MPD Operation Solved Drilling Challenges in a Severely Depleted HP/HT Reservoir

Abstract

Abstract A novel MPD setup has been tested and used at the Kvitebjørn field in the North Sea to make possible the drilling of 8.5" holes through reservoirs with heavily depleted zones. A central part of the concept has been to use an advanced dynamic flow and temperature model in combination with an automatic choke system to control open hole pressure very accurately. The paper describes briefly the operations, and discusses challenges and experiences related to making these complex components work reliably together. The system has proven its potential for minimal variations in pressure at a given open hole position, with accurate automatic pressure control in wells were margins are very small, smaller than frictional pressure losses added when circulating at drilling rate. This paper focuses in particular on the use of an advanced transient model for automatic choke regulation. Other aspects of the operation are described in more detail elsewhere, see Refs. 1–3. Challenges exceeded expectations, but after a test period with many improvements based on trial and error inside cased hole, the 8 1/2" sections were drilled successfully with good pressure control. Introduction The Kvitebjørn field in the North Sea, close to Statfjord and Gullfaks, started production in 2004. Conventional drilling continued after this, but the increasing pressure depletion caused severe losses on the ninth well. This event put an end to conventional drilling at Kvitebjørn, and the development of a comprehensive managed pressure drilling setup started. Main elements were:Running a real time, advanced dynamic flow model onlineAutomatic choke system with continuously updated pressure set-point from the flow modelContinuous Circulation SystemRotation Control HeadCesium Formate based designer mudUse of a Balanced Mud Pill for minimum pressure surges when pulling out drillstring and running in liner The overall setup, individual components, and experiences are described in detail in Ref. 1. This paper deals with the model part of the concept, with examples from wells A-13-T2 and A-12 on Kvitebjørn. Overall system setup An advanced dynamic flow and temperature model ran continuously with input from the rig system. Desired equivalent mud weight (EMW) at a given position was input to the model, which calculated the corresponding choke pressure. An accurate automatic choke system was used to control pressure according to the set-point calculated. A continuous circulation system was used to maintain constant flow rate and thus reduce downhole pressure and temperature variations to a minimum. The system worked well during drilling operations, and contributed to resolving the very challenging situation with an open string after a shallow drillstring washout.