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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.
Abdul-Majeed, Ghassan H. (University of Baghdad (Corresponding author) | Arabi, Abderraouf (email: firstname.lastname@example.org. Now with Al-Mashreq University)) | Soto-Cortes, Gabriel (University of Sciences and Technology Houari Boumediene)
Summary Most of the existing slug (SL) to churn (CH) or SL to pseudo-slug (PS) transition models (empirical and mechanistic) account for the effect of the SL liquid holdup (HLS). For simplicity, some of these models assume a constant value of HLS in SL/CH and SL/PS flow transitions, leading to a straightforward solution. Other models correlate HLS with different flow variables, resulting in an iterative solution for predicting these transitions. Using an experimental database collected from the open literature, two empirical correlations for prediction HLS at the SL/PS and SL/CH transitions (HLST) are proposed in this study. This database is composed of 1,029 data points collected in vertical, inclined, and horizontal configurations. The first correlation is developed for medium to high liquid viscosity two-phase flow (μL > 0.01 Pa·s), whereas the second one is developed for low liquid viscosity flow (μL ≤ 0.01 Pa·s). Both correlations are shown to be a function of superficial liquid velocity (VSL), liquid viscosity (μL), and pipe inclination angle (θ). The proposed correlations in a combination with the HLS model of Abdul-Majeed and Al-Mashat (2019) have been used to predict SL/PS and SL/CH transitions, and very satisfactory results were obtained. Furthermore, the SL/CH model of Brauner and Barnea (1986) is modified by using the proposed HLST correlations, instead of using a constant value. The modification results in a significant improvement in the prediction of SL/CH and SL/PS transitions and fixes the incorrect decrease of superficial gas velocity (VSG) with increasing VSL. The modified model follows the expected increase of VSG for high VSL, shown by the published observations. The proposed combinations are compared with the existing transition models and show superior performance among all models when tested against 357 measured data from independent studies.
Summary Understanding the behavior of two-phase flow is a key parameter for a proper oil/gas-production-system design. Mechanistic models have been developed and tuned to model the entire production system. Most existing two-phase-flow models are derived from experimental data with low-viscosity liquids (μL < 20 mPa·s). However, behavior of two-phase flow is expected to be significantly different for high-viscosity oil. The effect of high liquid viscosity on two-phase flow is still not well-studied in vertical pipes. In this study, the effect of high oil viscosity on upward two-phase gas/oil-flow behavior in vertical pipes was studied experimentally and theoretically. A total of 149 air/high-viscosity-oil and 21 air/water experiments were conducted in a vertical pipe with an inner diameter (ID) of 50.8 mm. Six different oil viscosities—586, 401, 287, 213, 162, and 127 mPa·s—were considered. The superficial-liquid and -gas velocities were varied from 0.05 to 0.7 m/s and from 0.5 to 5 m/s, respectively. Flow pattern, pressure gradient, and average liquid holdup were measured and analyzed in this study. The experimental results were used to evaluate different flow-pattern maps, mechanistic models, and correlations for two-phase flow. Significant discrepancies between experimental and predicted results for pressure gradient were observed.
Summary In this study, we investigate the effect of liquid viscosity (μL) on the slug/churn transition in gas/liquid flows in vertical pipes. A total of 80 experimental churn-flow data points from two different sources are compiled as a data set, covering liquid viscosities from 17.23 to 586 mPa·s. Air was used in these studies as a gas phase with two different liquids, aqueous glycerol and a commercial synthetic mineral oil, flowing in vertical pipes of 0.0192- and 0.0508-m inner diameter (ID). The data set is used to examine the existing slug/churn-flow-transition models and provide further insights into the effect of μL on the transition. The existing models are categorized into two groups according to their response of the slug/churn transition to the increase in liquid superficial velocity (Vsl) on the Vsg/Vsl flow map. The first category exhibits a decrease in superficial gas velocity (Vsg) with the increase in Vsl at slug/churn (the transition concave to the left). The other one predicts an increase in Vsg with increasing of Vsl (the transition concave to the right). Analysis of the data set reveals that on the Vsg/Vsl flow map, the slug/churn transition moves toward lower superficial gas velocities as liquid viscosity increases and occurs approximately at a constant Vsg for low to medium Vsl. The predictions of these models were tested against the data set and poor results were shown by most models. The best performance is given by the Abdul-Majeed (1997) model. A dimensional analysis is applied in the present study to develop a new slug/churn-transition model. This analysis indicates that the transition is related to three dimensionless numbers, namely gas- and liquid-phase Froude numbers, in addition to the inverse liquid-viscosity number. An improved revision to the Abdul-Majeed model is achieved using these three dimensionless numbers. The revision enables the model to predict the transition for low, medium, and high liquid viscosity. The revised model clearly outperforms all the existing models for the present data and viscous data from independent studies. Furthermore, the revised model exhibits the expected trend against changes in pipe diameter and gas density.
Summary Slug-liquid holdup is a critical slug-flow parameter, which affects average liquid holdup and pressure gradient in pipes. Most experimental slug-liquid-holdup studies in the literature were conducted either by use of low-viscosity liquid for all inclination angles or high-viscosity liquid for horizontal and slightly inclined pipes, indicating a lack of experimental data for vertical flow of high-viscosity liquid. Therefore, the objective of this study is to experimentally and theoretically investigate the effect of oil viscosity on slug-liquid holdup in gas/liquid upward vertical flow, and to develop a new closure model to predict slug-liquid holdup in vertical pipes. In this study, experiments were conducted in a 50.8-mm inner-diameter (ID) vertical pipe for six oil viscosities: 586, 401, 287, 213, 162, and 127 mPas. A new slug-liquid-holdup closure model derived from Froude and inverse viscosity numbers was developed in this study for highviscosity-liquid two-phase upward vertical flow. The proposed model was validated against independent experimental data and showed excellent prediction for high-viscosity data. Furthermore, the proposed model was compared with existing models that take into account the viscosity effects showing better performance. The new model was incorporated in the Tulsa University Fluid Flow Projects (TUFFP) unified model (all versions; Zhang et al. 2003b), improving the prediction of pressure gradient and average liquid holdup for high-viscosity upward vertical flow. Introduction Heavy oils are characterized by their high density and viscosity compared with light and medium oils. Al-Ruhaimani et al. (2016) experimentally and numerically studied the effect of high liquid viscosity on flow pattern, pressure gradient, and average liquid holdup.