickens the moment circulation stops and thins again the moment it restarts.
Filter cake formation
Plates deposit on the wellbore wall as a thin, low-permeability skin that limits fluid loss into the formation.
Ion exchange capacity
The surface carries exchangeable cations, which is both why sodium activation works and why salt and calcium contamination damages performance.
Environmental acceptance
An inert natural mineral, which is one reason water-based muds clear environmental approval more easily than oil-based systems.
What bentonite does in drilling mud
Drilling mud, or drilling fluid, is the only element in contact with the wellbore continuously from spud to completion. It carries cuttings to surface, cools and lubricates the bit, holds back formation pressure, and keeps the hole open. Bentonite is involved in all of those except cooling.
| Function | What bentonite contributes | What happens without enough of it |
|---|---|---|
| Cuttings transport | Raises viscosity so cuttings are carried rather than dragged | Cuttings fall back, the annulus packs off, torque and drag rise |
| Suspension at rest | Gel strength holds cuttings and weighting material in place when pumps stop | Barite sags and cuttings settle on the bit during a connection or trip |
| Fluid loss control | Builds a thin impermeable filter cake on the wall | Filtrate invades the formation; thick cake causes differential sticking |
| Wellbore stability | The cake plus hydrostatic head supports the formation face | Sloughing shale, washouts, hole collapse |
| Lubrication | The cake reduces metal-to-formation friction | Higher torque and drag, faster bit and casing wear |
The point most buyers miss. Bentonite is cheap relative to the additives that compensate for a poor one. Under-spec bentonite is usually corrected on the rig with extra polymer and extra fluid-loss additive, and the cost of those exceeds the saving on the clay many times over. The grade is a false economy to cut.
Sodium bentonite and calcium bentonite
Bentonite is classified by the dominant exchangeable cation sitting between the clay plates. That cation decides how far the plates can separate, and therefore how much the clay swells.
| Property | Sodium bentonite | Calcium bentonite |
|---|---|---|
| Swelling capacity | High — many times dry volume | Low |
| Viscosity built per tonne | High | Low |
| Thixotropy | Pronounced | Weak |
| Use in drilling mud | The standard choice | Only after sodium activation |
| Other uses | Drilling, civil engineering, sealing | Bleaching earth, foundry, absorbents |
Calcium bentonite can be converted by treating it with soda ash, which exchanges calcium for sodium. The result is called sodium-activated or peptized bentonite. It works, and a large share of the world’s drilling bentonite is produced this way, but an activated clay is generally less tolerant of contamination than a naturally sodic one and can lose viscosity over time in storage.
If a supplier quotes an unusually low price, ask whether the material is natural sodium bentonite or activated calcium bentonite, and ask to see the viscometer reading on the batch certificate rather than only on the data sheet.
Drilling bentonite grades and specification
We supply three grades. The figures below are taken directly from our technical data sheets, which you can download in full further down this page.
| Parameter | OCMA grade | Standard drilling grade | High yield |
|---|---|---|---|
| Viscometer reading at 600 rpm | 30 minimum | 40 minimum | 60 minimum |
| Plastic viscosity | 5 minimum | 5 minimum | 4 minimum |
| Yield point / plastic viscosity ratio | 6 maximum | Not specified | Not specified |
| Residue greater than 200 mesh (75 µm) | 6 % maximum | 3 % maximum | 6 % maximum |
| Moisture | 12 % maximum | 12 % maximum | 12 % maximum |
| Data sheet | Download PDF | Download PDF | Download PDF |
What the figures mean
Four numbers describe a drilling bentonite, and they are not independent of one another.
| Parameter | What it measures | Why it matters on the rig |
|---|---|---|
| 600 rpm reading | Viscosity built by a standard suspension of the clay in water | The headline number. A higher reading means fewer tonnes to reach the same mud viscosity |
| Plastic viscosity | Resistance to flow from particle-to-particle friction | Too low and the mud carries poorly; too high and pump pressure and bit hydraulics suffer |
| YP / PV ratio | The balance between structure and internal friction | A guard against a clay that gels so strongly it becomes hard to break circulation |
| Residue > 200 mesh | Grit and unground material left on a 75 µm sieve | Contributes no viscosity, wears pumps and bit nozzles, and loads the solids control equipment |
| Moisture | Water already in the powder as shipped | You pay freight on it, and a damp bag cakes in storage and will not disperse |
The values in the grade table are the guaranteed limits on the data sheet, not the measured result of a particular batch. Every shipment leaves with a batch certificate of analysis giving the actual figure against each test. Test definitions follow the standard drilling fluid procedures published by the American Petroleum Institute.
Which grade to ask for
If your contract or tender names OCMA, that decides it. OCMA is the specification written for the European market and a certificate against any other standard will not satisfy a tender that names it.
If no standard is named and the work is oil and gas drilling, the standard drilling grade is the usual answer. It reads 40 at 600 rpm against OCMA’s 30, and its residue limit is tighter at 3 % against 6 %, so it builds more viscosity per tonne and puts less grit through the solids control equipment.
