- natural flow,
- artificial lift,
- in-field separation with crude oil transfer pumps, gas to flare, or gas compression systems.
The development of commercial viable multiphase pumps has led to the development of an attractive fourth option.
Multiphase pumping is essentially a means of adding energy to the unprocessed effluent which enables the liquid/gas mixture to be transported over long distances without the need for prior separation. Typically the liquid/gas is transported to an existing processing centre for onward long distance transportation.
Interest for multiphase production, which leads to simpler and smaller in-field installations, is primarily dictated by the need for more a cost effective production system.
Sulzer helico-axial pumps, under normal operating conditions, are largely unaffected by process fluctuations at the pump inlet (changes in pressure, liquid or gas flow rate) and have demonstrated stable behaviour with their self-adaptive capability when subject to instantaneous changes in the gas/oil ratio.
With the recent field deployment of numerous multiphase pumps (MPP) new approaches to field development and production have proven themselves to be a cost-effective, easy to operate and reliable alternative in the development of marginal fields or remote wells.
Specifying a Multiphase Pump
Multiphase pump selection cannot be based solely on one defined operating point (main duty point) as is common for process pumps or compressors for refinery duties.
Field experience shows that actual operating conditions change during the field life and may be different from predictions.
Therefore multiphase pumps should be designed to be capable of facing different operating parameters by having a wide operating envelope.
Flow rates and pressure predictions are never certain. The design of production facilities is generally based on extrapolations from the results of exploration well tests, delineation wells and reservoir characterisation.
As a result, when the production wells are drilled and completed, the actual production data may be different from the prediction.
Furthermore during the field life the production data (oil flow rate, WC, gas fraction, pressure, temperature, etc.) will change. This long term evolution is induced by natural reservoir depletion.
Medium term fluctuations may also be encountered during transient operations (start-up, shut-down, well-testing, pigging operations, etc.) where large gas pockets or liquid slugs may form.
Well instabilities can also contribute to changing flow and pressure conditions at pump suction. Some wells, at end of life, with or without bottom hole activation can demonstrate an unstable cycling behaviour characterised by an active period (producing) followed by an inactive period. These fluctuations, which vary strongly from case to case, are less documented but have been reported and helico-axial multiphase pumps have been operating under such conditions. These well instabilities are characterised by relatively strong fluctuations of the instantaneous gas and liquid flowrates, and consequently the Gas Volume Fraction (GVF, see click here for definition and equations, automatic jump to your Jargon page) expressed in volume/volume at pump suction conditions with a periodicity which can vary between 15 minutes up to several hours.
Finally, short term fluctuations (over several seconds up to a few minutes) induced by slug flow can also affect the pump suction conditions.
In multiphase pumping applications terrain slugging and severe slugging are generally of lesser significance as both phenomena are more likely to happen downstream of the pump for relatively long flowlines. The main impact is a possibly fluctuating pump discharge pressure.
On the surface, pump suction lines are generally short and horizontal therefore prone mainly to hydrodynamic slugging. Obviously severe slugging can happen in the riser or directly in the pipework especially when the flow velocities are low.
Several methods (design and/or modification of flowline riser systems) are available to avoid severe slugging.
Ideally one could try to consider all these fluctuations at design stage of a multiphase boosting system. As this is often not considered at the pump specification stage.
Therefore a multiphase pump should be designed for variable inlet conditions. It should also offer the possibility of a large operating envelope (in terms of flow and GVF variation).
Operational flexibility has to be given priority to efficiency optimisation for a narrow band of operation.
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