Hydrate Blockage in Subsea Water Injection Wells - Causes and Preventive Procedures

摘要

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.