Identification of horizontal slug flow structures for application in selective cross-correlation metering

Accurate metering of slug flows is important in many industries that handle multiphase products. For the oil and gas industry the harsh environmental conditions mean that non-invasive and non-intrusive instruments are preferred. Cross-correlation meters, particularly those based on electrical tomography, offer a potential solution to this problem but sufficient accuracy has proved difficult to achieve, with the primary issue being that the measurement is dominated by the motion of interfaces rather than the bulk fluid. In the work reported here, results are presented for flows of oil and nitrogen gas in a horizontal pipe of diameter 10.2 cm. Superficial velocities of liquid and gas range from 1 m/s to 3 m/s and 0.4–3 m/s respectively. By analysing the structures of liquid slugs via tomography, it is found that three significantly different slug front structures occur. The high-speed and spatial resolution of Electrical Capacitance Tomography (ECT) enables independent measurement of individual slug fronts and tail as well as average slug velocity. Based on detailed measurements of slug structures and velocity profiles, we go on to show that using differential-based cross-correlation and the average velocity of slug front and tail, an overall accuracy of better than +/−5% is achieved for estimation of the mixture superficial velocity. This is an equivalent level of accuracy to that obtained using intrusive methods such as optical fibre probes, which are less suitable for oil and gas applications.