Cobble Zone and Lost Circulation in a Creek-Side Alluvial Well
A C-57 contractor is bidding a 120-ft, 6-inch PVC domestic well on a parcel 80 ft from a seasonal creek. The logged section is 0–18 ft sandy silt, 18–55 ft cobbles and gravel with a sand matrix, and 55–120 ft medium sand with thin clay lenses. The target is the 55–120 ft sand; the cobble zone is known to cave. Static water is 32 ft. A neighboring 100-ft well produces sand and has a specific capacity of 0.8 gpm/ft. The owner wants a 10-inch borehole and a gravel pack. The driller has both a mud rotary rig and a down-the-hole hammer with simultaneous casing. The county permit requires the annular seal to be verified and a 24-hour pump test. The bid must address hole stability, formation damage, and sampling.
1. Recommend a drilling method and borehole/casing sequence for the full interval. Explain why you would not simply use the down-the-hole hammer.
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Use mud rotary with temporary surface casing through the sandy silt, then continue with a low-solids bentonite or polymer mud through the cobble zone. The cobbles and sand matrix are unconsolidated; a hammer with simultaneous casing can advance, but air returns tend to blow fines into the formation, create voids behind casing, and make it hard to get a representative water-quality sample. Mud rotary gives a stable hole, allows cuttings sampling at 5-ft intervals, and keeps the 10-in borehole open for a 2-in gravel pack around 6-in PVC. Set 10-in temporary casing to about 20 ft and, if the cobbles cave, drive a second string to 60 ft. Drill to 120 ft, log water strikes, run 6-in PVC with screen in the 55–120 ft sand, gravel-pack from 120 ft to about 50 ft, and place the annular seal above the pack. Do not use the hammer unless the cobble zone proves too hard for the bit and lost circulation cannot be controlled.
2. What drilling-fluid and sampling controls would you require before the completion string is installed?
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Keep mud weight close to the minimum needed for stability; excess pressure forces filtrate and fines into the sand and lowers specific capacity. Use clean, low-solids mud, control sand content with a desander, and record viscosity, weight, and fluid loss at least every 30 ft. Do not add lost-circulation material in the screen interval; it can plug the gravel pack. Sample cuttings at 5-ft intervals and record the color, grain size, and any change in return rate. Note each water strike and its flow; if a zone is productive, isolate it with a packer or conduct a bail test before drilling ahead. At total depth, circulate clean until returns are stable, then run a caliper or video log if the hole is caving. These records justify the screen and seal depths and give the pump-test baseline.
3. At 50 ft the hole loses returns and the bit drops 3 ft. Outline immediate actions and the revised completion plan.
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Stop drilling and pull the string; a 3-ft drop with lost returns means a void or open gravel seam, not just a seep. Do not continue blind. Set a temporary 8-in casing through 55–60 ft and grout the outside if the formation will take cement; otherwise drive casing with a shoe. Then drill the remaining 60 ft with clean water and a small amount of foam, using a tricone or hammer if the cobbles require it. If returns are still lost below the casing, drill with water only and control the water level; do not use bentonite lost-circulation material near the producing sand. The completion plan changes to a 6-in PVC string with screen only below 60 ft, gravel pack from 60 to 120 ft, and a continuous annular seal from ground surface to 60 ft. Record the loss zone on the log and report it.