API and OCMA are different reference envelopes
API drilling-grade and OCMA-grade bentonite use related but non-identical laboratory limits. The technically correct comparison is parameter by parameter rather than describing one grade as automatically better or stricter.
Rheology
Both references use R600 ≥ 30. The main rheological difference is the YP/PV limit: API uses ≤ 3, whereas OCMA uses ≤ 6.
For example, a YP/PV value of 3.43 falls outside the API-oriented ratio limit while remaining within the OCMA-oriented ratio limit.
Filtration
API uses a maximum filtrate volume of 15.0 mL; OCMA uses 16.0 mL. API is therefore slightly tighter for this parameter.
Coarse residue
For residue above 75 µm, API uses a maximum of 4.0%, while OCMA uses 2.5%. Here the relationship reverses and OCMA is tighter.
A residue result of 3.1% can therefore be within the API reference but outside the OCMA reference.
How to select the appropriate grade
Selection should follow the purchaser specification, tender requirement, drilling program, water chemistry, laboratory qualification protocol and required rheological and filtration performance. The grade name should never replace actual laboratory data.
| R600 | ≥ 30 |
|---|---|
| YP/PV | ≤ 3 |
| Filtrate | ≤ 15.0 mL |
| Residue >75 µm | ≤ 4.0% |
| R600 | ≥ 30 |
|---|---|
| YP/PV | ≤ 6 |
| Filtrate | ≤ 16.0 mL |
| Residue >75 µm | ≤ 2.5% |