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Two-phase slug flow is a common occurrence in wells, riser pipes and pipelines of crude oil and natural gas systems. Current predictive tools for two-phase flow are based on either the mixture model or the mechanistic two-fluid model. The latter one, also called phenomenological model, requires the use of closure relations to solve the transfer of mass, momentum and energy between the phases, in the respective conservation equations, so that integral flow parameters such as liquid holdup (or void fraction) and pressure gradient can be predicted. However, these closure relations carry the highest uncertainties in the model, since they are obtained empirically or through the use of overly simplified assumptions. In particular, significant discrepancies have been found between experimental data and closure relations for the Taylor bubble velocity in slug flow, which has been determined through an in-house study to strongly affect the pressure gradient and liquid holdup predicted by the mechanistic models of (Orell and Rembrand, 1986), (Ansari et al., 1994), and (Petalas and Aziz, 2000). In this work, Computational Fluid Dynamics (CFD) and the Level Set (LS) interface tracking method (ITM), implemented in the commercial code TransAT®, are employed to simulate the motion of Taylor bubbles in slug flow. Therefore, a numerical database is being generated to develop a new, high-fidelity closure relation for the Taylor bubble velocity as a function of the fluid properties and flow conditions, rendered non-dimensional through the use of the Froude, Reynolds, Eötvös and Morton numbers, and pipe inclination angle. The simulations suggest that in inclined pipes the Taylor bubble velocity is strongly reduced if there is no lubricating liquid film between the bubble and the wall. A simple analytical model predicting the drainage of this lubricating film is also presented.
IntroductionTwo-phase slug flow is a common occurrence in wells, riser pipes and pipelines of crude oil and natural gas systems. Current predictive tools for two-phase flow are based on either the mixture model or the mechanistic two-fluid model (Brill and Mukherjee, 1999). In the latter one, slug flow is modeled as a sequence of fundamental units, also called slug units. Each unit contains a long bullet-shaped bubble, known as Taylor bubble, and a liquid portion with smaller homogeneously distributed bubbles, known as liquid slug.
Arabi, A. (SONATRACH, Direction Centrale Recherche et Développement, and University of Sciences and Technology Houari Boumediene (USTHB)) | Azzi, A. (University of Sciences and Technology Houari Boumedien (USTHB)) | Kadi, R. (SONATRACH, Direction Centrale Recherche et Développement) | Al-Sarkhi, A. (King Fahd University of Petroleum and Minerals) | Hewakandamby, B. (University of Nottingham)
Summary Intermittent flow is one of the most complex flow regimes in horizontal pipes. Various studies have classified this regime as two distinct subregimes: plug and slug flow. This classification has been made based on flow observations. In this work, the behavior of several flow parameters that characterize plug and slug flow are presented. Data from eight published works in the open literature were collected and studied to explain the behavior of both regimes. These data include pressure drop, void fraction, and slug frequency, as well as the lengths of liquid slugs and elongated bubbles for slug and plug regimes. It is observed from the evolution and analysis of these parameters that plug and slug flows have several different distinct features and should be considered as two separate regimes for the empirical modelization of the hydrodynamic parameters. The mixture Froude number, and to a lesser extent the liquid superficial velocity to gas superficial velocity ratio, seem to have significant impacts on the plug-to-slug flow transition. Introduction A variety of fluids (oil, natural gas, water, condensates, and condensed vapor) in the petroleum and gas industry can form two-phase flow.
Abstract A detailed understanding of wellbore flow is essential for production engineers in both the design of site equipment and optimisation of operation conditions. With the depletion of conventional resources, the need for unconventional extraction techniques to leverage untapped reserves has seen the generation of new downhole flow conditions. In particular, the extraction of natural gas from coal seams has led to scenarios where liquid removal from the reservoir can cause the development of a counter-current multiphase flow in the well annulus in pumped wells. In this work, high-fidelity computational fluid dynamics is used to capture the momentum interaction between gas and liquid phases in such a flow configuration, allowing for the evaluation and modification of closure relations used in upscaled models. The computational fluid dynamics model is based on a recently proposed formulation developed using phase-field theory in the lattice Boltzmann (LB) framework. It has been previously applied to the analysis of Taylor bubbles in tubular and annular pipes at a range of inclinations and flow directions. The robustness of the numerical formulation has been proven with a range of benchmark scenarios that extend upon previously reported results in the LB literature. Future investigations will look to apply the developed closure relations into the two-fluid model and compare with in-house experimental and mechanistic results. Using the multiphase lattice Boltzmann model, the drag force closure relations are investigated for bubbles covering a range of parameters. This assesses the accuracy of existing closures and provides confidence in the developed computational tool. Following on from this, the size of the liquid slug behind a Taylor bubble is analysed. Comparison of the results with pre-existing relations provides a means to modify current large-scale simulators to accurately capture the momentum exchange between gas and liquid phases in a wellbore. With the improved understanding of phase interactions developed in this study, upscaling work is to be conducted through the implementation of closure models within a two-fluid-type model, not unlike OLGA, as well as in a recent mechanistic model. The novelty of the high-fidelity computational model is in its ability to resolve high density ratio (liquid-gas) flows under complex, dynamic conditions within the lattice Boltzmann framework. Additionally, the development and validation of novel closure relations for mechanistic and two-fluid models improves the accuracy of predictions associated with wellbore operations, ultimately allowing for more optimised production.
