Modeling non-Equilibrium Crystallization of Gas Hydrates under Stratified Flow Conditions
Résumé
The strategy of transportability in multiphase flow systems for oil & gas production requires a solid understanding of the coupling between thermodynamics and hydrodynamics to design the flow pattern along the pipe sections. Engineers are using commercial software in order to evaluate the coupling between production flow rates, pressure drops, and flow patterns.
Along the flow, from the well head to the on-shore or off-shore facilities, engineers have also to estimate the risk of hydrates formation, and to propose a solution to prevent their formation. It is the classical conservative approach implying to insulate the pipes in some sections, or to inject specific thermodynamic additives to shift the crystallization thermodynamic conditions out of the operative conditions of pressure and temperature. These solutions do not modify radically the anticipated flow pattern.
Another less conservative solution is to accept the risk of solid hydrate formation, but to manage it by using other kinds of additives to disperse the solids and to prevent their agglomeration, sticking and plugging. It adds a new degree of complexity in the pipe design because the flow pattern becomes coupled to the solid content via kinetics, and inversely.
This contribution resumes our efforts to develop and connect the models of thermodynamics, hydrodynamics and kinetics of hydrate formation. Our case study is based on the description of stratified flow, but the modeling framework can be applied to any other geometry and flow condition. One of the key outcomes of this work is to give the equations and procedures to be implemented in in-house softwares so that academics can master completely their flow modelling, including the coupling with kinetics. In addition, this work also shows that the hypothesis of thermodynamic equilibrium is shifted from the bulk phase to the interfaces between the phases. In turn, the bulk composition becomes dependent on the kinetics of hydrate formation and the geometry of the system.
In the first part of the model, calculations are done to determine the partition coefficient (from thermodynamics) at the interfaces between the respective Liquid Water, Liquid Oil and Gas phases. Then, the calculations for the geometry of the system could be performed. This work reviews the literature models to model the flow patterns. It identifies the two current models. One based on a well-established flow mechanic approach, and the other based on a recent energetic approach. We propose a special focus on the energetic approach, up to now applied to diphasic flow only, and we give the general equation to applying it to a three-phase system. One of the main advantages of the energetic approach is that it avoids implementing closure relationships, which are generally expressed under the form of a shear stress at interfaces, which often remains a private known-how of commercial softwares. The final part deals with the kinetics, and uses the geometry (interfacial areas), for determining the crystallization of hydrates. The crystallization model is based on a non-equilibrium hypothesis where the thermodynamic equilibrium is assumed only at the local scale, that is, at the liquid-solid interface. Finally, some examples are given to explain how the composition of hydrates is affected by the flow pattern and vice-versa.
Domaines
Génie des procédés
Origine : Fichiers produits par l'(les) auteur(s)
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