Abstract

This paper describes the hydrate prevention concept for an actual North Sea subsea development, and how this concept was established.

Based on the outline of the development, an extensive program of hydrate characterisation tests was performed. These tests consist of semi-static experiments in visual high pressure PVT cells and dynamic studies in pressurised flow loops and flow simulators. Special emphasis has been put on the investigation of the potential of hydrate blockage and the future use of unconventional hydrate inhibitors.

Introduction

At present, Norsk Hydro is performing detailed design of the North Sea Troll oil production system comprising 18 subsea production wells located at around 310 m water depth. The wells are located at a distance of 3 to 10 km from a floating production platform. The subsea wells are tied in to manifolds by infield flow lines. The production fluids are then routed from the manifolds to the production platform via two parallel gathering lines. The subsea layout is shown in Figure 1.

The horizontal wells will produce a large amount of water and free gas. The water cut might exceed 80 % and the gas-oil ratio (GOR) may vary from the reservoir fluid GOR of 60 Sm3/Sm3 to a value above 200 Sm3/Sm3.

If the fluid temperature falls below the hydrate formation temperature during production, testing or shut-in, there is a possibility of hydrate formation which could lead to pipeline blockage by hydrate plugs.

In order to evaluate and select a hydrate control strategy, the following properties related to hydrate formation had to be established:

  • Physical properties, including composition, density, heat capacity, heat of dissociation, thermal conductivity and viscosity.

  • Equilibrium formation conditions

  • Inhibition, conventional and unconventional

  • Kinetics

  • Blockage potential

Physical Properties of Hydrates

Hydrates are non-stoichiometric, ice-like crystals formed by low molecular weight hydrocarbons enchlaterated in a framework of water molecules. Depending on the pressure and the composition of the hydrocarbon fluid, the hydrates may be stable at temperatures up to 30 C.

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