If the work is water well drilling, horizontal directional drilling or piling, high yield is normally the cheapest answer delivered. At a minimum of 60 at 600 rpm it builds roughly twice the viscosity per tonne of an OCMA grade, so freight and handling per barrel of mud fall sharply. It is polymer-extended, which is why it is not offered against OCMA.
Send us the tender wording, not the grade name. Specifications are often written loosely, and the difference between a material that passes technical evaluation and one that does not is usually a single line in the tender. If you send us the wording at the enquiry stage we will tell you which grade satisfies it before you commit.
Mixing rates and field use
Bentonite must be added slowly through a hopper into shearing water. Dumped in, it forms fish-eyes — lumps with a hydrated skin and dry powder inside — which never yield their viscosity and simply end up on the shakers.
| Application | Typical concentration | Notes |
|---|---|---|
| Spud mud | 20–30 lb/bbl (57–86 kg/m³) | Top hole, where hole cleaning matters more than fluid properties |
| Prehydrated bentonite slurry | 25–35 lb/bbl (71–100 kg/m³) | Mixed in fresh water and left to hydrate before adding to the active system |
| Maintenance in a water-based system | 10–20 lb/bbl (29–57 kg/m³) | Adjusted against the mud report rather than a fixed figure |
| HDD and water well | 15–50 lb/bbl | Wide range; depends on formation and whether the high yield grade is used |
These are starting points, not a mud programme. Final concentrations come from the mud engineer against the formation, the hole size and the mud report. A high yield grade reaches the same viscosity at a lower concentration than the figures above.
Prehydration, and why it matters in salty water
Bentonite hydrates properly only in fresh water. Calcium and salt collapse the plate spacing and the clay never develops its viscosity — mix it directly into seawater or a brine and most of the tonne is wasted. The standard answer is prehydration: make the slurry up in fresh water, let it hydrate fully, then add that slurry to the salty system. The plates are already separated and hold their structure.
Where the mud is heavily contaminated with calcium or salt, bentonite stops being the right viscosifier at all and polymer products take over. See PAC LV and PAC HV for filtration control, CMC as a lower-cost alternative, and KCL for shale inhibition.
Water-based and oil-based drilling mud
Bentonite belongs to water-based systems. In an oil-based mud the continuous phase is oil, water-swelling clay has nothing to swell in, and organophilic clays do the viscosifying instead.
| Feature | Water-based mud | Oil-based mud |
|---|---|---|
| Primary viscosifier | Bentonite | Organophilic clay |
| Cost | Lower | Higher |
| Environmental handling | Simpler disposal and approval | Restricted; cuttings need treatment |
| Lubricity | Moderate | High |
| Shale stability | Sensitive to reactive formations | Superior |
| Temperature tolerance | Lower | Higher |
Most wells are drilled water-based wherever the formation allows it, on cost and environmental grounds, and switch to oil-based only where reactive shale or high temperature forces the change.
Where drilling bentonite is used
Oil and gas drilling
The core market. Spud mud, prehydrated slurry and maintenance additions in water-based systems.
Water well drilling
High yield grades dominate, because freight per tonne is a larger share of delivered cost on small jobs.
Horizontal directional drilling
HDD bores rely on bentonite for cuttings removal and for holding an unsupported bore open over long crossings.
Piling and diaphragm walls
Bentonite slurry supports the excavation face until concrete is placed.
Tunnelling and pipe jacking
Face support in slurry shield machines, and lubrication of the pipe string.
Grouting and sealing
Permeation grouts, cut-off walls and well abandonment plugs where a low-permeability seal is required.
Packing, storage and shipping
| Packing | Unit weight | Notes |
|---|---|---|
| Paper or PP bag | 25–30 kg | The standard for rig supply. Palletised and shrink wrapped |
| Jumbo bag | 1 MT | Lowest packing cost per tonne where a forklift or crane is available at the destination |
Storage
Bentonite has no meaningful shelf life as a mineral, but it is hygroscopic and that is the whole storage problem. A bag that picks up moisture cakes into lumps that will not disperse, and the clay is effectively lost even though nothing has chemically changed. Keep pallets off the ground, under cover, with the wrapping intact, and use older stock first. The data sheets allow up to 12 % moisture as shipped, so there is not much margin before damp storage pushes a batch out of specification.
Customs
Bentonite falls under HS 2508.10. It is not classified as dangerous goods for sea carriage, though the safety data sheet is normally required with the booking and by destination customs.
Quality control and certification
Every shipment leaves with a batch certificate of analysis giving the lot number, production date and each test with its method and measured result. Third-party inspection at loading by SGS, Bureau Veritas, Intertek or any surveyor named in your contract is accepted.
Technical data sheets
Bentonite OCMA
600 rpm 30 min · PV 5 min · YP/PV 6 max · residue 6 % max · moisture 12 % max. PDF.
Bentonite standard drilling grade
600 rpm 40 min · PV 5 min · residue 3 % max · moisture 12 % max. PDF.
