April 2014 • Oil and Gas Facilities 73 Summary Onshore gas developments are often characterized by drilling, frac- turing, and production of wells before low-pressure gas-gathering systems are in place. As well production declines, liquid-loading is- sues begin to appear. Gas-well deliquefication (GWD) can be ac- complished with compression or in-well artificial-lift methods or both. Wellhead wet-gas compression is desirable in that it does not require well intervention to provide GWD, and it is especially useful in maintaining well production in the interim period before field- wide compression is available. Even when fieldwide compression is available, local wellhead compression is desirable at various lo- cations in a field as high-rate wells are added or for wells located at peripheral locations. The use of a twin-screw pump to provide boost for high-gas-volume-fraction (GVF) multiphase flow was in- vestigated experimentally. Tests were conducted with pressure rises ranging up to 250 psi for GVFs greater than 90%. Water and air were used as the working fluids. The pumping system is a commer- cially available 230-gal/min twin-screw pump (60 hp) with a design speed of 3,600 rev/min used in conjunction with a knock-out tank that recirculates liquid from the pump exit to provide seal flush. The amount of electrical power required to operate the pump, the inlet liquid- and gas-flow rates, the pressure rise, and the inlet and exit temperatures were recorded. From these data, the volumetric effi- ciency (flow rate), pump effectiveness, and mechanical efficiency were calculated. Because there is a fixed clearance between the ro- tating screws and the pump housing, there is a leakage from the high- to low-pressure regions of the pump that will reduce the volu- metric efficiency of the pump. It was found that the volumetric ef- ficiency decreased significantly with decreasing pump speed and increasing GVF. At full speed, the volumetric efficiency was be- tween 70 and 88% at ΔP=50 psi. Increasing ΔP to 250 psi decreased these values to 55 and 81%, respectively. The mechanical efficiency was relatively constant over the pressure-rise range, varying from a high of 48% at the lowest inlet pressure (10 psig) at 0% GVF to a low of 14% for both inlet pressures (10 and 50 psig) at 100% GVF. Overall, the testing demonstrated the ability of a surface twin-screw pump to provide wet-gas compression. Introduction The use of twin-screw pumps to accomplish GWD and enhance production of wet-gas wells has been under investigation for many years. Compression not only solves liquid-loading prob- lems, but also boosts production rates and increases the ultimate recovery. The ability to boost a wet-gas stream provides a number of advantages when compared with conventional single-phase gas compression. 1. Wet-gas compression boosts pressure to transport gas to market without the need to separate the phases, eliminating additional required equipment. 2. Uninterrupted flow, even under severe slugging conditions. 3. Simplicity, to lower operational expenditure (OPEX): easier to install, operate, and move, and reduces the downtime asso- ciated with a single-phase compressor with separation equip- ment. 4. Small footprint for large volume. 5. Lower capital expenditure without the need for separation equipment. 6. More flexibility to adapt to the well-condition change by means of a variable-frequency-drive (VFD) system. 7. Maintain a constant suction pressure by means of a VFD unit. As shown in Fig. 1, twin-screw pumps are rotary positive-dis- placement pumps. As its name indicates, the two-screw pump con- sists of two intermeshing screws. One screw is connected to the motor through the drive shaft and transfers the drive force to the other screw by timing gears. As the two screws counterrotate, they generate a series of C-shaped sealed chambers and push the fluid inside the chambers from the suction end of the screws (“Suction” in the figure) to the pump discharge. The twin screw operates by continuously displacing the inlet volume by moving it along the double screw toward a discharge point. The “no contact” design between screws, screw, and liner enables the pump to tolerate some sand or solids in the fluid. But the clearances between screws, screw, and liners also provide a path for the fluid to flow back from the discharge to the suction, which reduces the volumetric effi- ciency (flow rate) of the pump. Pressure is built up by successive slip flows between cavities, creating less and less gas volume re- lated to the liquid in each successive turn of the screws. Experimental Investigation of Wellhead Twin- Screw Pump for Gas-Well Deliquefication Gerald L. Morrison, Ryan Kroupa, and Abhay Patil, Texas A&M University; Jun Xu, SPE, and Stuart Scott, Shell; and Sven Olson, Leistritz Copyright © 2014 Society of Petroleum Engineers This paper (SPE 159910) was accepted for presentation at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 8–10 October 2012, and revised for publication. Original manuscript received for review 8 January 2013. Revised manuscript received for review 22 August 2013. Paper peer approved 3 September 2013. Fig. 1—Twin-screw-pump cutaway. Discharge Suction Drive Shaft Timing Gears