A world first "Controlled Annular Mud Level" type Dual Gradient Drilling (DGD) system was successfully applied on an ultra-deepwater well drilled in May -July 2012. Water depth was 2260m and the formation was generally carbonates with potential for severe or total losses. DGD was applied to prevent losses from occurring. Dynamic circulation pressure effects were eliminated. The DGD topside system was rigged up offline and when running the riser, the DGD pump was launched and attached to the riser for the last 400m. During drilling of the 17 ½" hole section, a Dual Gradient mud weight was selected and the riser level was maintained between 150 and 200m below flow line most of the time. On connections the riser level was kept unchanged. The level was decided according to PWD Readings. On several occasions it was demonstrated that lowering the riser level increased ROP whilst raising it again decreased ROP in hard rock. In the 12 ¼" hole section, drilled to well TD, the same approach was taken, but here the riser level was raised 50 m (approximately 75 psi) on connections to account for some of the reduced bottom hole pressure when shutting down the rig mud pumps. In this section some gas bearing formations were penetrated with a reduced riser level. No gas at all was seen in the riser top. During drilling of the reservoir and surrounding formations, no losses were seen and the hole was in good condition with no indication of fill or drag. The paper will present the technology applied and analyze the results achieved on this well.
Ultra deep water presents the most challenging conditions during drilling operations. When weak carbonates increase the likelihood of fracturing a formation, the challenge increases further still. High hydrostatic pressures, due to the long riser as well as the narrow gap between pore and fracture pressures, add to the complexity. Performing successful cementing operations is equally challenging. This paper describes the drilling of a well at approximately 2200m in the eastern GoM. The formations were carbonates with potential for high-to-total losses. The rig was a new Sixth Generation DPMODU. To manage the BHP and avoid or reduce losses, the rig had been equipped with a new type of MPD system. This is a pumped riser system, with the pump mounted approximately 400m below the bell nipple. The pump reduces the riser level in a precisely controlled manner to manipulate the BHP as required. Due to a very weak 20"shoe the 17 ½" section had a drilling window of only 0,3ppg. The MPD system was used to drill the section with nearly constant BHP, compensating for the ECD by managing the hydrostatic column level while drilling, and bringing the level back up for connections. No losses were seen. The same applies for cementing of the 13 3/8 casing. The main focus in this paper will be the 8,5"section where this MPD system again was necessary due to potential for severe losses. The section was drilled with riser level actively managed to control the BHP. On connections riser level was increased to account for the loss of annular friction, thus always keeping the BHP under control. It soon became evident that this formation could not have been drilled conventionally as weak zones were penetrated at the start of the section. Loss pressure was 8,4 ppg. Drilling with returns was still required in order to get the required geological information from the well, which was achieved. As drilling proceeded a normal-pressured zone was encountered. This paper will describe the planning and drilling operation of the mentioned ultra-deepwater well with focus on the very challenging 8,5" section
A previous attempt to drill an exploration well in ultra-deepwater in the GoM did not reach its objective due to an inability to maintain a Water Based Mud system light enough to maintain circulation. For the next round of exploration drilling, a system that allowed dynamic management of Annular Friction Loss (AFL) was required in order to stay within the narrow margin between pore and fracture pressure gradients.Conventional Rotating Control Device (RCD) based Constant Bottom Hole Pressure (CBHP) management systems were not suitable, as no drilling fluid light enough to manage the AFL with an added backpressure could be applied for this campaign.A "Controlled Annular Mud Level" type DGD system was successfully applied on this well in 2260 m water depth. This method compensates for the annular friction pressure by reducing the riser level according to the circulation pump rate. The system was also used to manage downhole pressure for cementing operations to mitigate losses and ensure good cement integrity with full returns during the cementing operation.The riser level was decided in the drilling program and adjusted according to PressureWhileDrilling (PWD) Readings. It could also be demonstrated that lowering the riser level increased ROP, whilst raising it again decreased ROP. The sensitivity for kick detection was greatly improved as the effect of rig motion on surface volumes was eliminated.The paper presents the project approach to deliver results and the technology applied. Results achieved on this well will be discussed, and how to further enhance the technology to other projects and increase the operational envelope of the system.
Ultra deepwater drilling is challenging. When easily fractured carbonates are encountered, the challenge increases further still. High hydrostatic pressures, as well as the narrow gap between pore and fracture pressures, add to the complexity. Performing successful cementing operations is equally challenging. This paper describes the drilling of a well at 2180m in the eastern GoM. The formations were carbonates with potential for high-to-total losses. The rig was a new Sixth Generation DP MDU. To manage the BHP and prevent losses, the rig had been equipped with a new type of DGD/MPD system; a pumped riser system, with the pump mounted around 400m below the bell nipple. The pump reduces the riser level in a precisely controlled manner to manipulate the BHP as required. Due to a very weak 20"shoe the 17 ½" section had a drilling window of only 0,3ppg. The pumped riser system was used to drill the section with nearly constant BHP, compensating for the ECD by manipulating the hydrostatic column level while drilling and bringing the level back up for connections. No losses were seen. The same applies for cementing of the 13 3/8 casing. In the 12,25" section the system was rested due to robust formations and a generous drilling window but was used again successfully to ensure a good 9 5/8" cement job. In the 8,5"section the pumped riser system was again necessary due to potential severe losses in weak reef-type carbonates. It soon became evident that this formation could not have been drilled conventionally as weak zones were penetrated at the start of the section. Loss pressure was 8,4 ppg. Drilling with returns was still required in order to get the required geological information from the well, which was achieved. As drilling proceeded a normal-pressured zone was encountered. The pumped riser system was used to balance the resulting pressures downhole with required returns to the surface. The system was also used to perform a good Plug and Abandonment operation after TD was called. This paper will describe the planning and drilling operation of the mentioned ultra deepwater well and present the operational results.
MPD -Managed Pressure Drilling technologies have been successfully implemented over the last decade. The technique is also frequently used on floating drilling units. MPD has primarily been understood as a technique where back pressure is applied in combination with a low density mud weight and a Rotating Control Device (RCD) when working onshore or on fixed platforms. However, within the IACD there is a sub-group defined as Dual Gradient Drilling (DGD). These techniques have mainly been used on floating drilling units and allow the use of a higher, overbalanced mud weight. EC-Drill is a technique that falls within the DGD segment. In its current form it is not providing "full dual gradient", instead it is eliminating the effect of annular friction pressure (ECD) and enables drilling with a close to constant bottom hole pressure (BHP). This may impact well design by allowing more optimal placement of casing points. The technique is ideal in low mud weight environments and depleted fields. EC-Drill is also a valuable tool in loss regimes and it may be used to ensure good cement jobs in deep water preserving the integrity of the well. A subsea mud pump is connected to the riser where it controls the well pressure by manipulating the fluid level within the riser. The change in bottom hole pressure is directly related to the change in fluid level -or suction pressure of the subsea pump. The system is not a part of the well control equipment and the mud is still the primary well barrier. The EC-Drill technology has been successfully deployed on jack-ups and on Modular Drilling Units (MODUs) in deep water. The Paper will present the technology and some case histories.
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