The drift velocity of a gas bubble penetrating into a stagnant liquid is investigated experimentally in this paper. It is part of the translational slug velocity. The existing equations for the drift velocity are either developed by using the results of Benjamin (1968) analysis assuming inviscid fluid flow or correlated using air/water data. Effects of surface tension and viscosity usually are neglected. However, the drift velocity is expected to be affected by high oil viscosity. In this study, the work of Gokcal et al. (2009) has been extended for different pipe diameters and viscosity range. The effects of high oil viscosity and pipe diameter on drift velocity for horizontal and upward-inclined pipes are investigated. The experiments are performed on a flow loop with a test section with 50.8-, 76.2-, and 152.4-mm inside diameter (ID) for inclination angles of 0 to 90°. Water and viscous oil are used as test fluids. New correlation for drift velocity in horizontal pipes of different diameters and liquid viscosities is developed on the basis of experimental data. A new drift-velocity model/approach are proposed for high oil viscosity, valid for inclined pipes inclined from horizontal to vertical. The proposed comprehensive closure relationships are expected to improve the performance of two-phase-flow models for high-viscosity oils in the slug flow regime.

Summary Oil/gas pipe flows are expected to exhibit significantly different behavior at high oil viscosities. Effects of high-viscosity oil on flow pattern, pressure gradient, and liquid holdup are experimentally observed, and differences in flow behavior of high- and low-viscosity oils are identified. The experiments are performed on a flow loop with a test section of 50.8-mm ID and 18.9-m-long horizontal pipe. Superficial liquid and gas velocities vary from 0.01 to 1.75 m/s and from 0.1 to 20 m/s, respectively. Oil viscosities from 0.181 to 0.587 Pa·s are investigated. The experimental results are used to evaluate the performances of existing models for flow pattern and hydrodynamics predictions. Comparisons of the data with the existing models show significant discrepancies at high oil viscosities. Possible reasons for these discrepancies are carefully examined. Some modifications are identified and implemented to the closure relationships employed in the Zhang et al. (2003) model. After these modifications, the model predictions provide better agreement with experimental results for flow pattern transition, pressure gradient, and liquid holdup. Introduction Gas/liquid two-phase flow in pipes is a common occurrence in the petroleum, chemical, nuclear, and geothermal industries. In the petroleum industry, it is encountered in the production and transportation of oil and gas. Accurate prediction of the flow pattern, pressure drop, and liquid holdup is imperative for the design of production and transport systems. High-viscosity oils are discovered and produced all around the world. High-viscosity or "heavy oil" has become one of the most important future hydrocarbon resources, with ever-increasing world energy demand and depletion of conventional oils. Almost all flow models have viscosity as an intrinsic variable. Two-phase flows are expected to exhibit significantly different behavior for higher viscosity oils. Many flow behaviors will be affected by the liquid viscosity, including droplet formation, surface waves, bubble entrainment, slug mixing zones, and even three-phase stratified flow. Furthermore, the impact of low-Reynolds-number oil flows in combination with high-Reynolds-number gas and water flows may yield new flow patterns and concomitant pressure-drop behaviors. The literature is awash with two-phase studies addressing mainly the flow behavior for low-viscosity liquids and gases. However, very few studies in the literature have addressed high-viscosity multiphase flow behavior. In this literature review, the state-of-the-art of two-phase flow is first summarized. Then, the studies addressing the effects of liquid viscosity on two-phase oil/gas flow behavior are reviewed.

Summary Slug frequency is defined as the number of slugs passing at a specific point along a pipeline over a certain period of time. Most experimental studies related to slug frequency in the literature were conducted using air and water. Data with a viscous liquid phase are scarce. Knowledge of the effect of liquid viscosity on slug flow is crucial to size pipelines and design preprocess equipment. In this study, the effects of high oil viscosity on slug frequency for horizontal pipes are investigated experimentally. The experiments are performed at oil viscosities between 0.181 and 0.589 Pa·s in a horizontal pipe. Experimental results are compared with the existing slug-frequency correlations. Experimental observations reveal that slug frequency appears to be a strong function of liquid viscosity. However, existing slug-frequency closure models do not show any explicit dependency on liquid viscosity. A closure model taking into account viscosity effects for horizontal pipes on slug frequency is proposed. The proposed slug-frequency model is compared against published data. The comparison between the proposed closure model and the limited published data shows that the former is a better alternative than existing correlations for high-viscosity oils. The proposed slug-frequency closure model can improve the performance of the existing mechanistic models for high-viscosity-oil applications.

