Abstract

The sub sea development of the Ormen Lange gas field located in the Norwegian Sea will use MEG (mono-ethylene-glycol) for hydrate inhibition. Gas, condensate and MEG + water will be transported through a multiphase carbon steel pipeline to the onshore process plant at Nyhamna located on the west coast of Norway.

pH adjustment and film forming corrosion inhibitor will be used to control the corrosion rate in the pipeline. Although the corrosion rate is low during production, iron will be produced due to corrosion and follow the MEG/water phase. Most of the iron will be removed as iron carbonate particles during the MEG regeneration process but about 10ppm iron as particles and ions is expected to circulate in the MEG-loop.

Production of formation water will result in carbonate scaling when pH stabilised MEG is used as corrosion control unless a scale inhibitor is used. Three commercial scale inhibitors have been tested for this system. Tests have been performed without iron, with iron ions and with iron carbonate particles present. The results from the tests showed that the scale inhibitor concentration had to be increased considerably when iron ions were present compared to experiments without iron. The scale inhibitors performed poorer at the same inhibitor concentration in tests where iron carbonate particles were present than in tests with iron ions. Scale inhibitor concentrations up to 400ppm were used in the tests without achieving satisfactory scale protection when iron particles were present.

In addition tests with lowering the amount of pH stabiliser have been performed. Tests with combining scale and corrosion inhibitors have also been performed. No scale inhibitors have shown satisfactory performance when iron carbonate particles were present in the tests.

Introduction

The Ormen Lange gas field is a sub sea development located in the Norwegian Sea 120 km from the shore in mid Norway. Norsk Hydro Produksjon is operator for the planning and development phase. Shell will take over as operator when Ormen Lange comes on-stream in October 2007.

Sub-zero seabed temperatures in combination with the high-pressure conditions in the carbon steel multi phase pipelines favour the formation of gas hydrates and ice. MEG (mono-ethylene-glycol) will be injected to prevent hydrates and ice from forming in the sub sea pipeline. The MEG will be pumped out to the injection point in carbon steel pipelines.

From the start of production the wells will also produce condensed water (CW), CO2 and organic acids, besides hydrocarbons. The pH of this water phase will be low due to the presence of CO2 and organic acids. pH adjustment and film forming corrosion inhibitor (CI) will be used to control the corrosion rate in the carbon steel pipeline1. The pH stabiliser will be added to the MEG.

The degree of pH adjustment varies from full pH stabilisation to partial pH stabilisation. When partial pH stabilisation is used, a film forming corrosion inhibitor has to be used in combination with pH stabilisation to achieve corrosion control in the pipeline.

Although the corrosion rate is low during production, iron will be produced due to corrosion and follow the MEG/water phase. Most of the iron will be removed as iron carbonate particles during the MEG regeneration process but about 10ppm iron as particles and ions is expected to circulate in the MEG-loop.

Later in the field life formation water (FW) breakthrough may occur in the wells resulting in a co-production of FW and CW. CaCO3 crystals may form if the formation water mixes with the pH-stabilised MEG. The high pH and the presence of MEG reduce the solubility of CaCO32. The crystals may form a layer on the pipeline wall.

The MEG that is pumped out from the onshore facilities is denoted Lean MEG, while Rich MEG is the Lean MEG after this has been mixed with the well fluids.

Several commercial scale inhibitors (SI) have been tested in dynamic tube blocking tests and static tests. Since iron is known to have influence on the performance of scale inhibitors3 their efficiency was determined with and without the presence of iron.

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