Two-phase flow in vertical wells is a common occurrence in oil and gas production. High-liquid viscosity two-phase upward vertical flow in wells and risers presents a new challenge for predicting pressure gradient and liquid holdup due to the poor understanding and prediction of flow behavior, specifically flow pattern. Current two-phase flow mechanistic models were developed, validated, and tuned based on low-liquid viscosity two-phase flow data for which they show accurate flow pattern predictions. The objective of this study is to investigate the effect of liquid viscosity on two-phase flow pattern in vertical pipe flow. Further objective is to develop new/improve existing mechanistic flow-pattern-transition models for high-liquid viscosity two-phase flow in upward vertical pipe flow. High-liquid viscosity flow pattern two-phase flow data was collected from open literature, against which existing flow-pattern transition models were evaluated to identify discrepancies and potential improvements. The evaluation revealed that existing flow transitions do not capture the effect of liquid viscosity. Therefore, two bubble/dispersed bubble flow pattern transitions are proposed in this study for two different ranges of liquid viscosity. The first proposed model modifies Brodkey (1967) critical bubble agglomeration diameter by including liquid viscosity, which is applicable for liquid viscosity up to 100 mPa.s. The second model, which is applicable for liquid viscosities above 100 mPa.s proposes a new critical bubble diameter based on Galileo dimensionless number. Furthermore, the existing bubbly/intermittent flow transition model based on Taitel et al. (1980) critical gas void fraction of 0.25, is modified to account for liquid viscosity. For the intermittent/annular flow transition, Wallis (1969) was found to be accurate for high liquid viscosity two-phase flow and able to capture the high liquid viscosity data better than existing models. A validation study of the proposed transition models against high liquid viscosity data and a comparison with Barnea (1987) model revealed sensitivity to liquid viscosity and better results in predicting high viscosity liquid flow pattern data.

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