Gas hydrates are crystalline solids that can plug subsea oil and gas flowlines and are considered a major flow assurance problem. The risk of hydrate plug formation is significantly increased when free water is present in the flowline. Transportability of hydrates has a strong dependence on the total amount of liquids (the carrier phase) present, along with the mixture velocity of the fluids. It is also shown that partially dispersed systems (water dispersed in oil and free water) tend to fail at lower hydrate fraction, compared to completely dispersed systems (no free water) for the same set of conditions. This paper presents the results and analyses of flowloop experiments on partially dispersed systems as a function of water volume fraction, velocity, liquid loading, and oil properties.
A mechanistic model for hydrate plugging of flowlines in partially dispersed system is developed based on flowloop data obtained at the University of Tulsa. Once the hydrates form at the water-oil interface, they soak up large amounts of water thereby depleting the system of a carrier phase which makes transportability very difficult. The plugging mechanism for these systems is a combination of various phenomena (wall growth, agglomeration, bedding/settling, etc.). Experimental data indicates that liquid loading and water cut play a very important role when it comes to transportability of hydrates. The data from the tests performed provide the basis for a mechanistic model for hydrate formation and plugging in partially dispersed systems. A correlation is being developed/modified to predict the onset of plugging in terms of easily measurable variables like the oil viscosity, mixture velocity, etc. Preliminary analysis of the results of experiments using different viscosity oils show that viscosity of oil is an important parameter that determines transportability, and higher viscosity oils tend to facilitate better hydrate transportability.
The work resulting from our experiments should be incorporated into flow assurance models and will ultimately help to advance operating company production strategies to reduce hydrate plugging risks in complex multiphase systems.