Hydrodynamic slug flow is the commonest flow regime observed in high viscosity liquid–gas horizontal pipelines over a wide range of different flow conditions. Hydrodynamic slugging tends to generate large vibrations that may impose structural instability or even damage oil production pipelines. For that reason, there is a need to investigate high viscosity slug flow regime to understand its complex characteristics. This is pertinent when considering that existing slug flow models used in the petroleum industry to design production pipelines are not suitable for predicting the behaviour of high viscosity oil–gas flow. In this study, the effects of liquid viscosity and flow variables on slug flow regime were investigated experimentally through the analysis of two key parameters—slug frequency and slug body liquid holdup, both measured with a gamma densitometer. Comparison of the measured slug parameters to existing correlations revealed that slug body liquid holdup correlations were in close agreement with high viscosity experimental data. However, none of the existing slug frequency correlations used was able to produce accurate predictions. A new empirical correlation for slug frequency was proposed. Compared with existing correlations, the newly proposed correlation performed much better in predict- ing slug frequency of high viscosity liquid–gas flows.
empirical correlation, gamma beam densitometer, hydrodynamic slug flow regime, multiple- linear regression, slug body liquid holdup, slug frequency, two-phase flow
 Colmenares, J., Ortega, P., Padrino, J. & Trallero, J.L., Slug fl ow model for the prediction of pressure drop for high viscosity oils in a horizontal pipeline, SPE International Thermal Operations and Heavy Oil Symposium, Margarita Island, Venezuela, 2001. doi: http://dx.doi.org/10.2523/71111-ms
 Gokcal, B., Wang, Q., Zhang, H.Q. & Sarica, C., Effects of high oil viscosity on oil/gas fl ow behaviour in horizontal pipes, SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 2006. doi: http://dx.doi.org/10.2523/102727-ms
 Matsubara, H. & Naito, K., Effect of liquid viscosity on fl ow patterns of gas–liquid two-phase fl ow in a horizontal pipe. International Journal of Multiphase Flow, 37(10), pp. 1277–1281, 2011. doi: http://dx.doi.org/10.1016/j.ijmultiphasefl ow.2011.08.001
 Gregory, G.A. & Scott, D.S., Correlation of liquid slug velocity and frequency in horizontal co-current gas–liquid slug fl ow. AIChE Journal, 15(6), pp. 933–935, 1969. doi: http://dx.doi.org/10.1002/aic.690150623
 Gregory, G.A., Nicholson, M.K. & Aziz, K., Correlation of the liquid volume fraction in the slug for horizontal gas–liquid slug fl ow. International Journal of Multiphase Flow, 4, pp. 33–39, 1978. doi: http://dx.doi.org/10.1016/0301-9322(78)90023-x
 Heywood, N.I. & Richardson, J.F., Slug fl ow of air–water mixtures in a horizontal pipe: determination of liquid hold-up by gamma-ray absorption. Chemical Engineering Science, 34, pp. 17–30, 1979. doi: http://dx.doi.org/10.1016/0009-2509(79)85174-x
 Manolis, I., Meendes-Tatis, M. & Hewitt, G., The effect of pressure on slug frequency in two-phase horizontal fl ow, 2nd International Conference on Multiphase Flow, Kyoto, Japan, 1995. doi: http://dx.doi.org/10.1016/b978-0-444-81811-9.50035-4
 Nydal, O.J., An experimental investigation of slug fl ow. PhD Thesis, University of Oslo, Norway, 1991.
 Abdul-Majeed, G.H., Liquid slug holdup in horizontal and slightly inclined two-phase slug fl ow. Journal of Petroleum Science and Engineering, 27, pp. 27–32, 2000. doi: http://dx.doi.org/10.1016/s0920-4105(99)00056-x
 Gomez, L.E., Shoham, O. & Taitel, Y., Prediction of slug liquid holdup: horizontal to upward vertical fl ow. International Journal of Multiphase Flow, 26, pp. 517–521, 2000. doi: http://dx.doi.org/10.1016/s0301-9322(99)00025-7
 Shea, R.H., Eidesmoen, H., Nordsveen, M., Rasmussen, J., Xu, Z.G. & Nossen, J., Slug frequency prediction method comparisons, Proceedings of the 4th North American Conference on Multiphase Technology, pp. 227–237, 2004.
 Gokcal, B., Al-Sarkhi, A.S., Sarica, C. & Al-Safran, E.M., Prediction of slug frequency for high-viscosity oils in horizontal pipes, SPE European Formation Damage Conference, New Orleans, Louisiana, USA, 2009. doi: http://dx.doi.org/10.2118/124057-ms
 Kora, C., Sarica, C., Zhang, H.Q., Al-Sarkhi, A. & Al-Safran, E.M., Effects of high oil viscosity on slug liquid holdup in horizontal pipes, SPE Canadian Unconventional Resources Conference, Calgary, Alberta, Canada, 2011. doi: http://dx.doi.org/10.2118/146954-ms
 Okezue, C.N., Application of the gamma radiation method in analysing the effect of liquid viscosity and fl ow variables on slug frequency in high viscosity oil–gas horizontal flow. WIT Transactions on Engineering Sciences, 79, pp. 447-461, 2013, ISSN 1743-3533. doi: http://dx.doi.org/10.2495/mpf130371
 Hernandez-Perez, V., Gas–liquid two-phase fl ow in inclined pipes. PhD Thesis, University of Nottingham, United Kingdom, 2007.