Huldra is a gas condensate field in the Norwegian sector of the North Sea operated by Statoil ASA. During drilling and completion of this field, high pressure and temperature conditions were encountered in the reservoir section, 67.5 MPa (9,790 psi) and 149°C (300°F). The difference between pore pressure and fracture pressure gradient was small in the reservoir. The Huldra gas stream contained 3–4% CO2 and 9–14 ppm H2S. The wells were drilled at 45°–57° inclination through the reservoir and completed with 300-micron singlewire-wrapped screens.
The initial plan was to drill all the wells with barite-weighted oil-based mud and complete in cesium/potassium formate brine. When the first production well was drilled a severe well kick was experienced while running the sand screens. The main reason for the kick was a loss of drilling fluid density due to barite sag during the wiper trip. A 1.85–1.94 g/cm3 (15.4–16.2 lb/gal) cesium/potassium formate-based drill-in fluid was therefore selected for the following wells, primarily for well control. This was the first time worldwide that cesium formate had been chosen as a drilling fluid.
The Huldra drilling operations using formate brine were characterised by good hole stability, low equivalent circulating densities and good hole cleaning. The excellent rheology and thermal stability of the drilling fluid led to rig-time savings from faster tripping speeds, faster casing-running speeds, less mud conditioning and fewer wiper trips. The rate of penetration was also good. The drilling fluid was circulated over a combination of 250-, 300- and 400-mesh shaker screens before completion screens were run. After running screens, the drilling fluid was replaced with filtered cesium/potassium formate completion brine.
Statoil has reported that the six Huldra wells drilled and completed with formate brines are all producing with excellent average productivity indices of around 1.9 million scf/day/psi. In fact, plateau production rates were achieved from the first three wells of the six-well project.
The most notable features of the Huldra drilling and completion operations using 1.85–1.94 g/cm3 (15.4–16.2 lb/gal) cesium/potassium formate brine were:
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