Commercial Bentonite

Commercial bentonites used in drilling muds are naturally occurring clays and are mined in many areas of the world. They contain the clay mineral smectite and may contain accessory minerals such as quartz, mica, feldspar and calcite.

Drilling Fluid Bentonites

Smectites are three-layer swelling clays of which the most predominant form is calcium, but also sodium. Because smectites have the ability to swell (hydrate) in the presence of water, these bento-nites are used to viscosify drilling fluids. Sodium bentonite, found exclusively in the northwestern part of the United States, is the most widely sought-after bentonite because its hydrational ability is considerably greater than that of calcium bentonite. Hence, the name "premium bentonite" is given to sodium bentonite. Because operational need for using bentonite varies around the globe, API established specifications to cover sodium and calcium bentonites.

Currently there are three API bentonites available (see Table 10). They are listed in order of degree of chemical treatment (beneficiation):

  • API Nontreated Bentonite (no treatment)
  • API Bentonite (some treatment level)
  • API OCMA Grade Bentonite (high treatment level)

Table 10

Comparison - API Bentonite Specifications


Nontreated Bentonite


OCMA Grade Bentonite

Water, cm3




Clay, g/350 cm3




Test for Spec:

600 rpm, minimum




YP/PV ratio, maximum




Filtrate Volume/30 min., maximum




+200 mesh, wt%, maximum




Moisture, wt%, maximum




Added SHMP to Slurry*:

PV, minimum

Filtrate Volume/30 min., maximum

10 12.5

Nontreated Bentonite, in its testing, is treated with sodium hexametaphosphate thinner (SHMP) to assure that it has not previously been treated during manufacture.

Nontreated Bentonite, in its testing, is treated with sodium hexametaphosphate thinner (SHMP) to assure that it has not previously been treated during manufacture.

Beneficiation is a process where chemicals are added to a low-quality clay to improve its performance. Soda ash is added for peptization (see Figure 15) and polymers are added to improve rheology and filtration control. All of this is done to make the clay pass API viscosity specifications.

Figure 15

Peptizing Calcium Clay to Sodium Clay

Figure 15

Peptizing Calcium Clay to Sodium Clay

Suspension Bentonite Viscosity

Bentonites that have been treated may exhibit poor performance in five respects:

  1. intolerance to hardness ions,
  2. incompatibility with other polymers in a mud,
  3. low thermal stability,
  4. limited shelf life, and
  5. polymer structure breakdown as it passes through the bit.

For some drilling operations, API or OCMA bentonite may be economical and not create problems. However, based on the five items above, their performance in many of the mud systems we currently use can be unpredictable and increase bentonite consumption.

API Bentonites

API Nontreated Bentonite - API Nontreated Bentonite is a premium quality material without any chemical additives for beneficiation. Being premium clays, they bring a premium price; however, they can be cost effective in terms of predictable performance and lower usage.

API Bentonite - API Bentonite is generally predominantly sodium bentonite; however, some degree of beneficiation is allowed under specifications for this material. The YP/PV ratio specification (3 maximum) limits this amount of beneficiation. Hence, API bentonite, under many conditions, performs similarly to the nontreated bentonite.

API OCMA Grade Bentonite - API OCMA Grade Bentonite is predominantly a calcium bentonite and as such, cannot meet specifications of either of the other two API-grade bentonites. Even with less stringent performance requirements, OCMA Grade bentonites require high levels of chemical treatment (beneficiation) which is reflected by the high YP/PV ratio (6 maximum) allowable. However, API OCMA Grade Bentonite is used in many areas where the drilling fluid will not encounter excessive temperature nor contamination, and where suspension and hole cleaning are the primary requirements.

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  • hollie
    What cations are in OCMA clay specs?
    7 years ago
  • simret
    Are ther any substutituts for bentonenite in drill fluid?
    7 years ago
  • jasmine
    How is beneficiation drilling fluid done?
    5 years ago
    What is yp test for bentonite?
    3 years ago
  • Kifle
    What is pv yp ratio for api bentonite?
    3 years ago
  • luigina
    What is beneficiation in drilling operation?
    1 year ago

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