Using Dry Bentonite to Seal Boreholes
Dry bentonite sealants have become very popular because they are easy to apply and require no mixing or pumping. They are a very effective method of grouting wells and other boreholes with a high-solids bentonite seal. They actually provide the highest solids bentonite seal available today.
The dry sealants include:
- Bentonite chips
- Bentonite tablets or pellets
- Coated bentonite tablets or pellets
Granular bentonite is sometimes included as a dry sealant, but it has limited use for grouting. It works well for a surface seal or in dry boreholes, but cannot be poured through water.
There are a number of applications for the three products listed above. Some of these include:
- Full-length annular seals
- Interface seals between gravel pack and slurry grouts
- Hole abandonment
- Grounding rods
- Piezometer installations
- Sealing off artesian flows
- Loss of circulation
- Seismic shot holes
- Surface seals
- Miscellaneous uses, including resealing after pitless adapter installations
Installation of These Products
In order for these products to be effective, they must be installed properly. Although they are easy to use, as mentioned above, they need to be applied with care.
The first order of business is to calculate the amount of product needed. Using the well dimensions (depth and diameter), the contractor can consult technical information from the manufacturer. This usually includes published technical information, information on product containers or manufacturer websites. Some websites even include calculators for figuring the amount of product needed. Usually, manufacturers recommend figuring 10 percent above the amount of product calculated to cover unexpected void areas in the borehole.
One very important factor to consider in the installation process is that, regardless of whose product you may be using, fines will accumulate in the container during transit of the material from the manufacturer to the drill site. These fines will hydrate immediately when poured into water and will cause sticking and bridging problems. These fines should be removed before the product enters the borehole, and this can be done by pouring them over a screen as they are introduced into the borehole.
These sealants should be poured slowly over the screen in to the borehole. A couple of minutes per 50 pounds of material is a good rule of thumb.
Manufacturers also recommend the use of a tagline, a weighted string, to assure that the material is falling into place and not bridging. If bridging occurs, the contractor should stop and correct the problem before continuing with the installation.
Normally, manufacturers recommend using these products in applications where annular space is 2 inches or more. There have been installations with less than 2 inches, but that is the general rule.
These products have been placed to depths ranging from a few feet to 1,500 feet, with some even going beyond that. The keys to successful installations at any depth are the care taken during the installation and the conditions of the borehole.
Selection of the proper product to use will depend on a number of factors. Some of these include:
- Hole dimensions
- Product quality
- Criticalness of the seal
Below is a discussion of each of the products, which will include characteristics of each and advantages and disadvantages of each.
This product is a very effective, high-solids seal. It is very popular because of its low cost compared to bentonite tablets or coated tablets. Bentonite chips are not as pure as bentonite tablets, which results in a slower swell and less swell than the tablets, and they have higher moisture content. This can be an advantage, in that the chips will not get sticky as quickly as tablets when introduced to water. Bentonite chips have a slower fall rate through water than tablets, due to their irregular shape. Recent testing has shown that chips do not have the same structural integrity as bentonite tablets when they are hydrated. The above factors need to be taken into consideration, and the difference in cost should be weighed against the conditions in the borehole and how critical the seal is to the application.
Bentonite tablets in their hydrated state are an extremely effective, high-solids seal. Their biggest disadvantage is their cost, as they are much more expensive than bentonite chips. Bentonite tablets are much more pure than bentonite chips, have a higher swell rate and provide a sturdier seal. They have a regular shape, which allows them to have a faster fall rate through water than bentonite chips. They also have less moisture content, which causes them to hydrate and become sticky more quickly than bentonite chips. This problem can be overcome by freezing the bentonite tablets using dry ice or nitrogen. Again, cost has to be weighed against how critical the seal and the conditions to be encountered.
Coated Bentonite Tablets
Coated bentonite tablets have become more popular in recent years because of the ease of installation. The bentonite tablet is coated with an environmentally safe coating, which delays the hydration of the bentonite for at least 30 minutes. This can take the worry out of placement and minimize bridging problems. The coated tablets provide a good seal and less hassle in installation. Their chief disadvantage is cost, which is more than uncoated bentonite tablets and much higher than bentonite chips. This cost is weighed by contractors and their clients against conditions in the borehole, and how critical the seal.
In summary, all three of these products are effective sealants. Each has advantages and disadvantages. The important thing is that contractors and their customers have a choice and will have effective seal whichever way they choose to go.
If you have questions or comments on the above information, please contact me through National Driller.