Drilling Fluids: Grouting Wells with Dry Sealants - Columns

The grouting of wells has become a large part of the drilling industry in recent years. Water well, environmental wells, mineral exploration boreholes, and ground source heat pump installations make up a large part of the grouting in our industry.

The purpose of grouting is to protect the ground water by preventing the co-mingling of aquifers in the wells and to prevent contamination from the surface. Regulations for grouting focus on grouts for surface seals and for aquifer protection. Depth requirements for grout vary from state to state.

Grouts and sealants that have been in use for a number of years include:

Bentonite chips
Bentonite tablets, coated and uncoated
Cement
Cement/bentonite
Granular bentonite
Granular bentonite and polymers
High-solids bentonite grout slurries

This article will focus on sealants that are applied largely in the dry state. These sealants usually are poured dry into the annulus of a well, and then hydrated to form a seal. These sealants also are often used in well abandonment.

Dry sealants include:

Bentonite chips
Bentonite tablets, coated and uncoated
Granular bentonite

Dry sealants have proven themselves to be very effective seals for wells and boreholes. In the Nebraska Grout Task Force Study about which I have written in earlier articles, bentonite chips outperformed other types of grouts used in the study.

Granular bentonite can form a very effective seal if it can be placed properly. It has a major disadvantage in that it cannot be poured through standing water in the borehole, as can bentonite chips and tablets. The reason for this is that it hydrates too rapidly and bridges in the borehole. It can, however, be placed in a dry annulus or borehole, or it can be placed with driven casing. It is important to remember that to ensure consistent grouting, the formation needs to be fairly consolidated when using this method. When granular bentonite is placed in a dry borehole, it is recommended that each bag be hydrated after it is placed by adding 3 gallons to 5 gallons of water. This also is true of bentonite chips and tablets.

The most popular dry sealant by far is bentonite chips. The reasons for this popularity are the effectiveness of the seal, the ease of use, and the price of the product.

Bentonite chips can be poured to significant depths through standing water. Depth recommendations by manufacturers tend to vary, but a fairly common recommendation is 500 feet. However, many contractors have placed bentonite chips much deeper – 1,000 feet or more. I am aware of chips being placed to depths of 1,500 feet to 1,600 feet.

Each grouting situation can be different, but a big factor for placing chips or any other grout is the care that is taken in placing them. To successfully achieve placement of chips, the common recommendations are to pour them across a screen with 1⁄4-inch openings to screen fines that have accumulated in transit, and to pour the chips slowly, approximately 2 minutes per 50-pound bag. It is further recommended that a tagline – a weighted string – be placed in the borehole to ensure that the chips make it to the area to be grouted with no bridging. If bridging is discovered, the bridging needs to be cleared before adding more chips. A majority of bentonite chips come in two sizes – 3⁄8 inch and 3⁄4 inch. The size selected is dependent on the size of the annulus or borehole.

Bentonite tablets also are a great sealing medium for wells and boreholes. The major disadvantage of tablets is that they are much more expensive than chips. This has to be weighed against the importance of the seal and the other advantages that bentonite tablets offer. Tablets are composed of granular bentonite that has been compressed into tablet form. Sizes include 1⁄4, 3⁄8 and 1⁄2 inches.

Some of the advantages of bentonite tablets:

Tablets are composed of a more pure form of bentonite than chips.
Tablets fall through water at a faster rate than chips.
Tablets have a greater swell than chips.
Tablets hydrate faster than chips.

Tablets have these advantages because of the purity of the bentonite used in them vs. that in chips and because they have a lower moisture content than chips. Tablets have a rounded shape that causes them to fall faster than chips, which are more irregular in shape. Recent testing on the falling rate of tablets vs. chips has shown that tablets fall an average of 1.1849 feet per second, and that chips fall an average of 0.9351 feet per second – a difference of 27 percent.

The faster hydration rate mentioned above can be an advantage in completing the grouting project, but it also can be a disadvantage if the tablets become sticky and bridge before they are placed. One solution for this problem is freezing the tablets before placing them. Another solution is to use coated bentonite tablets, which have become very popular in certain applications. Coated tablets have a coating that is environmentally safe, and this coating delays the hydration for 30 minutes. Coated tablets also are available with coatings that allow 60-minute delays in hydration. Coated tablets come in 1⁄4- and 3⁄8-inch sizes, with 3⁄8-inch being the most popular.

To summarize, dry sealants offer a very good method for sealing a well or borehole. The advantage of these dry sealants is that they have an extremely high solids content, which results in better sealing and less shrinkage. These sealants are very environmentally friendly, and do not require the pumping equipment needed for pumpable slurry grouts.
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