Simulating Slug Flow in Hilly-Terrain Pipelines

Abstract

A simulator has been developed to track two-phase slugs in pipelines transporting liquid and gas mixtures. The algorithm consists of close coupling of a one dimensional hydrodynamic slug flow model with an interface tracking methodology, and solving both simultaneously with an iterative procedure. The tracking scheme is based on propagating the fronts and backs of the liquid slugs to new locations during an incremental time step. New positions of the interfaces determine if a slug will enter the pipeline, exit the pipeline, collapse, merge with a slug ahead of it, or none of the above. The solution procedure determines the locations and the characteristics of all the slug units which exist in the pipeline at a given time. Data collected in hilly-terrain and horizontal pipes in a large-scale multiphase flow loop were used to validate the slug tracking simulator. The average absolute percent errors in predicting the maximum slug length and inlet pressure were 12.6 and 0.47 respectively. A case study with field data collected on a 14,762 feet, 16 in. pipeline showed that the simulator predicted the maximum slug length and inlet pressure with absolute percent errors of 11.6 and 4.3 respectively. The comparisons are good and provide confidence in using the algorithm and the simulator to model and track two-phase slugs in hilly-terrain pipelines. The simulator can be used to determine the characteristics of the slug unit used to design separators and slug catchers. It can also be used to analyze the impact of the flow and pressure transients on reservoirs, equipment, and structures, and study the effects of slugging on corrosion rates. The history of each slug in the pipe can be traced by determining if it will grow, shrink, collapse, or remain the same size as it traverses the pipe. During slug tracking, slug lengths are determined by the locations of interfaces instead of a correlation. Introduction The efficient and economic production of hydrocarbon reserves found in reservoirs located in marginal fields and hostile environments often require the transportation of unprocessed fluids. In the case where the fluids are hydrocarbon gas, hydrocarbon liquid, and formation water, a multiphase flow mixture is the result. A common occurrence in a pipeline transporting a multiphase flow mixture is the existence of flow patterns. These flow structures are characterized by a distribution of the interfaces separating the phases. Examples of flow patterns include churn, bubble, slug, and annular in vertical pipes; stratified, dispersed bubble, slug, and annular in horizontal multiphase flow. These flow structures are determined by,operational variables, such as flow rates of fluids and pressure;geometrical variables, such as diameter and angle of inclination of pipe; and,physical properties of the fluids being transported, such as density and viscosity. Flow variables in multiphase transportation are dependent on the distribution of the phases. These variables include liquid holdup, gas void fraction, pressure gradient, and heat and mass transfer coefficients. Thus, it is necessary to know not only when these flow patterns occur, but also the characteristics associated with each flow structure. The slug structure is a common and complex two phase flow pattern. It consists of a region of liquid with entrained gases, referred to as the liquid slug body; a gas bubble or pocket, and a liquid film. Figure 1 shows the slug flow pattern in the case of horizontal flow. In multiphase pipelines, slugs can be differentiated according to its mode of formation. The slug flow structure may be initiated by flow instabilities, such as the Kelvin-Helmholtz instability. These are termed hydrodynamic slugs. In other cases, the geometry of the pipeline plays an important role in slug formation. An example of this is a pipeline-riser pipe system. At low fluid flow rates, there is blockage at the base of the riser leading to an intermittent flow behavior termed severe slugging. Slugs formed as a result of the geometry are commonly referred to as terrain induced slugs.