Multiphase fluids typically produced from an oil well consist of hydrocarbon liquid, hydrocarbon gas, and an immiscible water phase. These fluids historically must be processed by a multiphase production system near the wells. This arrangement is needed because transfer of the multiphase fluids is achieved through the use of reservoir energy and, in most cases, this energy is insufficient to transfer fluids over any considerable distance. The inherent problem with processing multiphase fluids close to the wells is the high capital and operating cost experienced (both onshore and offshore). A solution to this problem is to obtain a pump that can handle unprocessed multiphase fluids and transport them over considerable distances; the multiphase pump can do this task. This pump is able to boost the pressure of wellhead fluids over considerable distances to a central processing facility, thereby eliminating several smaller local processing facilities. In addition to providing for economic savings from consolidation of surface and offshore facilities, the use of multiphase pumps makes the development of satellite fields more economically attractive. Multiphase pumps also aid in increasing well production rates by lowering required back-pressure on wells.
Multiphase pumps are applicable in services where the gas volume fraction (GVF) is as high as 95%. To determine the GVF, divide the actual gas flow rate by the total mixed flow rate. For a GVF above 95%, the volumetric efficiency decreases, and more fluids (gas) slip back to the pump inlet. Slower speed and higher pressure boost also increase slip, and thus decrease volumetric efficiency. A typical twin-screw multiphase pump is shown in Figure 10-12.