Experimental Investigation of Wellhead Twin-Screw Pump for Gas Well Deliquification

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Abstract Onshore gas developments are often characterized by drilling, fracturing and production of wells before low-pressure gas gathering systems are in place. As well production declines, liquid loading issues begin to appear. Gas well deliquification (GWD) can be accomplished using 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 is especially useful to maintain well production in the interim period before field-wide compression is available. Also, even when field-wide compression is available local wellhead compression is desirable at various locations 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 experimentally investigated. Tests were conducted with pressure rises ranging up to 250 psi for GVF's above 90%. Water and air were used as the working fluid. The pumping system is a commercially available 230 gpm twin-screw pump (60 hp) with a design speed of 3600 rpm used in conjunction with a knock out tank used to re-circulate liquid from the pump exit to provide seal flush. The electrical power required to operate the pump, liquid and gas inlet flow rates, pressure rise, and inlet and exit temperatures were recorded. From these data, the volumetric efficiency, pump effectiveness, and mechanical efficiency were calculated. Since there is a fixed clearance between the rotating screws and the pump housing, there is a leakage from the high to low pressure regions of the pump which will reduce the pump's volumetric efficiency. It was found that the volumetric efficiency decreased significantly with decreasing pump speed and decreasing GVF. At full speed the volumetric efficiency was between 70 and 88% at DP=50 psi. Increasing DP 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 and 100% GVF to a low of 14% 14% for both inlet pressures at 100% GVF. Overall, the testing demonstrated the ability of a surface twin-screw pump to provide wet-gas compression and to tolerate 100% gas input, which may results from slugging or periodic flow, for relatively large periods of time.