Abstract

The paper highlights the importance of developing a comprehensive gas hydrate model in flow assurance for the oil and gas industry. The comprehensive model should account for mechanisms present in different systems of oil/gas production. Since this kind of tool could be seen as complex and difficult to use, the main focus of the paper is to present an example of generation of "best practices" that can address the needs of a general flow assurance community, using a gas hydrate model developed for oil-dominated systems. The example illustrates the restart of an offshore well, using a typical geometry and fluid properties from the Caratinga Field located at Campos Basin, Brazil. Two extreme cases are studied to identify the worse case scenario, one considering a water-in-oil stable emulsion and the other where water is allowed to separate from the continuous oil phase. Then, the restart procedure of the case with higher risk of forming a hydrate plug is optimized to minimize hydrate formation and plugging risk. From the comparison of the two cases studied, it is concluded that systems with free water have a higher risk of plugging with hydrates. The optimization of the restart of a well shows that a slow restart of the well minimizes the formation of hydrates, decreasing the risk of plugging. The main conclusion is that the gas hydrate model can be used as a tool for the generation of "best practices" for gas hydrates in flow assurance.

Introduction

Flow assurance is a technical discipline of the oil and gas industry that focuses on the design of safe and secure operation techniques for the uninterrupted transport of reservoir fluids from the reservoir to the point of sale. Offshore explorations in deeper and colder waters impose more challenging scenarios to the flow assurance of the produced streams, requiring production facilities with longer subsea tiebacks for the transport of hydrocarbons from the wellhead to production/processing platforms. These facilities may operate at high pressures and low temperatures promoting the formation of gas hydrates, crystalline compounds formed by hydrogen-bonded water molecules in a lattice structure that is stabilized by encapsulating a small guest molecule, like methane or ethane (Sloan and Koh, 2008). One of the most challenging problems in flow assurance is the plugging of pipelines due to rapid formation of gas hydrates compared to other solid deposits (Sloan, 2005).

The formation of gas hydrates in an oil-dominated multiphase flow system containing water, oil, and gas, can be described using the conceptual model illustrated 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).

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