(226b) Dynamic Modelling and Analysis of Multi-Phase Flow in Oil and Gas Pipelines: Assessing Conditions for Flow Assurance in Transient Operation Scenarios

Deep water production and long-distance sub-sea flow lines are new challenges that the oil and gas industry is facing. As a natural consequence of the decline of major offshore production fields around the globe, new reserves being discovered tend to be smaller and located under more remote and severe conditions for successful and profitable operation. Under these circumstances, the key issue is to guarantee the continuous flow from the reservoir to the processing offshore platform or onshore facility.

Low temperatures and high pressures, conditions encountered in the production from deep water wells and sub-sea pipelines, can favour the formation of gas clathrate hydrates. These solid structures are highly undesirable during operation since they could deposit in the pipelines, thereby obstructing the flow or totally clogging the pipelines. Therefore, the control of the hydrate formation risk factors is fundamental for the flow assurance in the production system (Mehta et al. 2000, Mokhatab et al. 2007).

In view of the above, the prediction of the thermal-hydraulic dynamics is essential for the cost-effective design, operation and flow assurance of these production systems. For example, determining temperature fluctuations and pressure peaks caused by different transient scenarios is not only important for safety and environmental considerations (Chin and Xu, 2001), but also for the evaluation of equilibrium thermodynamic conditions of hydrate formation along the pipelines (Kwon et al. 2001). Moreover, while the system may be stable under steady-state operation, in transient situations the multi-phase thermo-fluid dynamics may affect the flow assurance. Such a situation may occur during restart operations after a shutdown (Golczynski and Nielsen, 2002; Leporcher et al., 2002).

This work presents the development of a multi-phase flow model that describes the transient thermal-hydraulic behaviour of transport lines of gas and oil. The hydrodynamic model is based on the mixture model (Ishii, 1975; Hibiki and Ishii, 2003) that considers the relative motion (drift velocity) between the gas and liquid phases, in combination with a dynamic energy balance. In addition, the flow model is coupled with a hydrate thermodynamic equilibrium (van der Waals and Platteeuw, 1959) calculation routine based on a Statistical Associating Fluid Theory (SAFT) equation of state, that determines whether hydrate formation could occur at each time instant and point along the pipeline by evaluating the difference between the equilibrium temperature (or pressure) and the actual pipeline temperature (or pressure). This conservative estimate of the possibility of hydrate formation, obtained in the absence of detailed kinetic information, would considerably improve the current flow assurance practice (i.e. inhibitor dosages, operational policies). In order to demonstrate the capabilities of the proposed framework, implemented in a state-of-the-art modelling tool (gPROMS), several relevant transient production scenarios (shut-in, restart, etc.) are evaluated.

References

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