Gravel packs are a well construction method that allows drillers to achieve maximum efficiency with minimum sand production. They are most often used in unconsolidated sand aquifers, such as most of the coastal plains areas of the world, or in unconsolidated alluvial formations. Often, these aquifers are made up of poorly graded and poorly cemented sands, and the wells will produce sand during pumping.
Some drillers just use a finer slot screen to keep out the sand. This works if the well doesn’t have to make much water for its intended purpose. If a formation is capable of making 100 gpm, but you are only going to install a 10 gpm pump, you can just put in a fine slot screen to shut off all the sand and part of the water. This is common in house wells where competitive pressures keep costs down, but it hurts the efficiency of the well.