WorldWide Drilling Resource

GEO Boring Thoughts by Todd Tannehill Owner, Mud & More, LLC High Solids Bentonite Grout vs. Cement on Casing This issue has a theme of drill pipe and casing, so I thought a fitting article might be to discuss the advantages and disadvantages of high solid bentonite grouts versus cement grouts on casing. This subject was brought up in my booth at a recent trade show. High solids bentonite grout is a term used to describe commercially packaged grouts containing 20% and 30% solid content. Many of the common one-bag and two-step grouts used for residential are 20% solid rated. Some commercial and specialty grouts are 30%. A grout designed for geothermal starts out at a 30% solid grout with no silica sand added, but is a 72% solid grout when 400 pounds of silica sand is added. Cementitious grouts come in a variety of types. Some are available with NSF certification while others are not. If you are just using a Portland blend from a local home store, chances are it does not carry the NSF certifications for drinking water. While this is very important in some states and local areas, it will not be our focus here. For years, we have been teaching the effects of cement grouts on plastic casing and the advantages of bentonite grouts. While my industry peers and I will not always agree with those from the cement industry, there are a few facts which are hard to dispute on each side. Cement - The advantages of cement grouts are the availability, structural strength, low permeability, and positive place- ment. A quick look at the bag price makes the cost appear less expensive than bentonite grout. Cement grouts are also a solu- tion to brackish water conditions as the cement holds up where bentonite will not. The disadvantages are general to cement grout overall, and some brands and types are better than others. These disad- vantages include that cement grouts shrink and crack as they setup and cure. For this reason, they fail to adhere to PvC well casing, creating a bonding problem. This can create a super highway for surface contaminates straight down the borehole. Because cement by nature is very dense, it can push out into the formation, or worse - back on your casing if the foundation has no give. A crushing situation. High pH, heat of hydration, objectionable ingredients, and lack of NSF approvals round out the list of disadvantages for cement. Bentonite - High Solid Bentonite grouts contain 20-30% solids and set up like stiff peanut butter. They can be pumped into place using a positive displacement pump. Using a 1¼-inch tremie pipe to install, they provide a complete seal from the bottom to the top of the well. Bentonite grouts mix easily on-site and are easier to cleanup than cement. The key is, you get a better seal because they will not shrink or crack below the water table. As a disadvantage, bentonite grouts appear to be costlier than cement. If you look further and compare the yield per batch, this is not so. Are there places for both types of grout? yes. Can a combi- nation sometimes be an option? yes. Under normal water conditions, the high solids bentonite grout will usually be a better choice. Todd Todd Tannehill may be contacted via e-mail to michele@ worldwidedrillingresource.com 19 WorldWide Drilling Resource ® MAY 2018 Thank you. you are the best! [Regarding May 2018 ad in WWDR .] Kevin McGinnis Cotey Chemical Corp. Lubbock, Texas