What API 13A is and why it matters
API 13A drilling clays are the bentonite and attapulgite grades defined in API Specification 13A, the American Petroleum Institute standard for drilling fluid materials. The specification sets minimum physical performance requirements and describes the laboratory procedures used to check them, so that a mud engineer can compare material from different sources on the same basis.
API 13A covers more than clays. It also includes weighting materials such as barite and haematite, and other products such as sepiolite, starch and carboxymethylcellulose. This guide focuses on the three clay sections buyers specify most often: Section 9 bentonite, Section 11 OCMA-grade bentonite and Section 12 attapulgite.
The standard is revised periodically, and section numbers and limits can change between editions. A purchase specification should name the edition that applies, and the supplier should report test results against that same edition.
The three clay sections at a glance
Each section relies on similar core tests but sets different pass limits. Attapulgite is tested in saturated salt water rather than deionised water, because that is the environment it is chosen for. The table summarises the API 13A requirement for each section.
Only the requirements discussed in this guide are shown. Confirm the full list of limits, and any additional requirements, against the edition named in your purchase order.
| Parameter | Section 9 Bentonite | Section 11 OCMA-grade bentonite | Section 12 Attapulgite |
|---|---|---|---|
| Test fluid | Deionised water | Deionised water | Saturated salt water |
| Viscometer dial reading at 600 r/min | ≥ 30 | ≥ 30 | ≥ 30 |
| Yield point / plastic viscosity ratio | ≤ 3 | ≤ 6 | — |
| Filtrate volume | ≤ 15.0 cm³ | ≤ 16.0 cm³ | — |
| Residue > 75 µm | ≤ 4.0 % by mass | ≤ 2.5 % by mass | ≤ 8.0 % by mass |
| Moisture | — | — | ≤ 16.0 % by mass |
What the test parameters tell you
Each test answers a practical question about how the clay will behave in a mud system. Knowing what they measure helps a buyer read a Certificate of Analysis with understanding, rather than simply looking for the word pass.
- Viscometer dial reading at 600 r/min: taken on a direct-indicating rotational viscometer after the suspension has been prepared and aged. A reading of at least 30 shows that the clay builds enough viscosity at the specified concentration to carry cuttings.
- Yield point / plastic viscosity ratio: yield point reflects the attractive forces between particles, while plastic viscosity reflects mechanical friction between solids. A ratio limit helps exclude material that relies heavily on additives to reach its viscosity target. Section 9 sets a tighter limit than Section 11.
- Filtrate volume: measured in a low-pressure filter press. It shows how well the clay forms a thin, low-permeability filter cake on the borehole wall, which limits fluid loss into permeable formations.
- Residue greater than 75 µm: the mass retained on a 75 µm sieve after wet sieving. High residue usually means grit, sand or poorly ground material that adds no useful viscosity and increases wear on pumps and solids-control equipment.
- Moisture (attapulgite): free moisture affects how much active clay is delivered per tonne, and how the powder flows and stores.
Fresh water versus salt water
The most important practical difference between these clays is how they respond to salinity. Bentonite is a smectite clay. Its platelets hydrate and swell in fresh water, and that hydration produces both viscosity and a tight filter cake. Dissolved salts, and calcium and magnesium in particular, suppress this hydration. In high-chloride make-up water, untreated bentonite yields little viscosity and its filtration control falls away.
Attapulgite works in a different way. Its needle-shaped particles do not depend on swelling. Viscosity comes from shearing the needle bundles apart so that they form a random, interlocking lattice that traps water. Because this mechanism is largely mechanical, attapulgite builds viscosity in fresh water, sea water and saturated brine alike. This is why API 13A tests Section 12 attapulgite in saturated salt water.
The trade-off is filtration. Attapulgite contributes little to filter cake quality, so salt-water muds built on it normally rely on starch or polymer additives for fluid-loss control. Where only saline water is available, some operators prehydrate bentonite in fresh water before adding it to the system. That adds a handling step, and its benefit declines as salinity rises.
How to choose the right grade
The choice usually follows three things: the make-up water, the well programme and the specification written into the drilling contract.
