Lost Circulation in Fractured Granite During Air-Hammer Drilling
An 8-inch air-rotary hammer hole is at 400 ft in fractured granite. At 260 ft the returns dropped from full to nothing; the hole now makes only dust and the bit bogs. Overnight the hole filled to 140 ft, and the static water level in a nearby monitor well is 55 ft. The driller has a water truck, foam, and casing on site. The target is a domestic supply for a mountain home; the owner wants at least 5 gpm. Previous holes in the area have hit water at 300-380 ft in steep fractures. The driller must decide whether to continue dry, switch to foam, set casing, or grout the loss zone. There is no permit condition limiting withdrawal, but the site is on a slope above a spring-fed stream.
1. Diagnose what happened at 260 ft and explain why continuing with dry air hammer is unlikely to succeed.
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The sudden loss of returns at 260 ft, followed by the hole filling to 140 ft, is the signature of a large open fracture or solution channel that is taking all the air and water. The hole is not making water fast enough to clean itself; the 140-ft water column is simply the static level in the borehole, and the monitor well's 55-ft level shows the fracture is not a high-capacity producer at this location. Dry air hammer needs annular velocity to lift cuttings; with the annulus open to a thief zone, air escapes into the fracture and cuttings fall back. The bit bogging is consistent with a debris pile at the bottom. Continuing dry will either plug the hole with cuttings or polish the fracture, and it risks sticking the hammer. The correct response is to change the circulating system, not to add more air.
2. Recommend a drilling plan from 260 ft to total depth, including how to handle the loss zone and what method to use to reach the target fractures.
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Set 6-inch casing from land surface to about 270 ft, through the loss zone, and grout the annulus to surface. Casing isolates the thief zone so the hole above can hold a water column and the hammer can clean the hole below. Then drill ahead with a 6-inch hammer using foam or a foam-water mixture rather than dry air; foam provides lifting capacity at lower annular velocity and is more tolerant of moderate loss. If losses continue below the shoe, use a biodegradable foam and monitor returns; if the hole cannot be kept clean, a second string may be needed, but that should be a last resort because it reduces the final diameter. At total depth, air-lift or surge the well to develop the fractures. Do not grout the loss zone unless it is clearly a shallow water-bearing zone that must be sealed for construction; grouting a producing fracture can destroy the well's yield. The target fractures at 300-380 ft remain ahead of the casing shoe, so the plan is consistent with local experience.
3. What development and yield-testing steps would you use on this well, and what would you do if the final yield is only 2 gpm?
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Develop with air lifting or surge blocking, working the water column across the open fracture intervals; foam residues should be flushed and the well disinfected afterward. Then run a step-drawdown test at 2, 4, and 6 gpm, followed by a constant-rate test at the intended rate for at least 4 hours, preferably 24 hours. Record specific capacity and recovery. If the well stabilizes at 2 gpm, do not simply install a large pump; a 2-gpm well with 200 ft of storage above the pump can still supply a house with a storage tank and a timer, but the pump must be sized to the well's safe yield. Check whether the low yield is a drilling-damage problem by redeveloping and comparing specific capacity before and after. If the fracture system is simply tight, options include deepening below 400 ft, hydrofracturing where allowed, or relocating. The spring-fed stream downslope warrants a check that the well is not intercepting shallow groundwater that sustains the spring.