Some reservoirs produce not only oil and gas, but also sand. A formation produces sand when the individual grains making up the reservoir rock are unconsolidated. In other words, the rock grains do not adhere to each other. Sand can flow into the well along with the fluids and can clog it up to reduce or stop production. Sand also damages the equipment in the hole. In reservoirs with sand, then, a completion sometimes includes a wire-wrapped screen, or screen liner, plus a gravel pack to keep sand out.
Gravel pack
Wire-wrapped screen is a relatively short length of pipe with holes or slots in its sides (the slotted liner) and a specially shaped wire wrapped around it. A gravel pack is made up of fine gravel, which is actually coarse sand that filters out the finer sand in the formation. To place the gravel pack, first the crew circulates a fluid into the hole to wash out sand at the bottom. Then the crew pumps the gravel down the tubing in a viscous (thick) fluid. Finally, the crew runs in the wire-wrapped screen.