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Modeling twin-screw multiphase pump performance during periods of high gas volume fraction
xiii, 106 leaves : illustrations ; 28 cm.
Multiphase pumping is a new technology used for reducing capital investments, increasing production rates, and improving recoveries in many on-shore projects. Interest is also growing in the subsea/offshore application of this technology. Multiphase pumping adds energy to an unprocessed effluent stream acting as a combined pump and compressor, permitting the recovery of oil and gas on an economical basis. In practice, multiphase production is characterized by wide fluctuations in the gas and liquid mass flow rates. During periods of substantial gas flow, temperature becomes the critical variable determining the performance of the pump. Without the liquid phase to remove the gas compression heat, temperatures in the discharge gas stream begin to rise causing a decrease in efficiency, a high temperature shut-in of the pump, and damage to the pump. While this behavior has been observed in lab experiments and in the field, a review of literature reveals that no mechanistic model exists for predicting the performance of a twin-screw pump under high gas volume flow conditions. Also, very few data are available under high GVF conditions. Both large-scale experiments and mechanistic modeling were performed as part of this research. This thesis models the behavior of a twin-screw multiphase pump under high gas volume fraction conditions. The model of the pump is based on fundamental principles of thermodynamic and fluid mechanics. The specific experimental tasks undertaken in this work include studying the volumetric pump performance during periods of high gas volume fractions, the observation of temperature increase with time under periods of 100% gas volume fraction and studying the effect of viscosity on high GVF performance. The results indicate that the pump can be operated during periods of 100% gas volume fractions. The volumetric efficiency of the twin-screw multiphase pump during periods of high to 100% gas volume fractions can be improved by increasing the viscosity of the process stream or by injecting high viscosity fluids into the pump casing. At 94-95% gas volume fraction the system transitions from isothermal conditions. And the system moves from isothermal to adiabatic (uncooled) conditions with time. A model has been generated that predicts temperature increase with time as a function of the key parameters of pump speed, differential pressure, mass flow rate, specific heat and slip flow. This model can be used to develop novel control philosophies for operating multiphase pumps during high gas volume fractions.
Issued also on microfiche from Lange Micrographics.
Includes bibliographical references (leaves 82-83).