Multiphase flow metering for the oil and gas industry using a visualisation technique

Abstract

Measuring multiphase flows accurately and reliably, particularly when three or more phases are involved, is a challenging and long-standing problem in the upstream oil-and-gas industry. Crude oil extracted from the reservoir is a complex mixture that contains oil, water, gas and sand. Before it is further transported downstream, the crude oil is typically processed in a pressure vessel that separates oil, water and gas. To reduce costs and increase safety, it is critical to accurately measure the streams flow rate (two- and three-phase) at this stage. The conventional metering systems require test separators, where the flow is diverted and measured. This method requires additional capital equipment, large maintenance cost and constant operator intervention. In this paper, a non-intrusive technology that continuously measures multi-phase flow is proposed to overcome the limitations of traditional systems. The novel multiphase flow metering system is based on the combined use of electrical capacitance tomography (ECT) images and knowledge base which is envisaged to provide continuous accurate, reliable, non-intrusive measurements at a minimal cost. In this technique, two ECT sensors are placed at two different locations in an oil pipeline. A hybrid technique is used to evaluate ECT images based on principal component analysis (PCA) and cluster analysis (CA) to identify the time interval, when a specific process condition is detected in both sensors. Once this information is obtained, volumetric flow rate and mass flow rate can then be calculated using the cross sectional area of the pipeline and the average velocity. Initial results with imaging sensors at two points indicate that an error of less than 5% can be achieved, which is acceptable for most applications in the oil and gas industry. This measurement method can be further extended by using three or more points for increased reliability and accuracy.