Summary Oil/water flow-pattern transitions in horizontal pipes have been studied both experimentally and theoretically. A new state-of-the-art oil/water test facility was designed, constructed, and operated. A transparent test section (5.013-cm inside diameter x 15.54 m long) can be inclined at any angle, to study both upward and downward flow simultaneously. Mineral oil and water were the working fluids (µo/µw=29.6, po/pw=0.85, and o=36 dynes/cm at 25.6°C). A new classification for oil/water flow patterns based on published and acquired data is proposed. Six flow patterns were identified and classified into two categories: segregated flow and dispersed flow. Stratified flow and stratified flow with some mixing at the interface (ST&MI) are segregated flow patterns. The dispersed flow can be either water dominated or oil dominated. A dispersion of oil in water over a water layer and an emulsion of oil in water are water-dominated flow patterns. An emulsion of water in oil and a dual dispersion are oil-dominant flow patterns. The oil/water flow-pattern transitions for light oils are predicted using the two-fluid model and a balance between gravity and turbulent fluctuations normal to the axial flow direction. Stability analyses reveal that the stratified/nonstratified transition must be addressed with the complete two-fluid model. Stratified flow is predicted by the viscous Kelvin-Helmholtz (KH) analysis while inviscid KH theory predicted the ST &MI flow pattern. For the dispersed flow pattern, the predicted drop sizes from the Hinze and Levich models are modified to account for the effect of the dispersed phase concentration. The controlling parameter for the coalescence phenomena is the water fraction. The model performance is excellent and compares well with published data. Introduction The need for reliable design methods for multiphase flow has been the driving force behind an extensive research effort in this area, especially for gas/liquid flow, over the past 30 years. Recently, the industry has turned its attention towards the understanding of the simultaneous flow of gas/oil/water mixtures. However, the limiting case where no gas phase is present has received inadequate attention. Dynamic flow characteristics of oil/water mixtures are important in applications such as designing water-lubricated pipelines, production strings in oil wells, and artificial-lift methods. Understanding of oil/water flow behavior in pipes can be crucial in determining the amount of free water in contact with the pipe wall that could cause corrosion/erosion problems. Oil/water flow behavior is also important in arriving at the correct interpretation of the response of production-logging instruments. The performances of separation facilities and multiphase pumps are a strong function of the upstream flow pattern. A knowledge of the distinctive features of oil/ water mixtures, together with those for gas/liquid systems, can be used in the future as a basis to understand the more complex case of gas/oil/water mixtures.

Wax deposition tests under single-phase and two-phase dispersed water-in-oil at 16 and 35% water cut (WC) were conducted in a mini pilot-scale flow loop by using South Pelto crude oil. The initial inner wall temperature (T̅ w,ini ) and the bulk fluid temperature (T b ) were controlled to be 85−87 °F and 105−107 °F, respectively, throughout the experimental program. The initial Reynolds number (Re ini ) of tests conducted in rectangular (TS1) and circular (TS2) test sections was in the range of 400−4300 and 1000−5300, respectively. A high temperature gas chromatograph with flame ionization detector (HTGC-FID) and a differential scanning calorimeter (DSC) were used to analyze wax deposits. The results revealed that the 16 and 35% water cut tests yielded deposits containing only a trace amount of water (less than 2% w/w). The crossover behavior of the thickness versus time trends for different flow conditions was observed. The initial thickness growth rate increases with the flow rate. After the crossover period ended, the deposit formed under a lower flow rate condition had a higher thickness. In this experimental program, velocity, wall shear stress, Reynolds number, and heat transfer coefficient are coupled together. The initial heat transfer coefficient was found to correlate to the 48 h deposit thickness best compared to the initial wall shear stress, the initial Reynolds number, and the initial velocity. The effect of shear on the deposit composition was observed.

Wax deposition in oil and gas pipelines is considered one of the most severe operational problems, and significant efforts have been made to prevent and remediate this flow assurance issue. Chemical wax control strategies have received considerable attention, especially in certain cases where applying the mechanical and thermal treatment is inconvenient and difficult. However, until now expensive and inefficient “trial-and-error” procedures were used in fields, possibly due to the lack of the fundamental understanding of how these chemicals work in different operating conditions. Here, a comprehensive review of the available literature on chemical wax control strategies is presented. The key performance parameters used in assessing the chemicals in the laboratory are systematically reported. All commonly used wax inhibitors, pour point depressants, and wax dispersants are described based on the types, affecting factors, working mechanisms, and testing facilities. Solvents that are used to dissolve the wax deposits or to reduce the viscosity of the fluid are also discussed. Finally, future challenges ahead for chemical wax control research are discussed.

A unified hydrodynamic model is developed for predictions of flow pattern transitions, pressure gradient, liquid holdup and slug characteristics in gas-liquid pipe flow at all inclination angles from −90° to 90° from horizontal. The model is based on the dynamics of slug flow, which shares transition boundaries with all the other flow patterns. By use of the entire film zone as the control volume, the momentum exchange between the slug body and the film zone is introduced into the momentum equations for slug flow. The equations of slug flow are used not only to calculate the slug characteristics, but also to predict transitions from slug flow to other flow patterns. Significant effort has been made to eliminate discontinuities among the closure relationships through careful selection and generalization. The flow pattern classification is also simplified according to the hydrodynamic characteristics of two-phase flow.

A unified hydrodynamic model is developed for predictions of flow pattern transitions, pressure gradient, liquid holdup and slug characteristics in gas-liquid pipe flow at different inclination angles from −90 to 90 deg. The model is based on the dynamics of slug flow, which shares transition boundaries with all the other flow patterns. By use of the entire film zone as the control volume, the momentum exchange between the slug body and the film zone is introduced into the momentum equations for slug flow. The equations of slug flow are used not only to calculate the slug characteristics, but also to predict transitions from slug flow to other flow patterns. Significant effort has been made to eliminate discontinuities among the closure relationships through careful selection and generalization. The flow pattern classification is also simplified according to the hydrodynamic characteristics of two-phase flow.