A study on the effect of high liquid viscosity on slug flow characteristics in upward vertical flow

Understanding two-phase flow behavior in upward vertical wells is critical for proper well design and operation. Current two-phase slug flow models and their closure relationships are based on experimental data with low viscosity liquids (mu(L) < 20 mPa s). However, recent studies show that two-phase flow behavior of high viscosity liquids is significantly different than that of low viscosity liquids. The objective of this paper is to experimentally and theoretically investigate the effect of high oil viscosity on slug characteristics in upward gas-oil vertical pipe flow. Slug flow characteristics such as translational velocity, slug frequency, and slug length were experimentally measured using a 50.8 mm ID vertical pipe for six different high oil viscosities, namely 586, 401, 287, 213, 162, and 127 mPa s. The experimental results were used to comparatively analyze the physical behavior of flow parameters and evaluate existing mechanistic models and closure relationships, where significant discrepancies are found in prediction of slug frequency and average slug length. Based on a dimensional analysis approach, a new empirical slug frequency closure relationship is developed for high viscosity liquid upward two-phase vertical flow. The new closure relationship is a function of both Froude and viscosity dimensionless numbers. A validation study of the proposed model showed significant improvement in slug frequency prediction for high viscosity when compared with existing slug frequency correlations.