Pseudo-slug flow is a sub-regime of intermittent flow that is characterized by short, undeveloped, frothy chaotic slugs, with translational velocity less than the mixture velocity of the fluids. Pseudo-slug flow does not comply with the basic characteristics of conventional unit-cell slug flow where liquid blocks the entire pipe cross-sectional area, and liquid is scooped at slug front, transferred to slug body, and shed back to liquid film. The liquid in pseudo-slug body is insufficient to reach the upper part of the pipe wall, resulting in only large wave with entrained gas bubbles at the bottom part of the pseudo slug body. Consequently, a significant reduction in the gas phase flowing area above the wave is formed, which increases the local gas velocity, entraining large volume of liquid droplets in the upper part of the slug body. Therefore, the pseudo-slug body can be divided into two regions, liquid film (wave) with entrained gas bubbles at the bottom, and gas core with entrained liquid droplets. The objective of this study is to develop a plausible physical model of the experimentally observed pseudo-slug liquid holdup phenomenon and model the physical and hydrodynamic behavior using a dimensional regression modeling approach.
This paper discusses liquid and gas entrainment mechanisms within pseudo-slug body based on experimental observation. Previous experimental results show that the proposed dimensionless groups; namely, Stokes, Slippage, and Poiseuille are strongly correlated to pseudo-slug body liquid holdup experimental data and are capable of describing the experimentally observed physical behavior. A linearized regression model is developed to combine the liquid holdup proportionally in both regions of the pseudo-slug body (mentioned above) and correlate them to the experimentally measured total pseudo-slug liquid holdup using wire mesh sensor. A validation study of the proposed model with
Abstract Much work has been done to properly characterize the two-phase flow of liquid and gas in pipes. The importance of these flows extends over several branches of Engineering, notably nuclear and petroleum engineering. During their transportation, these two phases are subjected to several factors - gravity, viscosity and density gaps, different relative velocities between the phases and turbulence, to name a few relevant ones - that act in the spatial distribution of the liquid-gas interfaces bringing forth those that are known as flow regimes or, most commonly, flow patterns. The type of flow known as slug flow stands out as one of the most frequent, or the most frequent thereof, among these patterns. It main characteristics are intermittency and the continuous succession of two well-defined structures: an elongated bubble, which is a bulky accumulation of gas wrapped in a thin liquid film and the liquid slug, a significant amount of liquid containing mostly liquid with gas bubbles of variable size in its interior. Those two-structures, together, constitute that what is known a slug unit or, from a more computational standpoint, a unit cell. Due to their complexity, modelling slug flows has been a challenge over the last four decades. In the present day, steady-state one-dimensional models based on the unit cell concept and more accurate physical representations based on two-fluid models or slug tracking models which embed transient flow capabilities are available. Nevertheless, and notwithstanding the efforts that have been carried out, those models still require closure relationships that could bring three-dimensional flow features into the one-dimensional model, and those relationships must be backed by trustworthy experimental data. With this scenario in mind, the present work brings forward a brief review on two-phase, liquid-gas slug flows in circular pipes, be they horizontal, vertical or inclined, and a comprehensive state-of-the-art on the closure relationships aimed at predicting flow parameters such as the front bubble velocity, slug frequency and liquid slug holdup. An analysis of the performance of these correlations with experimental data for horizontal slug flows from the Universidade Estadual de Campinas' 2-Phase Flow Group (2PFG/DE/UNICAMP) was carried out and its results are presented.