In a previous article, I discussed proper hole sampling, screen selection, and size and type of gravel to use to gain the maximum efficiency of a given well. Once you have selected your screen size, material, and gravel size, it is time to get it into place. While some drillers just "cut and dump" the gravel, this method has several drawbacks. It works pretty well on shallow settings, say less than 50', but if you attempt to emplace gravel much deeper, coarser grains tend to fall faster than fine ones, creating a stratified pack that probably isn't right for the formation. The better way is to emplace your pack via tremie line. Obviously, the tremie must be small enough to fit in the hole without pushing the casing against the opposite side of the hole. It must also be large enough to carry the sand. On small holes, 1" hose will work to about 200'. Larger, deeper holes work better with 2" pipe. Be sure to cut a "window" in the tremie line a few feet off the bottom to prevent plugging.
The casing and screens should be run with centralizers, at least in the screen section, to prevent direct contact between the screen and formation. After the casing is run to t.d., it is important to circulate and condition the mud. Reduce the natural sand content and viscosity as much as possible. This will make it much better and easier to develop a gravel pack. Sometimes you may have to pump off and build new mud with fresh gel to get it right. The object is good, clean, fresh mud, with as low a viscosity as you have nerve enough to run in a given hole. Ideally, clear water would be great, but it might not "hold the hole", so a little fresh gel probably won't hurt. You can sometimes circulate the casing while you run your tremie line to the bottom, saving time.