- Section 9 bentonite suits fresh-water spud muds and general water-based systems where viscosity, gel strength and filtration control are all needed from one material.
- Section 11 OCMA-grade bentonite suits contracts and operator specifications that call for it by name. Its YP/PV and filtrate limits are less stringent than Section 9, while its residue limit is tighter. It is often used in less demanding water-based applications.
- Section 12 attapulgite suits salt-water, sea-water and saturated-salt systems, and drilling through salt formations, where bentonite will not hydrate properly.
- Combinations are common: a programme may use attapulgite for suspension and prehydrated bentonite or polymer for filtration control.
- For oil-based and synthetic-based muds, neither clay works on its own. Organophilic clays, also called organoclays, are used instead.
What to ask a supplier for
A compliant grade is only as useful as the evidence behind it. Before placing an order, ask for the following.
It is also good practice to run your own API 13A tests on the first delivery, or to commission an independent laboratory, and compare the results with the supplier's CoA. Keep a retained sample from each approved lot for later comparison.
- A batch Certificate of Analysis (CoA) reporting each API 13A parameter with the measured value, the limit and the edition of the specification used.
- The name of the laboratory that carried out the tests, and its accreditation status.
- A Technical Data Sheet stating typical properties and guaranteed limits.
- A Safety Data Sheet (MSDS/SDS) covering handling, dust control and storage.
- A representative pre-shipment sample, and confirmation that retained samples are kept for each lot.
- Batch numbers printed on every bag or FIBC so that test results can be traced to the material on site.
- Packaging details: bag type and weight, palletising and moisture protection.
- Agreement on third-party inspection or independent testing where the contract requires it.
How Obsidian Minerals can help
Obsidian Minerals manufactures and exports drilling clays from India. The range includes sodium bentonite drilling grade to API 13A Section 9, bentonite drilling grade to API 13A Section 11 (OCMA grade) and attapulgite to API 13A Section 12. Bentonite is produced at sites in Gujarat and Rajasthan, and attapulgite in Gujarat. An organoclay drilling fluid grade is available for oil-based systems.
Production follows ISO 9001 certified processes, with testing in an NABL-accredited laboratory. Each shipment is supplied with a batch Certificate of Analysis, Technical Data Sheet and MSDS/SDS. Material is packed in 25 kg or 50 kg HDPE or paper bags, or 1 MT and 1.2 MT jumbo (FIBC) bags, with custom or private-label packaging on request. Samples are available on request, and the applicable API 13A edition is confirmed at order.
Frequently asked questions
What is the difference between API 13A Section 9 and Section 11 bentonite?
Both are bentonite grades tested in deionised water, and both require a 600 r/min dial reading of at least 30. Section 9 sets tighter limits on the YP/PV ratio (≤ 3) and filtrate volume (≤ 15.0 cm³). Section 11 OCMA-grade bentonite allows a YP/PV ratio up to 6 and filtrate up to 16.0 cm³, but has a tighter residue limit of 2.5 % above 75 µm. The right choice depends on the operator or contract specification.
Why is API 13A attapulgite tested in salt water?
Attapulgite is selected mainly for saline drilling fluids, so it is tested in the conditions it will meet in service. Its needle-shaped particles build viscosity by mechanical dispersion rather than by swelling, which lets it perform in saturated brine where bentonite hydrates poorly. The API 13A requirement for Section 12 is a 600 r/min dial reading of at least 30 in saturated salt water.
Can bentonite be used in salt-water drilling mud?
Untreated bentonite gives limited viscosity and filtration control in high-salinity water, because dissolved salts suppress its hydration. Some operators prehydrate bentonite in fresh water and then add it to the saline system, which helps at moderate salinity. For sea-water or saturated-salt muds, API 13A Section 12 attapulgite is usually preferred for viscosity, with starch or polymers added for fluid-loss control.
What documents should come with an API 13A drilling clay shipment?
At minimum, expect a batch Certificate of Analysis with measured values for each API 13A parameter, a Technical Data Sheet and a Safety Data Sheet. The CoA should state the specification edition, the test laboratory and the batch number, which should match the markings on the bags. Many buyers also request a pre-shipment sample and, for larger contracts, independent third-party testing.