Kristin is a HP/HT field developed on Haltenbanken offshore mid-Norway. The Kristin development consists of 4 templates with a total of 12 subsea wells. The inclination through the reservoir ranges from 28 to 85 degrees. The reservoir at approx. 4600mTVD (15092 ft) has an initial pore pressure corresponding to 1.96sg EMW (16,36 ppg) and a temperature of 172°C (342°F). With a water depth of approx. 360m (1181 ft), a MW of 2.05sg (17,11 ppg) is needed to be able to maintain a riser margin.
Three different drilling fluid systems have been used in the reservoir section: 1) Cs/K-COOH clear brine system. 2) Invert emulsion HP/HT OBM. 3) Invert emulsion HP/HT OBM with ultra fine weight particles.
Challenges such as ECD management, hole stability, formation damage, weight material sag and operating on subsea HP/HT wells during harsh winter conditions had to be addressed both in the planning and the operational phase.
In this paper the background for selection of the drilling fluid is briefly described and especially the rationale behind using three different systems. The paper highlights operational experiences to illustrate how the drilling fluid systems influenced and coped with the challenges of drilling subsea, high angle HP/HT wells.
The paper provides a discussion of the pros and cons of the different fluids systems. Finally the paper identifies some of the challenges that lie ahead as the production has started and the reservoir starts to deplete.