Slug frequency is defined as the number of slugs passing at a specific point along a pipeline over a certain period of time. Most experimental studies related to slug frequency in the literature were conducted using air and water. Data with a viscous liquid phase are scarce. Knowledge of the effect of liquid viscosity on slug flow is crucial to size pipelines and design pre-process equipment. In this study, the effects of high oil viscosity on slug frequency for horizontal pipes are experimentally investigated. The experiments are performed at oil viscosities between 0.181 and 0.589 Pa•s in a horizontal pipe. Experimental results are compared with the existing slug frequency correlations. Experimental observations reveal that slug frequency appears to be a strong function of liquid viscosity. However, existing slug frequency closure models do not show any explicit dependency on liquid viscosity. A closure model taking into account viscosity effects for horizontal pipes on slug frequency is proposed. The proposed slug frequency model is compared against published data. The comparison of proposed closure model against limited published data shows that it is a better alternative than existing correlations for high viscosity oils. The proposed slug frequency closure model can improve the performance of the existing mechanistic models for high viscosity oil applications. Introduction Slug flow is one of the most observed flow patterns that characterize the gas-liquid flow in multiphase transportation pipelines. The most distinctive characteristic of slug flow is its intermittent nature due to a unique phase distribution. It occurs over a wide range of gas and liquid flow rates. Extensive experimental studies have been conducted to understand the mechanism of slug formation. However, very few studies have addressed effect of high viscosity behavior in slug characteristics. Gokcal et al. (2006) observed slug flow to be the dominant flow pattern for the high viscosity oil and gas flows. They found that slug frequency increased while the slug length decreased as the liquid viscosity increased. Prediction of slug frequency is important for design of transportation pipelines and gas-liquid receiving facilities. Slug frequency is required as an input in mechanistic models to predict slug flow characteristics such as pressure gradient and liquid holdup accurately.

Summary The translational velocity, velocity of slug units, is one of the key closure relationships in two-phase flow mechanistic modeling. It is described as the summation of the maximum mixture velocity in the slug body and the drift velocity. The existing equation for the drift velocity is developed by using potential flow theory. Surface tension and viscosity are neglected. However, the drift velocity is expected to be affected with high oil viscosity. In this study, the effects of high oil viscosity on drift velocity for horizontal and upward inclined pipes are experimentally observed. The experiments are performed on a flow loop with a test section 50.8 mm ID for inclination angles of 0° to 90°. Water and viscous oil are used as test fluids. Liquid viscosities vary from 0.001 to 1.237 Pa•s. A new drift velocity model is proposed for high oil viscosity for horizontal and upward inclined pipes. The experimental results are used to evaluate the performances of proposed model for drift velocity. The calculated drift velocities are compared very well with the experimental results. The proposed model could be easily implemented into translational velocity equation. It should improve the existing two-phase flow models in the development and maintenance of heavy oil fields. Introduction High-viscosity oils are produced from many oil fields around the world. Oil production systems are currently flowing oils with viscosities as high as 10 Pa•s. Current multiphase flow models are largely based on experimental data with low viscosity liquids. Commonly used laboratory liquids have viscosities less than 0.020 Pa·s. Multiphase flows are expected to exhibit significantly different behavior for higher viscosity oils. Gokcal et al. (2008b) observed slug flow to be the dominant flow pattern for the high-viscosity oil and gas flows. The knowledge of the slug flow characteristics is crucial to design pipelines and process equipments. In order to improve the accuracy of slug characteristics for high-viscosity oils, new models for slug flow are needed such as translational velocity. Translational velocity is composed of a superposition of the bubble velocity in stagnant liquid (i.e. the drift velocity, vd, and the maximum velocity in the slug body). The research efforts have been focused on the drift velocity in horizontal and upward inclined pipes.