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.

This article presents results from the Controlled Mud Pressure (CMP) field trial that encompassed well control on a rig equipped for dual gradient drilling. The tests were carried out after successfully drilling three laterals with a partly evacuated riser with a controlled mud level. The paper focus on analysis of the results to quantify the ability to: detect in-/out-fluxes of gas and liquid; circulate out gas with both an open and closed annular preventer; suppress migration of drilled gas into the evacuated part of the riser during drilling.The CMP test objective is to verify the ability of the CMP equipment to detect and controllably circulate out simulated influxes. Five tests are documented: one with liquid in-and out-fluxes introduced through the choke line using the cement pump, and four with gas introduced through the drill string. Nitrogen was used to emulate gas kicks, which were circulated out of the well with both an open riser and with a closed annular preventer and a dedicated return line, connected to the subsea pump module and a topside-mounted choke. To address the challenge with drilled gas accumulating in the evacuated part of the riser, small portions of Methane were injected into the drill string while circulating. Gas sensor readings at the shakers and in the flow-line were used to monitor gas concentration in the mud and above the mud mirror, respectively.The main findings from the field trial are: volume imbalances due to abrupt changes in flow rate into the well are quickly detected; it is possible to circulate small gas kicks out of the well through the subsea pump module and dedicated well control equipment when closing in the well with the annular preventer, given that the pump handles a mixture of mud and gas, and can withstand a high differential pressure to sea; it is not advisable to vent large gas kicks into the evacuated part of the riser without closing the blowout preventer (BOP). In addition, gas migration velocities in water-based mud with varying flow rates and gas content are reported.The article directs attention to deep water well control using specialized equipment for dual gradient drilling. It also contains valuable analysis of field trial results, which contribute to the understanding of gas migration, and the possibilities and restrictions, introduced with dual gradient drilling.

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This paper presents ongoing work as well as plans and ideas for future development of a toolbox of managed pressure drilling systems that solves major challenges when drilling in both mid-and deep waters. The toolbox contains three applications based on field proven Controlled Mud Level (CML) technology. Additional benefits can be achieved by combining controlled mud level technology with other technology elements such as a fast closing annular or a sealing element in the riser, usually referred to as a rotating control device (RCD).The first application is called CML. A subsea pump module (SPM) is used to pump the mud returns to surface in a separate mud return line (MRL). The SPM is used to regulate and manage the bottomhole pressure by adjusting the mud level in the riser. High bottomhole pressure or Equivalent Circulating Density (ECD) due to friction and other effects can be avoided by reducing the mud level in the riser accordingly. One of the objectives with this technology is to avoid or reduce losses both during drilling and other operations such as cementing.In the second application called ECD-Management, CML is combined with a fast closing annular installed in the riser just above the SPM. This enables extending the operating envelope of the technology to a point where loss of circulating capability could potentially cause an underbalance scenario in the well. The hydrostatic pressure from the drill string or MRL can be trapped within a few seconds by closing the fast closing annular element, e.g. on unplanned rig pump stops or formation pack off situations. A second annular will also be installed to be used during connections in order to save time filling up the riser to compensate for the ECD effect (friction losses in the annulus).The third application in the toolbox is called ECD-Control. Here a subsea RCD or a sealing element is installed in the riser and the SPM can then be used to manipulate the pressure below the RCD to compensate for ECD variations. The riser is always topped up with mud and hydrostatic overbalance is maintained.The paper will present simulation results comparing the different technologies and address benefits and challenges with the different methods.

Well control has been a major obstacle for introducing dual gradient systems for deep water drilling, involving both new and more complex procedures and equipment. Here test results using a new dual gradient drilling (DGD) system to control simulated well control events are presented. After successfully drilling a Statoil well with Enhanced Drilling's dual gradient solution (EC-Drill) at the Troll (Kjøsnes et al. 2014) field on the Norwegian continental shelf, the well was plugged and made ready for 48 hours of controlled testing. Several tests were run, each designed to determine the functionality of the system; to detect and handle liquid loss, gas and liquid influx, gas migration in the riser, and circulation of gas through the subsea pump module. The detection tests were carried out with a reduced riser level and an open annular, to quantify the capability of the system to detect volume imbalances while drilling. Three circulation tests with increasing amount of gas injected through the drillstring were performed. After the influx was detected, the annular was closed, and the flow was routed from the well through a bypass line to the subsea pump module. A topside choke in the mud return line was actively used to apply backpressure with the objective to control bottom-hole pressure, while circulating the gas from the well. In the last test gas was allowed to migrate into the marine drilling riser with a reduced riser level and open annular. The paper will present the planning and preparations as well as the results.