Liquid/gas systems use 75% to 100% of the volume of drilling fluid required (for a 120-ft/min annular velocity) with typical fluid systems. These systems may be water/air, water/nitrogen, oil/gas, or oil/nitrogen.
The ratio of gas to liquid is in the range of 10/1 to 50/1. Typically, for an 8¾-inch hole, the fluid volume will be 300 gpm and the gas volume will be 1 MMcf/d (700 scfm) to 2 MMscf/d (1400 scfm).
Liquid/gas systems are essentially fluid systems with an air, nitrogen, or natural gas boost. All of the solids-control fluid problems discussed elsewhere in this manual are part of the gaseated system. There normally are no problems separating the gas from the liquid with a simple separator(Figure a.).
Gas/liquid separation in the well on trips and connections and during drilling is the primary problem with all gas-eates systems. The separation causes volume and pressure surges that can be quite violent. In an 8¾-inch hole, using 300 gpm of fluid and 1000 scfm of gas, there can be separations in which there will be no flow at the flowline for as long as 5 minutes, and then 30 bbl of fluid will surge out in half a minute. On a connection or trip, surge can be doubled or tripled in volume, since the system has almost completely separated. As holes get larger and deeper, the surges become larger and more violent. No shaker system can economically handle this kind of volume change. In a regular rig system with open tanks, one or two extra fluid tanks need to be added to the system below the separator to handle the surge volume. The separate fluid tanks should include mixers to keep the cuttings in suspension and a centrifugal pump that will pump the average circulating volume of drilling fluid to the shaker along with cuttings. Once the drilling fluid has passed through the shaker, it can be sent to the regular fluid system and treated as is appropriate to the type of system (Figure b.).
In a commercial closed pressurized circulating system, the surge problem is controlled by the large capacity of the separator tank. Drilling fluid is supplied to the shale shaker by a pump that provides a constant controlled volume from the separator tank.