Grout Ticket Does Not Match the Annular Volume
A crew is sealing the annulus of a 72-foot bedrock well. The borehole is 8 inches in diameter to 72 feet; 6-inch steel casing with 6.625-inch outside diameter is set to 70 feet, and the casing shoe is 2 feet into shale. The annular space is to be grouted from the shoe to grade with neat cement. The crew mixes 10 bags of 94-pound cement, each yielding about 1.15 cubic feet of slurry, and pumps through a tremie. Pump pressure rises early, the tremie is raised 15 feet to keep moving, and no grout returns reach the surface. The driller wants to backfill the top 10 feet with sand and move to the next job because the cement ticket shows more than the calculated hole volume. The well is for a restaurant and will be sampled for bacteria next week.
1. Calculate the theoretical annular volume and the grout volume needed at 1.5 times the annulus. Is 10 bags enough?
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The 8-inch hole has an area of about 0.349 square feet. The 6.625-inch casing has an outside area of about 0.239 square feet. The annular area is therefore about 0.110 square feet. Over 72 feet, the theoretical annulus is 0.110 times 72, or about 7.9 cubic feet. At 1.5 times the calculated volume, the job needs about 11.9 cubic feet of grout. Ten bags at 1.15 cubic feet per bag gives 11.5 cubic feet, which is already below the 1.5 factor even before any loss to the formation. Because returns never reached the surface, the actual placed volume is unknown and is likely less than the ticket. The driller cannot treat 11.5 cubic feet as proof of a full seal.
2. What is wrong with raising the tremie 15 feet and backfilling the top with sand?
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The tremie must stay submerged in grout so the slurry is placed from the bottom up and does not mix with water or cave material. Raising it 15 feet can create a void, allow dilution, or leave a bridge of cuttings above the shoe. No returns plus rising pressure suggests the grout is going into a lost-circulation zone or the annulus is bridged, not that the seal is complete. Backfilling the top 10 feet with sand would leave an unsealed upper annulus, which is exactly where surface water and shallow groundwater can enter. Sand is not an approved annular seal. The crew should re-establish a tremie below the grout surface, pressure-grout or stage-grout the annulus, and verify returns or use another approved method before moving on.
3. What should the driller do before the restaurant well is sampled and placed in service?
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The driller should first correct the seal, not sample around a defective annulus. After the grout has set, the crew should verify that the upper annulus is continuously sealed to grade, remove any sand backfill, and repair voids if they are found. The well should then be developed or redeveloped as needed, disinfected, and flushed before the bacteriological sample is collected. The sample should be collected after disinfection and with proper technique, and the driller should record the actual grout volumes and any corrective work on the well log. If the seal cannot be verified, the well should not be placed in service until it is repaired, because a restaurant serving the public cannot rely on a well with a questionable annular seal.