A produced hydrocarbon stream from a wellhead encounters formation of solid gas hydrate deposits, which plug flowlines, and are one of the most challenging problems in deep subsea facilities. This paper describes a gas hydrate model for oil-dominated systems, which can be used for the design and optimization of facilities focusing in the prevention, management and remediation of hydrates in flowlines. Using a typical geometry and fluid properties of an offshore well from the Caratinga Field located in the Campos Basin in Brazil, the gas hydrate model is applied to study the hydrate plugging risk at three different periods of the well life. Additionally, the gas hydrate model is applied to study the performance of the injection of ethanol as thermodynamic hydrate inhibitor in steady-state flow and transient shut-in/restart operations. The application of the transient gas hydrate model proved to be useful in determining the optimal ethanol concentration that minimized the hydrate plugging risk.
Offshore explorations in deeper and colder waters impose more challenging scenarios to the flow assurance of the produced streams. High pressures and low temperatures of operation of production facilities with longer subsea tiebacks will promote the formation of natural gas hydrates; crystalline compounds formed by hydrogen-bonded water molecules in a lattice structure that is stabilized by encapsulating a small guest molecule (e.g., methane, ethane) (Sloan and Koh, 2008). Gas hydrates form in the presence of appropriate quantities of gas and water, and are considered one of the most challenging problems in deep subsea facilities, due to its rapid formation compared to other solid deposits (Sloan, 2005).
A transient gas hydrates model, that predicts when and where hydrate plugs will form in flowlines, will have significant utility for the flow assurance engineer in the oil and gas industry. By predicting gas hydrates formation and transportability, the gas hydrate model can be applied to design and optimize oil/gas transport facilities, focusing on prevention, management or remediation of gas hydrates in flowlines. This paper briefly presents a gas hydrate model specially designed for oil-dominated systems, which has been incorporated as a plug-in module in a transient multiphase flow simulator (Turner et al., 2005). The gas hydrate model developed for oil-dominated systems follows the conceptual model presented in Figure 1, where hydrates form at the interface of water droplets entrained in the continuous oil phase. In the oil phase, these hydrate-encrusted water droplets can agglomerate increasing into larger hydrate masses, leading to an increase in the slurry viscosity, which can eventually form a plug (Turner, 2005).
