Sand Aquifer: Screen Slot, Gravel Pack, and 200-gpm Demand
An irrigation well is drilled with bentonite mud to 140 ft in a confined sand aquifer. The borehole is 12 inches; 8-inch PVC casing is set to 90 ft, and a 40-ft screen is planned from 90 to 130 ft. Water level is 30 ft. Sieve analysis of the formation shows 60% fine sand (0.15–0.25 mm), 30% medium sand (0.25–0.50 mm), and 10% coarse sand (0.50–1.0 mm). Mud weight is 9.2 ppg and sand content is 2% at total depth. A step test at 150 gpm produces 40 ft of drawdown; the client wants 200 gpm for center-pivot irrigation. The state requires a gravel pack when the aquifer is finer than 0.5 mm and a screen sized to retain 90% of the pack. The driller proposes a 0.040-inch slot screen and development by overpumping at 250 gpm.
1. Evaluate the proposed 0.040-inch slot. What screen slot and filter-pack gradation would you specify, and why?
Read the worked answer
The formation is 90% finer than 0.50 mm, so a gravel pack is required. The formation d50 is about 0.22 mm, between the fine and medium sand fractions. A common filter-pack rule is 4–6 times the formation d50, so the pack should be about 0.9–1.2 mm. The screen should retain at least 90% of the pack; that puts the slot near 0.020–0.025 inch (0.5–0.6 mm). A 0.040-inch slot is too coarse: it would pass much of the pack and allow formation sand into the well. I would specify a continuous-slot screen, 0.020–0.024 inch, with a 2–3 inch gravel envelope, and check the slot against the actual pack sieve, not just the formation sample. I would also confirm screen length and open area to keep entrance velocity low.
2. Assess the proposal to develop by overpumping at 250 gpm. Recommend a development program and verification criteria.
Read the worked answer
Overpumping at 250 gpm is not a reliable development method in a mud-drilled sand well. It can compact the gravel pack, pull fines into the screen, and cause sand production without removing filter cake. I would develop first with a surge block or mechanical surging, working the screen in stages, then air-lift or bail to remove mud and fines. If a dispersant is used, it must be compatible with the formation and approved for the use. Alternate surge and pump cycles, start at a low rate, increase gradually, and stop if the pack starts moving. Verification: very low sand content in the discharge (target below about 1 ppm), improving specific capacity, clear water, and no drawdown increase at the same rate. Check that the gravel pack level is still above the screen.
3. Estimate whether 200 gpm is realistic. Use the step-test data and recommend a pump setting, noting any conflict with the screen interval.
Read the worked answer
Specific capacity at 150 gpm is 150/40 = 3.75 gpm/ft. A straight-line estimate at 200 gpm gives 53 ft of drawdown, so pumping level = 30 + 53 = 83 ft. That is only 7 ft above the top of the screen at 90 ft. A pump set below the pumping level would have to sit at or below 90 ft, inside the screen, with little submergence. That is a real constraint: the well may make 200 gpm hydraulically, but the pump cannot be set deep enough without risking cavitation and sand entry. Run a 4-hour constant-rate test at 200 gpm, measure drawdown and specific capacity, and if it holds, set the pump at about 95–100 ft with a shroud or as recommended by the pump manufacturer. If drawdown is non-linear or sand appears, limit the well to 150–170 gpm or deepen the well and lower the screen.