Cem Sarica

A Comprehensive Mechanistic Model for Upward Two-Phase Flow in Wellbores

A comprehensive model is formulated to predict the flow behavior for upward two-phase flow. This model is composed of a model for flow-pattern prediction and a set of independent mechanistic models for predicting such flow characteristics as holdup and pressure drop in bubble, slug, and annular flow. The comprehensive model is evaluated by using a well data bank made up of 1,712 well cases covering a wide variety of field data. Model performance is also compared with six commonly used empirical correlations and the Hasan-Kabir mechanistic model. Overall model performance is in good agreement with the data. In comparison with other methods, the comprehensive model performed the best.

A Study of Oil-Water Flow Patterns in Horizontal Pipes

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 (μ o /μ w = 29.6, ρ o /ρ w = 0.85 and σ = 36 dynes/cm @ 25.6 °C). Only horizontal flow tests were conducted. 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. Conductance probe data and high speed photographs were found to be adequate flow pattern identification tools while wall pressure fluctuations are not. Pressure drop decreases when the transition to dispersed flow is crossed. Slippage is only relevant for segregated 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. Linear and non-linear analyses reveal that the stratified/non-stratified transition must be addressed with the complete two-fluid model. Stratified flow is predicted by the viscous Kelvin-Helmholtz analysis while inviscid Kelvin-Helmholtz theory predicted the ST & MI flow pattern. Both the viscous Kelvin-Helmholtz analysis and structural stability criterion are satisfied simultaneously. For the dispersed flow pattern, the predicted drop sizes from the Hinze and Levich models are modified in order 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.

Unified model for gas-liquid pipe flow via slug dynamics: Part 1: Model development

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.

Unified Model for Gas-Liquid Pipe Flow via Slug Dynamics—Part 2: Model Validation

In Zhang et al. [1], a unified hydrodynamic model is developed for prediction of gas-liquid (co-current) pipe flow behavior based on slug dynamics. In this study, the new model is validated with extensive experimental data acquired with different pipe diameters, inclination angles, fluid physical properties, gas-liquid flow rates and flow patterns. Good agreement is observed in every aspect of the two-phase pipe flow.

A unified mechanistic model for slug liquid holdup and transition between slug and dispersed bubble flows

Abstract A unified mechanistic model for slug liquid holdup is developed based on a balance between the turbulent kinetic energy of the liquid phase and the surface free energy of dispersed spherical gas bubbles. The turbulent kinetic energy is estimated by use of the shear stress at the pipe wall and the momentum exchange (mixing term or acceleration term) between the liquid slug and the liquid film in a slug unit. The momentum exchange term varies significantly with pipe inclination and enables the model to give an accurate prediction of slug liquid holdup for the entire range of pipe inclination angle. The model has been compared with experimental data acquired at TUFFP for slug flows at all inclinations and good agreement has been observed. The model can also be used to predict the slug–dispersed bubble flow pattern transition boundary over the whole range of inclination angles. From comparison with previous experimental results, the model predictions are accurate for gas superficial velocities larger than 0.1 m/s.

A simplified transient model for pipeline-riser systems

Abstract Low liquid and gas flow rates in a pipeline-riser system can cause occurrence of the undesirable severe slugging phenomenon. An improved model for the prediction of the transient flow behavior in this system is presented. The improved model and other existing models for severe slugging are tested against a broad range of data. The present model predicts more accurately the system variables such as pipeline pressure transients, liquid accumulation, slug length, and cycle time for all flow conditions.

Investigation of Holdup and Pressure Drop Behavior for Oil-Water Flow in Vertical and Deviated Wells

Two-phase flow of oil and water is commonly observed in wellbores, and its behavior under a wide range of flow conditions and inclination angles constitutes a relevant unresolved issue for the petroleum industry. Among the most significant applications of oil-waterflow in wellbores are production optimization, production string selection, production logging interpretation, down-hole metering, and artificial lift design and modeling. In this study, oil-water flow in vertical and inclined pipes has been investigated theoretically and experimentally. The data are acquired in a transparent test section (0.0508 m i.d., 15.3 m long) using a mineral oil and water (ρ ο /ρ w = 0.85, μ o /μ w = 20.0 & σ o-w = 33.5 dyne/cm at 32.22°C). The tests covered inclination angles of 90, 75, 60, and 45 deg from horizontal. The holdup and pressure drop behaviors are strongly affected by oil-water flow patterns and inclination angle. Oil-water flows have been grouped into two major categories based on the status of the continuous phase, including water-dominated and oil-dominated flow patterns. Water-dominated flow patterns generally showed significant slippage, but relatively low frictional pressure gradients. In contrast, oil-dominated flow patterns showed negligible slippage, but significantly large frictional pressure gradients. A new mechanistic model is proposed to predict the water holdup in vertical wellbores based on a drift-flux approach. The drill flux model was found to be adequate to calculate the holdup for high slippage flow patterns. New closure relationships for the two-phase friction factorfor oil-dominated and water-dominated flow patterns are also proposed.

