Abstract

With an initial reservoir pressure of 911bar and a downhole temperature of 170oC 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.

Introduction

Kristin is a HPHT gas condensate field located in the Haltenbanken basin about 20km south-west of the Åsgard field. It consists of two main reservoirs, Garn and Ile, and a third but smaller formation, Tofte. Kristin has an initial reservoir pressure of 911bar and a downhole temperature of 170oC. The field has 12 production wells which are linked to the semi submersible production platform (SEMI) via 4 subsea templates as shown in Figure 1. The templates are located between 6 to 7km away and from which the gas are produced via 6X 10" ID flowlines to the SEMI. From the very beginning CaCO3 scale has been identified as the main production problem when formation water starts to break through. Frequent well interventions will be required for scale remedial and inhibitor treatments. This prognosis is based on the water sample analysis from the appraisal well 6406/2–5 (Table 1) and the predicted production profiles from reservoir simulation. As a result a number of measures had been taken during the field development and construction phase. These included the installation of a purpose built chemicals storage and injection unit on the platform and the laying of 2 dedicated service lines (3.5" ID mild steel) linking them with the various templates. A comprehensive chemical screening program was started at the same time. The long lead time of the program reflected the difficulties in finding the suitable chemicals which would be effective, thermally stable and environmentally acceptable. The screening program included both the scale inhibitor for squeeze treatment and the dissolver for scale removal. Finally, despite incurring significant costs and delay, special arrangement was made to obtain a fresh downhole water sample from R-4H, one of the first production wells that was drilled in Kristin. The information from this new sample was invaluable. It showed good agreement with the results from well 6406/2–5 (Table 1) and confirmed all the measures that had been put in place would be needed in tackling the scale problem. However, small anxiety persisted as how the chemicals would perform in the field under such harsh conditions remained unclear. In November 2005 Kristin started its gas production and in April 2007 formation water breakthrough was detected. Two combined scale dissolver and squeeze treatments were performed shortly in August 2007. In this paper a case history of these very first treatments on Kristin will be presented.

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