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.).

Figure 1. Closed separator system for underbalanced drilling

Gas is separated within the tank and sent to the flare stack. There is sometimes a free-water knockout tank between the separator and the flare stack. The oil is recycled through the system to pick up cuttings that have settled and is then pumped to the shale shaker, effectively canceling the surging effect. Since the viscosity of the oil is very low, fine screens can be used. The oil and fines are usually sent to a centrifuge for removal of the fine solids.

A buildup of ultra-fine solids in the oil can damage the reservoir because these solids form a sludge that plugs the pore spaces and can plug pipes and pumps when it settles. Eventually the oil has to be replaced to dilute the ultra-fines.

Sample Collection with Aerated Systems

Surging makes precise sample collection difficult. With all gaseated systems, samples need to be caught at the flowline. There is too much mixing and recirculating to get precise depth logged samples from the shale shaker. The best solution is to use the closed tank and screen system shown in Figure 2.

Figure 2. Cross section of a pressurized sample catcher