Effect of perforation density on single phase liquid flow behavior in horizontal wells

Horizontal wells can have very complex flow geometries, in part due to interaction between the main flow stream and the influxes along the wellbore, and also due to completion type. In this study, the flow behavior in horizontal wells with a single perforation and with multiple perforations of perforation densities equivalent to 1, 2 and 4 shots per foot were investigated. A new test facility was designed and constructed to simulate single phase liquid flow in horizontal wells. Experiments were conducted with Reynolds numbers ranging from 5,000 to 60,000 and influx to main flow rate ratios ranging from 1/5 to 1/100 for the single injection case and from 1/100 to 1/2000 for the multiple injection case and for the no influx case. A general horizontal well friction factor expression was developed using the principles of conservation of mass and momentum. Horizontal well friction factor correlations were developed by applying experimental data to the general friction factor expressions. It was observed that the friction factor for a perforated pipe with fluid injection can be either smaller or greater than that for a smooth pipe, depending on influx to main flow rate ratios. The proposed friction factor correlation can be used in any horizontal well model which considers pressure variation along the wellbore.

Effect of Completion Geometry and Phasing on Single-Phase Liquid Flow Behavior in Horizontal Wells

Horizontal wells can have very complex flow geometries, in part due to interaction between the main flow stream and the influxes along the wellbore, and also due to completion type. An experimental facility was used to investigate the effects of completion geometries and the density and phasing of injection openings in horizontal wells. Three test sections with perforation densities of 5, 10 and 20 shots per foot and phasings of 360°, 180° and 90° were. Four test sections were used for slotted liners, including one single slot case and three multiple slot cases. The numbers of slots for multiple slot cases were 18, 18 and 36 on 4-ft. long sections with slot phasings of 360°, 180° and 90°, respectively. A total of 1,257 tests were conducted for no fluid injection, no main flow at the test section inlet, and with fluid injection for Reynolds numbers ranging from 4,000 to 60,000 and for influx to main flow rate ratios ranging from 1/5 to 1/2000. Experimental results show that completion geometry, phasing and density in addition to Reynolds number and influx to main flow rate ratio have dramatic effects on the pressure behavior and therefore the production behavior of horizontal wells. A general friction factor expression for horizontal wells with multiple injection openings was developed based on the conservation of mass and momentum. A commercial Computational Fluid Dynamics (CFD) computer program was used to determine the length of the flow developing region in a horizontal well after the flow is disturbed due to radial influx. Applying experimental data and the result of CFD simulation to the general friction factor expression resulted in new simple correlations for horizontal well friction factors. Very good agreement was found between the friction factor correlations and experimental data. A field example is presented to show the importance of using a proper friction factor correlation to calculate the pressure drop in a horizontal well.

An Experimental Study on Mechanics of Wax Removal in Pipeline

Pigging has been recognized as the most cost-effective method for preventing flow restriction by wax deposits in subsea flowlines. However, the pigging mechanics for wax removal in pipelines is still very poorly understood. A unique test facility was designed and constructed for experimental studies on the mechanics of wax removal in pipelines. The test facility consisted of a test section, a support structure, an apparatus to pull the pig through the test pipe, and a computer-based data acquisition system. The test section was 6.4 m(21 ft) long and was made from 0.0762 m(3 in.) inner diameter schedule-40 steel pipe. The mixture of commercial wax and mineral oil was cast inside the test section at different wax thickness and oil contents. A series of experiments was performed to investigate the wax removal mechanics with three different types of conventional pigs. i.e., cup, disc, and foam pigs. The experiments showed that a typical wax removal process using a pig followed four distinct phases. namely wax breaking, plug formation, accumulation. and production phases. Wax accumulation can be very significant and is expected to he the dominating factor for the force required for moving a pig in long pipelines. As wax thickness and hardness increases, the required force to move the pig increases. The shape and material of the pig have a profound effect on the wax removal performance. While the disc pig provides the most efficient wax removal, the force requirement is excessive, especially for thicker and harder wax deposits. The wax removal performance of a cup pig is very similar to that of a disc pig. However, the cup pig can withstand higher load without mechanical damages than the disc pig. The foam pig offers the poorest wax removal performance.