WorldWide Drilling Resource

40 FEBRUARY 2015 WorldWide Drilling Resource ® Inspecting Geothermal Grout by Peter Tavino Litchfield Geothermal In last month’s WWDR article, we covered drilling and loop insertion. We now recommend to inspectors the im- portant features they should check dur- ing the bentonite grouting operation. Wise system owners or inspection agents will observe the tremie line is pushed all the way to the bottom of the borehole, so a good thermal lock is achieved and aqui- fers are protected. A story about uninspected bore- holes in Las Vegas circulated a few years ago. The unscrupulous drilling contrac- tor was busted when a large excavator dug 20 foot deep to show paper bags were stuffed down the holes, and only the top 20 feet were grouted. Before starting, the inspector should verify the U-bend loop is taped or wired to the casing, so it will not float out due to high grout pump pressure. Be sure grout water overflow will not flood a gar- den or catch basin. Check the length of the tremie line on the first day by unrolling on the ground and pacing it off. If new pipe, check the 2-foot printed length which is visible. Blue tape placed at 460 feet on a 500- foot-long line will wear off after a few uses, so keep it refreshed if it is your guide. If the tremie gets hung up, insist it be raised and twisted, and try push- ing it down again. A roller guide as shown, and an angular cut at the tip helps; as does filling it with heavy grout to make sinking easier. Once the tremie line is at the bot- tom and filling begins, count sand bags rolled into bentonite bags, at the pre- scribed rate. A recent 450-foot-deep 6-inch hole with 1¼-inch loop used 8 double batches, as we expected. Water content is difficult to verify if insurance does not allow the inspector to board the rig to see into the hopper. I send test samples to the lab from out of the casing, not out of the hopper as requested. Results from years of data show slower mixing with bentonite (such as TG Select), proper gallons of acidic water, and approved silica sand achieve better thermal conductivity results ~ 0.97 BTU/Hr for 1.0 specified and 0.8 designed, is great. If a large formation (especially where water was encountered) is using and wasting too much grout, sealing and plug- ging agents can be dropped into the hole slowly - not through the tremie! Inspectors should monitor this limited use, and note the approximate eleva- tion in a report that summarizes time spent, number of bags used, and where a sealing agent was added. A lot of grout settles, and topping off usually fixes it. If settlement is severe, drop in a plugging agent and add more grout. If minor settlement occurs the next day during trench fusion, when the casing is cut, adding a few feet of sand or cuttings instead of returning with the grout rig is reasonable. Be sure a sink- hole will not form after backfilling the trench. Covering the hole with a sealing agent so the settlement cannot be in- spected is imprudent. Next month, we address heat fusion and coring, and link seal inspection. The statements and comments in this article are based on information and references believed to be true and factual. If you have any questions or comments, please forward them to me in care of WWDR . Pete Peter Tavino may be contacted via e-mail at michele@ worldwidedrillingresource.com Kevin represented Hydro Resources. Photos courtesy of Joan Daugbjerg. Tom, Atlas Copco. Clay & Donna with Amarillo Gear Company LLC. Happy crew at the Grundfos ® booth. 2014 Drilling professionals-in-training. Marcus, Laibe Corporation. Approximately 400 billion gallons of water are used in the United States per day. Nearly half of this water is used for thermoelectric power generation.