Well Design and Construction
Well Design and Construction
This domain tests the physical well: casing, annular seal, screen, wellhead, and the details that keep surface water and shallow contamination out. Under Appendix 5-B, casing must be watertight, extend above grade or floor, and be grouted to prevent annular migration. Bedrock wells typically require casing seated at least 10 ft into competent bedrock, with the annular space sealed. Screens are selected from formation samples: slot size should retain most of the formation while passing fines during development.
Core rules
- Casing: use approved material, adequate wall thickness, and centralizers so the annulus is continuous.
- Grout: neat cement or bentonite slurry placed by tremie from the bottom up; never dump grout from the top.
- Wellhead: sanitary seal, vented cap, and a pitless adapter or above-grade discharge; slope grade away from the casing.
- Screen and gravel pack: match slot size to the sieve analysis; a pack that is too fine chokes the screen, too coarse lets formation sand through.
Worked example
You are sealing the annulus of an 8-in borehole with 4-in nominal casing to 60 ft. Annular area = π/4 × (8² − 4²) = 37.7 in² = 0.262 ft². Volume = 0.262 × 60 = 15.7 ft³. Neat cement yields about 1.15 ft³ per sack, so 15.7 ÷ 1.15 ≈ 14 sacks; add 10% for loss and mixing variation, giving about 15–16 sacks. The reasoning matters: use actual hole and casing outside diameters, and confirm the required seal depth and grout type with the approved design.
Traps
- Calculating grout from casing ID instead of borehole diameter minus casing OD.
- Leaving a void by pulling the tremie too fast or grouting in lifts.
- Setting screen by guess rather than formation sample.
- Forgetting centralizers, which let the casing lie against the borehole wall and create an unsealed channel.