A Simple Model for Hydrodynamic Slug Flow

Abstract

Hydrodynamic slug flow is the prevailing flow regime in oil production, yet industry still lacks a comprehensive model, based on first principles, which fully describes hydrodynamic slug flow. This paper describes a very simple, time-dependent, two-phase (gas-liquid) model which is capable of producing hydrodynamic slugging from first principles. The new method can be applied for evaluation of slugging potential for oil pipelines. The model is capable of producing slug lengths and frequencies, as well as slug hold-ups. The model has been compared to the published Prudhoe Bay field data gathered by Brill et al. The model has been able to predict the transition from homogeneous to slug flow, and also give information about slug lengths and frequencies. Such a simple model could be a very excellent jumping off point for building more complex models capable of predicting slug distributions from first principles, without any need for additional user input. No such model currently exists. Theoretical Background While hydrodynamic slug flow is an inherently transient phenomenon, it has historically been modeled as a ‘pseudo steady-state’ which ignores the fundamentally transient nature of the flow. Even in instances where the individual slugs are introduced and tracked in a Lagrangian frame (so-called ‘slug tracking’)1, the results have been somewhat disappointing, in that the ultimate slug distributions are heavily influenced by user input. This paper examines another approach - where the fundamental transient nature of hydrodynamic slug flow is accounted for in the model. In order to formulate a model for slug flow, we must first develop a ‘point model’ for an individual pipeline segment, or computational cell, in a pipeline. This pipeline segment must then be joined with other pipeline segments upstream and downstream of it to form a ‘steady-state’ model for the pipeline. Lastly, these steady-state solutions must be implemented into a transient scheme; a proper model of hydrodynamic slug flow absolutely requires that slugging not be treated as a pseudo-steady-state, but as an inherently transient phenonenon.

Multiphase Flow Modeling for Gas Hydrates in Flow Assurance

“…The framework for the current work is a simple multiphase flow model developed by Danielson (Danielson, 2011). This model forms the basis of the multiphase flow modeling approach we have used to incorporate additional phases, including hydrates.…”

“…We have extended a two-phase hydrodynamic slug flow model (Danielson, 2011) to include hydrates as a third phase in the system, and have studied the effect of hydrate formation on the flow behavior of the system. In order to establish the reliability of any model, it is necessary to validate the model against experimental observations.…”

Development of a Tool to Assess Hydrate-Plug-Formation Risk in Oil-Dominant Pipelines

“…where k Fit is an empirical fitting factor (0.002) derived on the basis of flow loop and field experiments Davies et al 2009;Sloan et al 2011); C 1 and C 2 are kinetic constants estimated to be 37.8 and 13,600, respectively Bishnoi 1983, 1985;Boxall et al 2009;Davies et al 2009); T eq,i is the hydrate-equilibrium temperature for conditions in the current timestep (i); T i is system temperature in the current timestep; Dt is the size of the timestep; M w,Gas is the molecular weight of the gas; and n con represents the number of moles of gas consumed in the current timestep. The surface area, S Area , for the water droplets emulsified in the oil is calculated from…”

“…1) to describe hydrate plug formation in oil-dominated systems: water droplets may be emulsified into the oil phase because of fluid shear (Boxall et al 2012), where the adsorption of natural surfactant to the water/oil interface will decrease mean droplet size under similar shear conditions (Zerpa et al 2010); hydrate grows at the water/oil interface around entrained droplets; hydrate shells and particles cohere to form larger aggregates and may deposit on the pipeline wall; and large aggregates and deposits enable jamming-type failure scenarios. This conceptual model was adapted into a 1D hydrate-reaction algorithm and implemented in a commercial multiphase-flow simulator, demonstrated by Davies et al (2009) and Boxall et al (2009).…”

Volume Averaging of Multiphase Flows With Hydrate Formation in Subsea Pipelines

“…In a stratified flow, the hydrate formation can quickly change the flow pattern to slug flow. Danielson (2011) used fundamental multiphase flow concepts and developed a model to predict the slug flow.…”