Not all Bentonite is created equal

Yesterday I spent a couple of hours with an HDD (horizontal directional drilling) contractor who was having some issues drilling his pilot hole. Around 200 metres out in sand and gravel he experienced high rotational torque and poor returns and was concerned that he was going to lose his tooling in the hole. I asked him about the mud he was using to which he replied, “oh just some bentonite and a bit of PAC.”

 This is not the first time I’ve heard this statement, but that word ‘just’ got me thinking about the misconception that all bentonite is the same.

 Bentonite is often referred to as the “Clay of a thousand uses” or “Miracle Mud”. It is used in myriad applications from paints and pet litter to pharmaceuticals and detergents, but it is the way that bentonite behaves when mixed in water that makes it such an effective component of a drilling fluid. 

Avoiding a deep dive into clay chemistry, bentonites are classified according to several types (e.g. calcium, sodium, potassium) but in the drilling industry sodium bentonite is the material of choice. Sodium bentonites have incredibly small particle sizes referred to as colloids. The particles themselves are small enough that the action of the water molecules is enough to keep them in suspension and when fully hydrated, sodium bentonites swell and expand providing large surface areas >750m2/gram.

Whilst they are composed of the same fundamental building blocks, calcium bentonite does not exhibit the same desirable properties as its sodium cousin. These major differences in swelling, suspension and thixotropy can affect your drilling fluid (mud) and the ‘functions of the drilling fluid’ which in turn can have a significant impact on and the overall success and profitability of your project.

Sodium Bentonite vs Calcium Bentonite

Sodium bentonite (montmorillonite) is a naturally occurring, plate-like, swelling clay containing the clay minerals of smectite and other minerals, such as quartz, mica, feldspar and calcite. It is a premium, high yield clay product sourced from large deposits in Wyoming (hence the name Wyoming bentonite), North Dakota, South Dakota and Utah in the USA and is used to determine the API’s (American Petroleum Institute) International Standard for Drilling Grade bentonite. 

Sodium bentonite clay platelets are very thin (measured in nano-metres) and have a flexible 3-layer structure with permanent negative charges on the top and bottom surfaces and positively charged on the edges.

The structure and chemical composition mean that when the clay is mixed in water, the dry stacks of platelets begin to hydrate and swell as the water is attracted to both the negative clay surface and the positively charged ions in between the layers. As the individual platelets separate (disperse) in the water, their surface area and colloidal activity increases producing a viscous fluid that can suspend solids and seal up the bore/well bore. 

Calcium bentonite (calcium-type montmorillonite) is found all over the world with India, Turkey, Greece and Australia in the top ten of producers. Untreated calcium bentonite is not suitable for drilling fluid, so it is typically treated (beneficiated) during the milling process by adding sodium carbonate (soda ash), long-chain synthetic polymers, carboxymethylcellulose (CMC), starch or polyphosphates.

Whilst theses additives help achieve certain quality standards, they are susceptible to water hardness, mechanical shear-degradation, bacteria and other factors that can render these additives and your drilling fluid ineffective. Some of these additives may also preclude their use due to environmental restrictions.

Sodium bentonite swells through a process of osmotic and crystalline swelling providing a 20-fold increase in volume. Calcium bentonite on the other hand undergoes crystalline swelling only resulting in a 2-fold increase in volume.

Why is this important? Better swelling (hydration) means increased surface area and colloidal activity which translates into improved viscosity, better suspension capability, superior gel strengths, greater filtration control and a higher yield.

It’s important to note that to hydrate properly, all bentonite needs to be mixed with good makeup water (hard and salty water affect hydration) and sheared with appropriate equipment for at least 30 minutes. Failure to mix properly will result in slower hydration and this can often lead to over addition of bentonite as the operator seeks to achieve their desired level of viscosity. Bear in mind that after this initial hydration, sodium bentonite will continue to swell over the next 8-10 hours as shown in the graph. So, if you have added too much on the initial mix, it’s possible to find yourself with mud that is too viscous and will require dilution in order to be pumped. 

Calcium bentonite will often give good initial viscosity, but this is due mainly to the polymer additives and not the bentonite itself. The viscosity that comes from the beneficiating polymers give the driller a “feel good factor” and they will generally confirm this with a simple visual observation or a Marsh Funnel.

These methods can be useful but without special testing equipment such as a rheometer, it is impossible to determine whether the fluid viscosity will actually clean the hole. Measuring the fluid with a rotational rheometer provides data over a range of shear rates 600, 300, 200, 100, 6 and 3 rpm all of which can be used in hydraulics modelling. A summary of this type of testing is shown below. 

