Wet-gas compression in twin-screw multiphase pumps



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Multiphase pumping with twin-screw pumps is a relatively new technology that has been proven successful in a variety of field applications. By using these pumps to add energy to the combined gas and liquid wellstream with minimal separation, operators have been able to reduce capital costs while increasing overall production. In many cases, such as subsea operations, multiphase pumping is the only viable option to make remote wells economic. Despite their many advantages, some problems have been encountered when operating under conditions with high gas volume fractions (GVF). Twin-screw multiphase pumps experience a severe decrease in efficiency when operating under wet-gas conditions, GVF over 95%. Field operations have revealed severe vibration and thermal issues which can lead to damage of the pump internals, requiring expensive maintenance. The research presented in this thesis seeks to investigate two novel methods of improving the performance of twin-screw pumps under wet-gas conditions. The first involves increasing the viscosity of the liquid stream. We propose that by increasing the viscosity of the liquid phase, the pump throughput can be increased. Tests were conducted at high GVF using guar gel to increase the viscosity of the liquid phase. Along with results from a multiphase pump model the pump behavior under wet-gas conditions with increased liquid viscosity was evaluated. The experimental results indicate that at high GVF, viscosity is not a dominant parameter for determining pump performance. Possible reasons for this behavior were proposed. These results were not predicted by current pump models. Therefore, several suggestions for improving the model?s predictive performance were suggested. The second method is the direct injection of liquid into the pump casing. By selectively injecting liquid into specific pump chambers, it is believed that many of the vibration issues can be eliminated with the added benefit of additional pressure boosting capacity. Since this method requires extensive mechanical modifications to an existing pump, it was studied only analytically. Calculations were carried out that show that through-casing liquid injection is feasible. More favorable pressure profiles and increased boosting ability were demonstrated.