Bentonite high yield
600 rpm 60 min · PV 4 min · residue 6 % max · moisture 12 % max. Product ID 31012. PDF.
Frequently asked questions about bentonite for drilling mud
What does bentonite do in drilling mud?
Four things. It raises viscosity so cuttings are carried to surface, it builds gel strength so cuttings and barite stay suspended when the pumps stop, it forms a thin impermeable filter cake that limits fluid loss into the formation, and that same cake stabilises the wellbore wall and reduces friction. No other single additive in a water-based mud covers all four.
What is the difference between the three bentonite grades?
The viscometer reading at 600 rpm, which is how much viscosity the clay builds. OCMA grade reads a minimum of 30, the standard drilling grade a minimum of 40, and the high yield grade a minimum of 60. The standard grade also has the tightest residue limit at 3 % against 6 % for the other two. All three are limited to 12 % moisture.
What is OCMA bentonite?
OCMA is the drilling fluid materials specification written for the European market, and a grade certified against it is what a tender naming OCMA requires. Our OCMA grade reads a minimum of 30 at 600 rpm with a plastic viscosity of 5 minimum, a yield point to plastic viscosity ratio of 6 maximum, residue above 200 mesh of 6 % maximum and moisture of 12 % maximum.
Should I use sodium or calcium bentonite for drilling?
Sodium. Sodium bentonite swells several times its dry volume and builds the viscosity, gel strength and filter cake a mud system needs. Calcium bentonite swells very little and is not usable as supplied. It can be converted by treating it with soda ash, which is called sodium activation, and a large share of commercial drilling bentonite is produced that way, but an activated clay is generally less tolerant of contamination than a naturally sodic one.
How much bentonite per barrel of drilling mud?
A spud mud is typically 20 to 30 lb/bbl, which is 57 to 86 kg per cubic metre. A prehydrated bentonite slurry runs higher, around 25 to 35 lb/bbl, and maintenance additions to an active system are lower, around 10 to 20 lb/bbl. A high yield grade reaches the same viscosity at a lower concentration. These are starting points only; the mud engineer sets the final figure from the formation, the hole size and the mud report.
What does the 600 rpm reading on a bentonite data sheet mean?
It is the dial reading of a standard rotational viscometer turning at 600 revolutions per minute in a suspension of the clay made up to a fixed concentration in water. It is the single best measure of how much viscosity a tonne of the clay will build. A grade reading 60 builds roughly twice the viscosity of one reading 30, so you need roughly half the tonnage for the same mud.
Can bentonite be used in oil-based mud?
No. Bentonite swells in water, and in an oil-based mud the continuous phase is oil, so there is nothing for it to hydrate in. Oil-based systems use organophilic clay — bentonite whose surface has been treated with an organic compound so it disperses in oil instead.
Why does bentonite not work in seawater?
Salt and calcium collapse the spacing between the clay plates, so the clay cannot swell and never develops its viscosity. Mixed straight into brine, most of the tonne is wasted. The standard solution is prehydration: make the slurry up in fresh water, let it hydrate fully, then add it to the salty system. Where contamination is severe, polymer viscosifiers replace bentonite altogether.
Why does the residue figure matter?
Residue above 200 mesh is grit and unground material that does not disperse. It builds no viscosity, so you are paying freight on dead weight, and it wears mud pumps and bit nozzles while loading the solids control equipment. Our standard drilling grade is limited to 3 % against 6 % for the OCMA and high yield grades.
What is the HS code for bentonite?
HS 2508.10, which covers bentonite as a clay. National tariff schedules add further digits, so confirm the full code with your customs broker before shipping.
What is the minimum order and how is bentonite packed?
One full 20 foot container. Bentonite ships in 25 to 30 kg paper or PP bags, palletised and shrink wrapped, or in 1 MT jumbo bags where handling equipment is available at the destination. Loading is from Jebel Ali or Mersin on FOB, CFR or CIF terms.
How should bentonite be stored?
Dry, under cover, pallets off the ground and wrapping intact. Bentonite does not age chemically, but it is hygroscopic: a bag that absorbs moisture cakes into lumps that will not disperse, and the material is effectively lost. The specification allows up to 12 % moisture as shipped, so damp storage pushes a batch out of specification quickly. Use older stock first.
Related drilling products and technical pages
Other additives in the water-based mud system: drilling grade barite as the weighting material, PAC LV and PAC HV for filtration control, CMC as a lower-cost filtration polymer, and KCL for shale inhibition. The full range is in our drilling fluid products.
Where a mud needs a sealing and stabilising additive rather than a viscosifier, see gilsonite in drilling fluids.
Request a quotation for drilling bentonite
Send us the quantity, the grade or the specification your contract names, the packing you need and the discharge port. If the tender wording is unclear about which standard applies, send us the wording itself and we will tell you which grade satisfies it before you commit. Batch certificate of analysis with every shipment, third-party inspection accepted at loading.
Dubai: No. 2305, Burlington Tower, Business Bay — +971 4 566 4998
Ankara: Karşıyaka District, 749 Street No. 30, Gölbaşı 06830 — +90 312 544 5481
