Combining and Performing Scale-Dissolver and Squeeze Treatments in Kristin - A Subsea HP/HT Gas/Condensate Field
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 114079, "The Challenges Associated With Performing and Com bining Scale-Dissolver and -Squeeze Treatments in Kristin - A Subsea HP/HT Gas/ Condensate Field," by R. Wat, SPE, K. Wennberg, SPE, R. Holden, and B. Hustad, StatoilHydro, and S. Heath, M. Archibald, SPE, and K. Singdahlsen, Clariant Oil Services, prepared for the 2008 SPE International Oilfield Scale Conference, Aberdeen, 28-29 May. The paper has not been peer reviewed.
With an initial reservoir pressure of 911 bar and downhole temperature of 170°C, Kristin was the first high-pressure/high-temperature (HP/HT) field completed with and produced by use of subsea solutions. The calcium carbonate (CaCO3) scale was a major production problem, with the high drawdown from the reservoir and the high level of bicarbonate and calcium in the formation water. A case history of scale-dissolver and squeeze treatments on Kristin is presented in which special attention was paid to constraints on injecting chemicals at an adequate rate against an HP/HT well.

Introduction
Kristin is an HP/HT gas/condensate field in the Haltenbanken basin 20 km southwest of the Åsgard field in the Norwegian North Sea. It has two main reservoirs, Garn and Ile, and a third smaller formation, Tofte. The field has 12 production wells linked to the semisubmersible production platform through four subsea templates, as shown in Fig. 1. The templates are 6 to 7 km away, and the gas is gathered through six 10-in.-inside-diameter (ID) flowlines. Early on, CaCO3 scale was identified as the main production problem when formation water would break through. Frequent well interventions would be required for scale remedial and inhibitor treatments. This prognosis was developed on the basis of water-sample analysis from appraisal Well 6406/2-5 and predicted production profiles from reservoir simulation. Several measures were taken during field development and construction. A purpose-built chemical-storage and -injection unit was installed on the platform, and two dedicated service lines (3.5-in.-ID mild steel) were laid linking it with the various templates. A comprehensive chemical-treatment screening program was initiated.

Chemicals Selection
The chemicals-qualification program started in 2001, soon after the field-development plan was sanctioned and the engineering/construction phase began. For the scale dissolver, an acid/corrosion-inhibitor mix, the main challenge was to achieve adequate dissolution efficiency without compromising system-corrosion limits. The corrosion tests were conducted in a carbon dioxide/hydrogen sulfide environment at 170°C. The screening program was more complicated for the squeeze chemical, and tests evaluated products on the basis of inhibition performance against CaCO3 (dynamic-tube-blocking test), thermal stability, fluid compatibility, formation-damage potential, and corrosiveness. Two products were selected, one as the preferred product and one as the backup. These products were tested further to determine their performance matrix [i.e., the minimum inhibition concentration (MIC)] under various conditions.