Depletion of the Kristin HPHT field (911 bar, 172º C / 13,213 psi, 342º F) is occurring, challenging the drilling of future increased recovery wells. The main concern is uncertainty of the reservoir pressures in development areas of the producing intervals. Initially, the operational window range for the bottomhole pressure (BHP) varied from a pore pressure of 1.97 SG to a fracture pressure of 2.14–2.16 SG. With depletion, the difference has become narrower since fracture pressure is decreasing.
This paper describes the need to implement managed pressure drilling (MPD) techniques in the Kristin field to overcome the problems related to a narrow drilling window. MPD is a technique that allows the use of lower-density drilling fluid, minimizing the overbalance pressure. Thus, BHP can be easily controlled and changed by applying surface-back pressure using a closed and pressurized circulation system and an automated choke. Circulating and static BHP can be increased rapidly, by applying back pressure at surface, if a higher than expected reservoir pressure is encountered. Conversely, BHP can rapidly be reduced, by reducing surface back pressure, should losses be seen down hole.
The paper will discuss the use of a drilling fluid with a density giving a hydrostatic pressure lower than the original pore pressure to facilitate drilling operations and allow better control of the BHP for drilling future wells in the Kristin Field.
The Kristin development wells may be the first wells globally to use MPD techniques in a harsh weather offshore environment on a floating drilling installation. The paper will also discuss some of the new equipment which has been designed to allow the implementation of MPD techniques in a harsh weather offshore environment from a semi submersible rig.
Introduction
The initial drilling program for Kristin is finished with 11 wells drilled and completed. Due to the high initial reservoir pressure, the pore pressure declines rapidly with production. Looking ahead at the possible need to drill increased recovery wells, several challenges to drilling were identified as a result of depletion, particularly with respect to the conventional requirement of keeping the BHP during drilling above the original pore pressure. MPD allows the safe use of lower mud weights much closer to the conventional drilling window limits, but in order to implement such drilling techniques on a floating drilling installation in the harsh weather environment of the Norwegian Sea, several issues had to be addressed.
The equipment utilized to date for MPD from floaters is not suitable for use on a HPHT well with Norwegian Sea types of weather conditions. Some new rig-up methods had to be developed which are described in this paper (Santos et al, 2003 & Lage et al, 2005). To ensure successful implementation of totally new technology, a step by step approach is outlined (William, 2003). Finally in the last section the methods that can be used with the MPD system are described and it is shown how the drilling of further wells can be safely achieved with ongoing depletion.
