The Kristin field is the most extreme HPHT field in Norway with a reservoir pressure of 911 bar and a temperature of 175°C. This gives an equivalent pore pressure of 1.96 sg. With additional safety margin, trip margin and riser margin, it requires mud weight up to 2.07 sg to balance the pore pressure. Combining very high mud weight, high temperature, sub sea operation, gas reservoir and high inclination create a formidable challenge to tools, equipment, procedures and personnel. In October 2004 Kristin finished drilling the first high angle sub sea gas producer in the world under such HPHT conditions. The positive pressure test of the 7" liner marked the successful completion of the drilling phase. The achievement on Kristin introduces a new term in to the oil and gas industry. HAHPHT wells, high angle high pressure high temperature wells. The well R-3 H was drilled 4 m short of 6000 m with an inclination of 75° without any well control incidents. Drilling of the well was completed in 82 days, 2 days ahead of schedule. This paper will highlight some of the challenges and experiences of drilling HAHPHT subsea wells. As this paper is written Kristin has finished drilling of 4 high angle HPHT wells and 2 low angle wells.
Drilling high inclination wells itself, are maybe not the most impressing these days. But the combination of all the special challenges on a high angle HPHT subsea gas producer makes it unique. Typical challenges are:
Casing design and casing wear.
Annulus pressure management.
Well growth due to high temperature.
Temperature with regards to downhole tool equipment.
High density drilling fluids.
Small operational window and ECD management.
Wellbore stability and rock mechanics.
Extreme weather conditions.
Sensetive cement operations.
15 K sub sea and BOP equipment.
In this paper some of the challenges mentioned above will be discussed. Some of the challenges are not directly related to high angle wells, but are special subsea challenges. The figure below shows a typical high angle well on The Kristin field.
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