Experimental Investigation of Wellhead Twin-Screw Pump for Gas-Well Deliquefication
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 deliquefication (GWD) can be accomplished 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 fieldwide compression is available. Even when fieldwide 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 investigated 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 commercially 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 efficiency (flow rate), pump effectiveness, and mechanical efficiency were calculated. Because 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 that will reduce the volumetric efficiency of the pump. It was found that the volumetric efficiency decreased significantly with decreasing pump speed and increasing GVF. At full speed, the volumetric efficiency was between 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.
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 problems, 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.
- Wet-gas compression boosts pressure to transport gas to market without the need to separate the phases, eliminating additional required equipment.
- Uninterrupted flow, even under severe slugging conditions.
- Simplicity, to lower operational expenditure (OPEX): easier to install, operate, and move, and reduces the downtime associated with a single-phase compressor with separation equipment.
- Small footprint for large volume.
- Lower capital expenditure without the need for separation equipment.
- More flexibility to adapt to the well-condition change by means of a variable-frequency-drive (VFD) system.
- Maintain a constant suction pressure by means of a VFD unit.
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