Abstract

The identification of hydrate risk region is the first task for deepwater gas field flow assurance. In order to accurately characterize the effect of liquid water on hydrate risk region, the dynamic hydraulics risk region was defined based on the transport characteristics and differences of various forms of liquid phase in gas-dominated multiphase pipe flow system and the further coupling of hydrate formation thermodynamics, hydrate formation dynamics and hydraulics of multiphase pipe flow in this paper. The transition from static thermodynamic region to dynamic hydraulic region is conducive to reduce the uncertainty of hydrate risk management.

Introduction

The pipeline is the channel of gas well production liquids to production platform or onshore terminal for deepwater gas field. Deep water pipeline has a long distance and faces low temperature and high pressure environment. The potential risk of hydrate formation and blockage in pipeline is high, and the pipeline flow safety is facing severe challenges (Di Lorenzo, 2018; Wang, 2018). The injection of hydrate inhibitors is the major method for hydrate risk management. The construction process of deepwater production system is complex, and hydrate inhibitors cannot be directionally injected into hydrate risk region. As shown in Fig. 1, hydrate inhibitors can only be injected into a pipeline by the umbilical, and then transported to the hydrate risk region completely depending on the multiphase flow system in the pipeline (Sloan, 2010). Therefore, the primary task of hydrate flow assurance in deepwater gas field is the accurate identification of hydrate risk region.

The hydrate risk region depends on the distribution and variation characteristics of the three factors of hydrate formation in the pipeline multiphase flow system, which are liquid water, temperature and pressure (Sum, 2018). Therefore, the prediction accuracy of hydrate risk region depends on the coupling mode and degree of hydrate formation thermodynamics, hydrate formation kinetics and hydrodynamic characteristics of multiphase flow in pipeline. At present, the hydrate risk region is defined and predicted based on the coupling between the thermodynamic conditions of hydrate phase equilibrium and the hydraulic thermodynamic parameters of multiphase flow in the pipeline. This method only takes the temperature and pressure of the three factors of hydrate formation as the related variables, and ignores the influence of the third factor liquid water. The calculation of the inhibitor injection concentration is also based on this method, as shown in the graph in Fig 1 (Notz, 1994).

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