When sodium bentoinite is mixed with water it forms a fluid with so-called thixotropic properties. This basically means that it reacts as a fluid when mechanical stress (pumps or mixers) are applied, but under quiescent (static) conditions, its viscosity increases and it forms a gel.

Gelation occurs through interactions between the positive and negative charges on the edges and surfaces of the microscopic bentonite clay platelets. These interactions have a major effect on the viscosity of the fluid that forms a gel network within the fluid, which suspends solids when the pumps are off and the fluid is at rest. When the pumps are switched on, the gel structure is broken, and the mud becomes liquid again. The decrease in fluid viscosity as the pump rate increases is known as shear thinning and is a key feature of a good drilling fluid. Fluid moving faster is thinner as it is pumped down the drill string and out of the bit and slows down and becomes thicker as it travels through the annulus, carrying the drilled cuttings to the surface. 

Calcium bentonite like most bentonite slurries do exhibit a thixotropic behavior, but they are dependent on the concentration used. Sodium bentonite shows a significantly greater degree of thixotropy (x100) than that of calcium bentonite at the same concentrations.

 As discussed earlier, there is a significant difference in the degree to which the two materials swell or the amount they yield. A bentonite’s ‘yield value’ is the number of barrels (1 US oil barrel = 158.987 litres) of bentonite slurry with a specific viscosity (15cps) yielded when one ton (2,000 lb/907.1847 kg) is added to freshwater.

Bentonite yield values vary greatly ranging from less than 25 barrels/ton for untreated calcium bentonite to >220 for some treated sodium bentonites.

High yield sodium bentonites are treated in much the same way as calcium bentonite to increase their yield point, viscosity and gel strength. But where sodium bentonite is treated to improve performance, calcium bentonite is treated to meet minimum performance criteria. Treating high yield bentonite basically means you need less product to achieve the same viscosity as untreated bentonite. The graph opposite shows the difference in viscosity produced by treated sodium bentonite and untreated calcium bentonite. To obtain a funnel viscosity of 40 seconds/quart for example, over 400% more calcium bentonite is required compared to sodium bentonite.  

Minimizing formation damage is one of the main functions of a drilling fluid. Whether you’re drilling an HDD bore, water well or oil and gas, drilling exposes the formation to the drilling fluid, and any solids and chemicals contained in that fluid. Some invasion of fluid filtrate (the water phase of the drilling mud) and/or fine solids into the formation is inevitable however this can be minimized with a dispersed sodium bentonite mud system.

Under hydro-static pressure, the thin, flexible bentonite platelets act as a sealant on the walls of the bore as they overlap each other and form what is known as a filter cake. In our mud schools, we sometimes use the example of building a wall inside the hole where the bentonite platelets are the bricks and the PAC (polyanionic cellulose) is the mortar between them. 

Calcium bentonite does not disperse into separate platelets like sodium bentonite does, instead remaining as aggregated thicker stacks of platelets. These aggregated platelets form a very thick, highly permeable cake which reduces the amount of annular space available for the drilling fluid to flow and allows fluid invasion into the formation. This can lead to over pressuring of the formation with potential for frac-out and hole collapse in unconsolidated or friable formations. HDD contractors should also be mindful that a thick filter cake doesn’t only impact drilling, it can also lead to problems when installing pipe. A thick filter results in a greater surface area contacting the product pipe pushing it into the roof of the bore which can lead to a jammed pipeline and an incomplete pulling operation.

Sodium bentonite produces a thin, slick cake which reduces torque and drag and the potential for the drill pipe to become stuck under pressure. Its low permeability isolates the formation from fluid invasion and reduces the potential for differential sticking in highly permeable zones. 

 A properly designed and maintained drilling fluid is a critical component of a drilling operation and a good bentonite is often a critical component of that drilling fluid. I mentioned earlier that bentonite is often referred to as the “Miracle Mud” and as a Mud Engineer, I have to agree. When you consider all the functions of a drilling fluid, and how those functions impact the success or failure of the drilling operation, sodium bentonite outperforms all other products on the market. 

So, the next time you say “oh it’s just some bentonite and PAC” ask yourself these questions:

1. Why is this bag of bentonite $3-4 cheaper than Wyoming sodium bentonite?
2. Is this really sodium bentonite or just branded that way because it has been treated with soda ash in an attempt to mimic true sodium bentonite?
3. Does this cheaper bentonite yield as much as the sodium bentonite or will I end up using three or four times as much product?
4. How much could I save on storage, trucking, mixing time and actual time drilling if I can use 25-30% less product simply by choosing true sodium bentonite?
5. Is it worth risking my downhole tooling, infrastructure damage and reputation for the sake of saving a few dollars per sack on bentonite?
6. Should I try running true Wyoming sodium bentonite to see if I get a better result?

And the final question you can ask yourself – maybe not all bentonite is created equal?