Lost Returns and Cobbles in a Glacial Valley
A landowner in a glacial valley needs a 20 gpm domestic well for a new home. The proposed hole is 8 inches in diameter to 160 feet. A nearby well log shows 12 feet of topsoil, 80 feet of saturated sand and gravel with cobbles, then 60 feet of fractured shale. The water level is 18 feet. At 70 feet the crew loses all drilling-fluid returns, the hole starts caving, and the bit repeatedly stalls on cobbles. The client wants to finish in two days. The crew has a mud rotary rig, a cable-tool rig, and a truck-mounted air hammer. The company also has 20 feet of 10-inch temporary steel casing and 160 feet of 6-inch PVC screen and casing. The driller must choose a method and a casing plan that will not leave a pathway for surface water or shallow groundwater to enter the well.
1. Which drilling method should the crew use through the caving sand and gravel, and why?
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The safer choice is mud rotary with temporary surface casing, not the air hammer. The hole is caving in saturated sand and gravel and has already lost returns, so an air hammer would tend to blow the hole open, increase collapse risk, and provide little wall support. Mud rotary can build a filter cake and maintain a positive head against the formation, provided the crew mixes enough solids and controls penetration rate. Cobbles may still deflect the bit, so the crew should slow rotation and may need cable-tool assistance or a casing advancer if the cobbles cannot be penetrated cleanly. The 10-inch temporary casing should be set through the topsoil and shallow zone, sealed, and used as a stable collar. The crew should not continue open-hole air drilling through the caving interval just to save time.
2. What casing and grouting plan would protect the well and seal off shallow groundwater?
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The plan should use the temporary 10-inch casing to hold the upper hole, then set 6-inch PVC casing through the sand and gravel and at least 5 to 10 feet into the shale. The producing interval in the sand and gravel should be screened and gravel-packed, while the casing above the screen isolates the shallow water. The annulus around the permanent casing should be grouted from the casing shoe to grade with an approved grout, placed by tremie from the bottom up. The surface seal is critical: if the upper 20 feet are not continuously sealed, surface water and shallow groundwater can move down the annulus. The crew should verify grout returns or use stage grouting if returns are lost, and should not rely on bentonite chips alone in a flowing or caving hole.
3. After development, the well tests at 8 gpm, not 20 gpm. What should the driller recommend before declaring the well complete?
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An 8 gpm well may still be usable, but the driller should not promise 20 gpm. First, complete development and a controlled yield test to see whether the rate stabilizes and whether drawdown is reasonable. The driller should compare static and pumping levels, check for turbidity or sand production, and inspect whether the screen and gravel pack are properly sized. If the shale was not adequately penetrated or the screen interval is too short, the driller may need to deepen the hole or modify the intake. The driller should report the tested rate honestly and discuss storage, a larger pressure tank, or a different use with the owner. The well log and test data should match the actual result, not the original target.