Abstract

Hydrate blockage has been a problem in Campos Basin subsea producer wells for a long time. Although hydrate formation was not expected in sea water injection wells there were some occurrences in Campos Basin. The formation of hydrate requires the presence of water and gas at low temperature and high pressure condition. As the subsea sea water injection wells always present water at low temperature and high pressure, the missing link to form hydrate is gas. Thus, to explain the occurrence of hydrate blockage in these wells one has to explain how and when gas enters into the well. A study was carried out to determine the causes of gas inflow into the injection strings and to make recommendations on the proper procedures and equipments to avoid hydrate blockage. The literature survey and the Campos Basin occurrences showed initially that gas segregation, water hammer effects and crossflow were the most probable causes for gas inflow during injection plant shutdowns or long time waiting for injection. The crossflow was ignored because the formations are homogeneous in all the cases. The water hammer effects were analyzed with a numerical simulator developed in the study. The final analyses revealed that gas segregation was the cause of hydrate blockage in the water injection wells studied. Other relevant conclusions are that down hole valves, such as deep subsurface safety valves and backflow valves are useless to prevent gas inflow and that water hammer effects can be managed in the sea water injection plant. This paper presents recommendations for water injection wells startups, regarding hydrate prevention, which can be useful for any subsea water injection project.

Introduction

Natural gas hydrates are solid structures composed of water and gas. Depending on the thermodynamic conditions, water molecules will form a solid cage entrapping gas molecules (Freitas et al. 2002). Hydrate blockage has been a problem in Campos Basin subsea producer wells since 1994. In Campos Basin the sea temperature decreases with WD showing values around 13°C at 300 m WD, 8.5°C at 500 m WD and varies in the range of 3.5°C to 4.5°C from 1,000 m to 2,000 m WD. In this scenario, hydrate may occur in water depth (WD) beyond 300 m (984 ft), even with small amounts of water and gas.

The hydrate blockages often occur during operations such as production shutdown/startup. From 1994 to 2006 hydrate blockage was the main cause of rig intervention in the most prolific field of Campos Basin (Figure 1). Their occurrences in completion and workover have decreased in Campos Basin because of the preventive measures adopted. Besides causing production losses and requiring expensive rig interventions, hydrate blockages can put in risk the production system safety and the environment. This is because they can occur in the wet Christmas tree (WCT) valves, and in the subsurface safety valve control lines (Freitas et al. 2002). Analyses have shown that lack of attention to details in planning the operations and not following program and good practices are by far the main causes of the hydrate problems (Rodrigues et al. 2007).

The first occurrence of hydrate blockage in water injection wells (WIW) in Campos Basin was in August 2000. Since then there were eight occurrences in seven wells, all requiring costly rig interventions. At the most penalized field it was decided to install downhole valves to prevent gas migration from the formation open zones up to the WCT valves. The recurrence of hydrate blockage in subsea WIW was the reason for this study.

There are no publications about hydrate blockage in WIW to the best of our knowledge. However, there are good references about some probable causes of hydrate blockage that were raised in the early phase of this study. This is the case of water hammer (WH) effects.

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