Abstract
With an initial reservoir pressure of 911bar and a downhole temperature of 170°C Kristin is the first HPHT field in the world that has been completed and produced using subsea solutions. CaCO3 scale has been identified as the major production problem due to the expected high draw down from the reservoir together with the high level of bicarbonate and calcium in the formation water. In April 2007 breakthrough of formation water in two of the wells were detected and the subsurface safety valve in one of them showed increasing inertia. In August/September 2007 the first combined scale dissolver & squeeze treatments were carried out in these two wells. This culminated after more than 5 years of testing of a large variety of chemicals and operational planning. The jobs were successfully carried out using the unique HP injection system on board that is dedicated for such well intervention. In this paper a full case history of these first treatments on Kristin will be presented. It starts with a brief summary of the difficult path to qualify various chemicals for the challenging conditions. This is followed by the early detection, diagnosis and data interpretation processes when the formation water first broke through. The paper will include the operational planning with special focus on the constraints to inject chemicals at an adequate rate against a HPHT well. The challenges in delivering and maintaining separation of the different chemical pills to a well lying 7km away will be highlighted. Despite the best intension some compromises on the treatment design have to be made in order to maintain safety and system integrity and these will be discussed. Finally the paper will conclude by presenting the results from these treatments, the assessment of the operations, the experiences being learnt and the area identified for future improvement.
