Since that is unlikely in the near future, the settling tank is extremely important. If the shaker screen were always adequate, never developed a tear that passed oversize solids through, never had to be bypassed during drilling, etc., the major justification for a settling tank (or “shale trap,” or “sand trap”) would disappear. Some information has been published on the subject, but the following points are cardinal:
A special case of liquid/gas fluid is the diesel/nitrogen or synthetic-oil/ nitrogen mixture. These systems have low viscosities and are used primarily in reentries or horizontal slim holes. They tend to require more fluid than water-based fluid systems. Annular velocities of 150–200 ft/ min are common in the horizontal section of the hole. In the drilling of slim holes, hole volumes are less than they are in the drilling of conventional holes, but strong surging occurs. A closed pressurized separator system is used when drilling with an oil/gas system (Figure 19.1.).
Foam in the hole is an emulsion of air or gas in water, but at the flowline a proper foam breaks to a mixture of droplets of water in an air stream. With proper foam breaking at the end of the flowline, there is a quickly separating mixture of gas or air with a small amount of water and a small skim of foam (Figure 1.). During use of a shale shaker, the screen will generally appear ‘‘wet’’ with a skim of foam. This is the result of the chemistry of the system, and while it appears wet with foam, the water volume is very small.
Many of the components in drilling fluids can affect the efficiency of solids-control devices. As discussed in the previous section, fluid rheology, shale inhibition potential, wetting characteristics, lubricity, and corrosivity can all affect both the properties of cuttings and the performance of solids-control equipment. Key components that affect those properties include colloidal materials, macropolymers, conventional polymers, and surface-active materials.