Multiphase flow metering has become very important to the development of marginal oil and gas fields, since the economics of producing these fields do not justify the construction of new platforms; instead, they are tied back to existing infrastructure. In such cases, several fields are tied back to common facilities requiring each unprocessed stream to be measured prior to comingling. Multiphase flow metering is also a promising alternative to traditional test separators in those cases where having a large vessel is not feasible, and in the production of heavy oils where separation represents a big challenge.
The increasing need for energy, combined with depleting worldwide light oil resources, has generated substantial interest in heavy oil reserves. Heavy oil applications involve low API gravity oils which are also very viscous. The production, transport, metering, and processing of heavy oils present a number of challenges associated with the increased resistance to flowing given the high-viscosity characteristics of heavy oils and the tendency to form emulsions. The hydrodynamics of high-viscosity oils presents various challenges in multiphase flow metering, including dependence of the discharge coefficient on the viscosity of the oil at low Reynolds numbers on Venturi-type meters used in commercially-available multiphase flow meters (MPFMs). Other issues are also related to the tendency of heavy oils of forming emulsion and the effect of complex fluids on the performance of the meters.
This paper presents a review on heavy oil, multiphase flow metering, and the challenges faced by multiphase flow meters in heavy oil applications. The purpose of this review paper is to provide relevant information that can be used in the selection of multiphase flow meters for heavy oil applications. While other studies on the subject have focused on one specific problem or flow metering technology, this paper provides a general overview of the various issues across existing flow metering technologies. Special focus is given to in-line multiphase flow meters, since they provide some advantages over other types of MPFMs in heavy oil applications. In-line MPFMs have the ability to measure the oil, water, and gas flow rates directly in the multiphase flow stream without separation of the phases. The inherent characteristics and physical properties of heavy oils impose additional constraints to MPFMs, as well as the enhanced recovery methods employed for the production of heavy oils. The main issue is associated with the tendency of heavy oil to form complex mixtures (emulsions, foam, etc.) as they can lead to inaccurate measurement of phase fraction and flow rates. The additional water or steam added to the reservoir to increase recovery rates can result in flow instabilities, high water cuts, and changes on the continuous phase. Thermal production methods can also cause inaccurate readings and compromise the integrity of the meters.