Under-reaming and Screening

Under-reaming and Screening

In under-reaming, the straight hole must be drilled large enough to accept the outer casing and allow sufficient diameter for grouting. Once the outer casing is set and cemented, the under-reamer is lowered to the bottom of the casing. Drilling then resumes as the under-reamer penetrates the formation, with its cutting blades gradually opening over about five feet of penetration until fully extended. Drilling continues as straight hole drilling to the bottom.

The false head must remain on the well during this process, even though the upper portion has been cased. Removing the head would cause the uncased bottom section to collapse.

After drilling is complete, the required screens and casings are set to the bottom. Gravel pack material is added between the screen and the hole wall. Well-washed, graded rounded gravel is typically used, selected based on formation sand analysis. The screen slot opening size is then determined by the gravel size.

The gravel grains must be large enough not to enter the screen but small enough to hold back the formation sand. Angular or flat gravels are not recommended as they pack together into a solid mass, restricting water flow and potentially ruining the well. Rounded gravel maintains open spaces between grains for smooth water flow into the screen without turbulence.

After gravel placement, the area above is backfilled with impervious fill to grade. Once complete, the well is developed for capacity and clarity.

For double-cased wells, the impervious fill above the gravel pack is usually not added until after development. During development, the gravel pack compresses, and having it accessible allows the crew to add more gravel as needed.