Drilling wells in high-pressure, high-temperature (HPHT) reservoirs is often characterized by a narrow operating window between formation pore pressure and fracture pressure. Depletion further reduces this window. Managed Pressure Drilling (MPD) provides methods for operating within safe limits in the narrow HPHT windows. Exceptional control over downhole pressures can be achieved with advanced MPD technologies that are uniquely suited for the HPHT environment. Such control can extend achievable HPHT targets, yet still have the flexibility to deal with the troubles that so often arise in these difficult environments.
The advanced MPD system developed for StatoilHydro's Kvitebjørn HPHT field are presented along with experiences from their use in the field. This includes:
Running a real-time, online, advanced dynamic flow model
Automatic dual redundant choke system with continuously updated pressure set-point from the flow model
Continuous Circulation System (CCS)
Pressure Control While Drilling (PCWD)
Caesium Formate mud system - A designer mud containing formation strengthening particles.
Balanced Mud Pill (BMP) - An innovative fluid technology developed for performing a precision top kill, producing minimal pressure surge when pulling the drillstring and running liner.
Kvitebjørn is located in the Northern North Sea on the Norwegian Continental Shelf, southeast of the Gullfaks Field (Fig. 1). It is classified as a HPHT gas condensate field. The reservoir consists of sandstones in the Mid-Jurassic Brent group and lower Jurassic (Cook Sst). The top reservoir is at approximately 4,070 m TVD. Early production during development drilling has induced pressure depletion, creating a convergence between pore pressure and fracture pressure in the reservoir. The initial pore pressure was 775 bar (1.93 SG) and fracture pressure was 875 bar (2.19 SG). The reservoir temperature is 155°C and the water depth is 190 m.
Nine wells had been drilled into the reservoir prior to introducing the MPD technique. The gas/condensate production started in September 2004 after the second well had been drilled and completed. On the last conventionally drilled well, 34/11-A-2, 140–170 bar of depletion was encountered and massive losses were experienced. Drilling was suspended before reaching TD due to the well-control situation created by these mud losses.
The A-2 incident marked the end of the traditional drilling programme as no further drilling on Kvitebjørn would be possible, unless a method could be found to safely operate within Kvitebjørn's reduced "Drilling Window". Prior to drilling the A-2 well, the Kvitebjørn platform produced at maximum capacity, 20.7 MMsm3 gas and 8 Msm3 condensate. After the A-2 incident, the Kvitebjørn production was reduced in an attempt to limit the rate of depletion to complete the primary drilling programme. Production from the field was reduced by 50% in December 2006 and then completely shut down by May 2007 when depletion approached 200 bar.