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Abstract
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.

The Wells on the Kristin Field
Faced with the challenges of developing a subsea HP/HT field the Kristin wells were conceptually designed to be 2D wells with a maximum inclination of 60 degrees.

The wells are close to vertical down to the 12¼” section, where they build inclination to 60 degrees. The casing program is of standard North Sea type design, ref Table 1, where the 14” and 9 7/8” casing is introduced to cope with the HP/HT aspects.

The reservoir, which is located in a Middle Jurassic structure below the Base Cretaceous unconformity, consists of three separate sands with highly varying properties. Thus the original plans called for 60 degree wells penetrating the low permeability sand, and wells with inclination less than 40 degrees penetrating the high-permeability sands.

As the understanding of the reservoir matured, it was realized that the reservoir length exposed to the production wells had to be increased for the low-permeability sand. This meant high inclination wells through the reservoir.

The principles of the basic well design were maintained, the production casing was landed at approx. 4500 mTVD, in the shale above the reservoir and in safe distance to the reservoir. An 8 ½” hole was then drilled through the rest of the shale and into the reservoir. All building from 60 degrees and up to 85 degrees was to be performed in the 8 ½” section, see Table 1.

The introduction of the high angle wells increased the total 8 ½” section meterage to be drilled on Kristin with 80%.

Another complication arose when essential parts of the completions equipment were delayed, thus making it impossible to complete the wells immediately after the drilling was finished. The wells therefore had to be temporary abandoned after drilling, thus leading to the introduction of 7” liner in the reservoir.

New complexities were added to the project and in consequence new challenges were induced at all levels in the field development project. For the drilling fluid this meant that larger volumes were required and that the total usage and cost would increase.

The Fluid Selection
The most important factors the choice of drilling fluids have been:

• Well control
• Formation damage
• Formation evaluation
• Drilling progress
• Fluid stability
• Well completion strategy