Six reservoir sections were drilled and completed with cesium/potassium formate brine in the HPHT Huldra field in the North Sea.
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.
Change of plan
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.
Conclusions
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:
- Six reservoir intervals successfully drilled and completed in a challenging HPHT environment with cesium/potassium formate fluids – all without a well control incident
- Unlike weighted oil-based mud, the formate fluids provide an extremely good well control environment, which improves safety and reduces risk
- Use of formate fluids for both drilling and completion gave a very simple transition between drill-in and completion fluids
- Each one of the six Huldra wells drilled and completed with formate brines have excellent productivity indices of around 1.9 million scf/day/psi, with the first three even reaching plateau production rates
No alternative
Huldra’s project manager says: “For the specific conditions of the Huldra field there is no realistic fluid alternative for successfully drilling and completing the wells”.
This case history and all others are downloadable here.
Literature
Saasen, A., Jordal, O. H., Burkhead, D., Berg, P. C., Løklingholm, G., Pedersen, E. S., Turner, J. and Harris, M. J. (2002, February 26). Drilling HT/HP Wells Using a Cesium Formate Based Drilling Fluid. Proc., IADC/SPE Drilling Conference, Dallas, Texas, USA. SPE-74541-MS. doi.org/10.2118/74541-MS.
Howard, S. K. and Downs, J. D. (2005, April 5–7). Formate Fluids Optimize Production Rates. Proc., AADE 2005 National Technical Conference and Exhibition, Houston, Texas, USA. AADE-05-NTCE-05.
Downs, J. D., Howard, S. K. and Carnegie, A. (2005, December 5). Improving Hydrocarbon Production Rates Through the Use of Formate Fluids – A Review. Proc., SPE International Improved Oil Recovery Conference in Asia Pacific, Kuala Lumpur, Malaysia. SPE-97694-MS. doi.org/10.2118/97694-MS.