Prevention of Crystallization of Gas Hydrates.Management of the risk with solids:Modeling the flow pattern
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 presents 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 procédésOrigine | Fichiers produits par l'(les) auteur(s) |
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