collar (fig. 9-9) joins the bits and drill pipe and helpsstabilize the bit and keeps the hole uniform and straight.This collar is 10 feet long and has a diameter larger thanthat of the drill pipe, yet small enough to clear the wallof the hole. The kelly sub, or wear sub, connects thekelly to the drill pipe. When it becomes worn, it can bechanged instead of the entire kelly.In areas where surface formations are soft or sandy,it is necessary to set the casing to keep the walls of thehole from caving and to prevent the hole from beingenlarged by the washing action of the circulating fluid.Sometimes, the hole is reamed or enlarged to set thecasing on a firm foundation. To run the casing, youshould attach a sub to the hoisting plug and threaded intoa length of the casing. Some casings are not threadedand must be welded together. The welded casing can bewashed and cooled down by the mixing hose from themud pump and then run into the hole in the same manneras the drill pipe. If the hole has caved so the casing willnot go down, attach the swivel to the casing with a suband circulate fluid through the casing. This is calledwashing down and is sometimes used to wash the casingto the bottom of a caving hole.The kelly and drill pipe are hollow to allow fluidmixtures of clay and water, air, foam, and so forth, to bepumped through them to the bit. The fluid circulatesthrough the drill pipe and out through the bit, where itsweeps under the bit and picks up the material loosenedby it. It then carries the material to the surface throughthe space between the drill pipe and walls of the well.Fluid from the well overflows into a ditch and passesinto a settling pit, where the cuttings settle. The fluidfree from coarse material and containing onlyfine-grained clay in suspension, flows into another pitand is picked up by the pump for recirculation in thewell.Enough mud-laden fluid (drilling mud) must becirculated to cool and clean the cutting tool properly andto rise in the hole fast enough to carry the cuttings withit. The weight and viscosity of the fluid, aided by theFigure 9-9.-Drill steel, collars, and subs.9-8
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