The bentonite used for drilling fluids, grout and sealants is primarily mined in the area encompassing Wyoming and surrounding areas in the western United States. This sodium-based bentonite is an ore whose major constituent is the clay mineral, sodium montmorillonite. Montmorillonite is a three-layer mineral consisting of two tetrahedral silicate layers sandwiched around a central octahedral aluminate layer. This clay “sandwich” is held together by the bonds between shared oxygen atoms, with each metal atom gaining a partial electron from the multiple oxygen atoms directly surrounding it. This bentonite was formed by the deposition of volcanic ash in a marine environment during the Cretaceous period. Over time, this ash was highly compressed and changed chemically to produce this dense platelet structure. This characteristic is what makes bentonite such an essential tool when drilling wells. In fact, each cubic inch of bentonite has enough platelet surface area to cover 66 football fields.

It is important to realize that all bentonite removed from the ground is not the same. In one deposit there may be several grades of bentonite that can be used for different purposes. The most sought after is a high-yield bentonite referred to as yellow bentonite. It is soft to the touch, has a yellowish tinge and typically has a very high yield, or expansion coefficient, that is required to produce bentonite drilling fluids. While this is the most sought after bentonite, it is also the most limited in its supply. To add to that, there are several grades of this “yellow” bentonite. Typically, underneath this vein of yellow bentonite is a blue bentonite that is much harder and has a bluish color profile. This bentonite is used for many products, including the exceedingly popular bentonite chips.

In order to determine the viability to mine a bentonite pit, a complete geotechnical investigation must first take place. Core samples are taken in a grid pattern to produce a three-dimensional map of the bentonite deposit. Various factors are considered in order to determine the feasibility of mining the proposed pit. These considerations include distance from the processing plant, depth of the deposit and grade of the deposit. The rule of thumb is the greater the depth of the deposit, the higher the price per ton to extract the bentonite. Deep deposits typically require blasting, while shallow deposits may only need a layer of overburden removed. The cost per ton of material to mine coupled with both the grade of the bentonite and the distance to the plant determine whether it is feasible to mine a pit. As the resource becomes depleted and the available pits become deeper and further from processing plants, market price increases. Other factors determining mining costs include remediation of the pits as well as other environmental considerations, including the maintenance of habitat for the endangered sage grouse. Responsible bentonite companies go beyond the regulations to help create more suitable habitat for the sage grouse, ensuring the viability of the area for both economic resources and the animals that live on this unique landscape. 

A bentonite plant processes and mixes each batch, testing it to ensure consistency.

A bentonite plant processes and mixes each batch, testing it to ensure consistency. Source: Cetco Drilling Products

Once a pit is mined, the bentonite is separated by quality and size and stockpiled at the plant. At this point, it is ready to process. Some products are very simple, such as bentonite chips. They are simply sorted by size and bagged. The bigger challenge is producing bentonite drilling fluids and bentonite grouts. It is imperative to determine the exact characteristics of the stockpiled high-yield bentonite in order to have a consistent product. As noted earlier, the grade of the bentonite is different not only based on the location of the pit, but within the pit itself. Many products have a mixture of high-yield bentonite and proprietary additives. As the mined bentonite has varying characteristics based on variables such as pit location and deposition within the pit, the proper mix of bentonite and additives must be maintained to ensure a consistent product. To add to this, other grades of bentonite are blended into the mixture in order to stretch the reserves of the high-yield bentonite. It is important to remember that this is a finite resource and it is the responsibility of a bentonite company to ensure the longevity of the resource.

In a recent plant tour, I was fascinated with the specialty mixing room. In this room, all the production of specialty products requiring proprietary blends of bentonite and additives are produced. As the characteristics of the bentonite vary from pit to pit and also from veins in the pit it is mined, the recipes to produce a consistent product are far from being “cookie cutter.” In order to maintain this level of excellence, a batch is mixed and then tested before any product is even bagged. An in-house lab performs a series of tests, including chemical composition and rheological profiles, on each and every batch. If the results do not conform to the required specifications, then changes are made to the batch until all specifications are met. Once these specifications are met, the product is ready to be bagged. This strict quality control policy ensures that uniform product quality is maintained.

Each batch is assigned a unique lot number that is marked on every bag. While the testing adds time to the manufacturing process, it ensures that every bag of product used by a contractor is within the same published specifications as the bag used the day, week or month before. A sample of every batch is kept and all the test results are filed. In the event that a contractor has a concern with a product, the sample from the lot number as well as the test results can be pulled to evaluate the contractor’s concern.

 There is a lot of planning from the mining process to the manufacture of bentonite drilling products to ensure the best product is available to the contractor in the field every day. Whether you are drilling a horizontal bore under a river, grouting geothermal boreholes for a commercial building or drilling a municipal water well, you can rely on the fact that the bentonite drilling product you are using is ready for your challenge.