WorldWide Drilling Resource

8 MARCH 2021 WorldWide Drilling Resource® Bentonite Seals for Environmental Drilling by Stewart Krause, Senior Sales Manager, Wyo-Ben, Inc. With the dramatic rise in environmental drilling of the 80s, came the need for additional products for well seals. Pellet and chip bentonite was already being used for their desirable swelling and low permeability properties in monitor well construction. Typically, monitor wells had a gravel pack at the bottom with a bentonite seal of either pellets or chips above and neat cement grout to the surface. As the engineering community began moving to PVC casing, some shortcomings of neat cement began to arise and/or were amplified. Cracking, shrinking and heat of hydration became serious problems. Heat of hydration, for example, caused the PVC to distort and expand. Once the cement set, the casing would cool and contract, causing the casing to pull away from the cement grout, leaving a micro annulus along the interface. At the same time, the environmental drilling industry was moving heavily into hollow stem auger well construction. This allowed the well to be completed inside the auger with a high degree of accuracy and minimal formation damage, but also required the grout seal to be placed inside the auger. Hydrating a column of bentonite pellets or chips inside the auger and then pulling the augers back resulted in more than one well being pulled out. With wells being drilled deeper, the need for grouts to be pumped into place came front and center. By this time, the bentonite industry had done a lot of testing for permeability and pumpability. Knowing more bentonite solids in a slurry will decrease the permeability and increase long-term stability, the challenge was to make it pumpable. Keep in mind, there were a very limited number of companies making small grout mixing and pumping equipment, which also compounded the challenge. Bentonite companies were challenged by the engineering side wanting higher solids flexible grouts, and contractors wanting something with better placeability. The industry target finally settled on 20 and 30% solids by weight, which meant two different products. Pumpable bentonite grouts generally fall into two distinct categories - inhibited or dispersed. Inhibitedbentonite grouts use inhibition technology, which means applying some sort of coating to the bentonite to inhibit water wetting or activation for a short period of time. The most common inhibition method is to add a polymer to the water prior to the addition of a granular bentonite. Another method is to use a bentonite slurry to suspend and coat a granular bentonite, allowing it to be pumped into place. Both methods take advantage of using lower surface area granular bentonite to extend the working time. Granular bentonite has less surface area than powder, thus hydrating slower, allowing more time to mix and pump. Dispersedbentonite grouts rely on dispersants (thinners) to keep the slurry pumpable at high bentonite solids concentrations. The idea is to keep the slurry pumpable while not compromising the gel strengths that make it set, or the in place permeability. For the manufacturer, choosing the correct dispersant and application rate is key. This is why manufacturers stress mixing the product as it was designed. Case in point, taking a product designed to be mixed at 30% solids and mixing it to 20% solids makes it much easier to pump, but produces a very low gel strength, thus a poor performing, unstable setting grout. Bentonite grouts do have limitations, and understanding those limits is key to a successful project. Bentonite grout slurries are susceptible to some of the same contaminants we run into with bentonite-based drilling fluids. High levels of chlorides, calcium, and hydrocarbons will affect the performance of the in place grout. We now know pumpable bentonite grouts don’t perform well long-term in the vadose zone where permeable formations are present, but work very well in saturated conditions. Bentonite grouts do not exhibit structural strength, so if compressive strength is desired, they are not the tool you are looking for. From the early development period through today, we have seen a lot of changes in how we use bentonite grouts and continue to refine how we make and place them to perform desired tasks. From ground source heat pumps and well seals to grounding rods, bentonite grouts work for us every day. ENV April 12-13, 2021~ The Louisiana Ground Water Association will hold its Annual Convention & Trade Show once again at the Paragon Casino in Marksville, Louisiana. The festivities begin Monday at 6 p.m. for cocktail hour, followed by dinner at 7 p.m., then Bingo at 8. There will be continuing education credit seminars on Tuesday, along with trade show exhibits of products and services for the industry. Find the registration form at See more events at online issue. Are you planning to go? WorldWide will be looking for you! Join WWDRR