The Barents Sea offers unique drilling challenges related to issues such as biogenetic gas in shallow formations, thermogenic gas seeps up to the seabed from underlying formations, shallow formations with abnormal pressure, shallow reservoirs, low-fracture-pressure formations in part of the overburden, and naturally fractured/karstified carbonate reservoirs. This paper discusses cementing challenges when drilling wells in the Barents Sea and the experience gained using managed pressure cementing (MPC) practices. When drilling the surface hole in potentially slightly overpressured formations, the riserless mud recovery (RMR) technique was used. For the first time on the Norwegian Continental Shelf (NCS), MPC was used when cementing the surface casing. RMR compensates for drilling the overpressurized zones without a riser and blowout preventer (BOP), and MPC allows for pressurization and monitoring of the pressure on the subsea wellhead toward the formation during the cement curing stage. Once the marine riser and BOP were installed, controlled mud level (CML) technology was used during drilling, running casing/liners, cementing operations, and other activities. CML enables manipulation of the fluid level in the riser and therefore helps optimize downhole pressure to avoid losses and maintain an overbalance. CML has proven to be particularly useful during cementing of liners in naturally fractured reservoirs and during setting of balanced cement plugs in an open hole. As a result, high circulation rates can be achieved and conventional high-density cement slurries can be used. MPC using either RMR or CML was employed for the first time in the Barents Sea. Examples of how cementing operations were planned and executed are described and results are presented.
In 2015, while coring in the carbonate reservoir in the second appraisal well on an oil and gas discovery in the Barents Sea (386 m water depth), the drill string fell 2 meters and a total mud loss was experienced leading to a well control incident. As a result, since 2016, the operator has introduced and used the Controlled Mud Level (CML) system. To date this system has been used on seven wells including two further appraisal wells on the same field and five exploration wells in the area. In 2017 it was decided to drill a horizontal well in the same carbonate reservoir and to perform an extended production test in close proximity to the original loss well. Since it is not possible to predict where large voids (karsts) and natural fractures could be encountered, contingency to handle high losses, had to be implemented for the horizontal well. During the well planning, further risk reducing measures were implemented, including the use of wired drill pipe to improve the management of the wellbore pressure profile. This paper describes the planning processes leading up to the operation and the highlights of the operation itself together with the lessons learned. It elaborates on how wired pipe, used in combination with the CML system, added value to the operation. It shows how it was possible to drill the reservoir section with a low overbalance while managing severe losses associated with open karsts and natural fractures and still maintaining the fluid barrier. Despite the severe losses encountered it was possible to safely drill and complete the well without any well control event by use of the CML system.
Riserless Dual Gradient Drilling (DGD) using a specialized subsea pump placed on the seafloor during top hole drilling has been widely used on offshore subsea wells prior to installing the blow out preventer (BOP). Riserless DGD systems, kown as riserless mud recovery (RMR), have been developed to allow riserless sections to be drilled with weighted mud while taking returns back to surface. This allows the operator to set surface casing strings deeper, thereby reducing the total number of liners/casing strings in the well. This paper addresses the deepest RMR operation on the Norwegian Continental Shelf to date. The water depth was 854m and the 26″ hole section was successfully drilled to 1972mMD (1913mTVD). The riserless mud recovery system enabled the 20″ shoe to be set at a sufficient depth to allow elimination of the 16″ liner, thereby reducing casing strings, cost, well complexity and most importantly setting up the 13-5/8″ string cementation for success. This was mission critical from a well integrity perspective in the context of a big bore 9-5/8″ upper completion. Following the successful drilling of the 26″, the 20″ casing was installed and cemented. Foam cement was pumped using the riserless mud recovery system in cuttings transfer mode to prevent accumulation of returned cement at the well head. The 20″ casing was successfully tested after installing the BOP and marine riser. The paper describes the riserless mud recovery equipment and the planning of the well, plus the engineering of a multi-stage subsea pump and the subsequent seabed deployment in order to drill the longest and deepest 26″ section on the field. A 26″ directional BHA with a 1.30sg surface mud weight (1.18sg downhole mud weight) KCL glycol mud at flow rates of up to 4800 lpm was used to drill to TD. At TD the well was circulated at a lower rate to 1.38sg surface mud weight at 3200lpm to provide trip margin over the effective mud density required for wellbore stability.
The objective was to be prepared for a total and sudden loss scenario while drilling and coring a challenging well in the Barents sea. A dual-gradient Controlled Mud Level (CML) system with Controlled Mud Cap Drilling (CMCD) mode was installed on the rig to manage minor and/or total losses. Prior to spud of the section, an advanced dynamic simulator with the actual well configuration loaded was used to conduct offline training, and prepare the drilling team and involved service personnel for the operation. Experience from previous wells in the area identified the risk of drilling into karstified carbonate zones with the potential of leading to total and sudden losses. An advanced dynamic simulator was used to reflect the details of the CML system to be used. The rig crew together with the CML operator and other involved service personnel were trained on how to manage a total loss scenario by switching from CML to CMCD mode. All relevant operational procedures were used as a basis for creating training scenarios and operational preparations for the exercises. This paper will briefly present the simulator set-up, the operation/training procedures and results from the training. Feedback from the operation itself will also be described including lesson-learned from utilizing a full-scale dynamic simulator with the actual well loaded during preparation for operation.
Since 2016, Lundin Norway has used Enhanced Drilling's EC-Drill System to enable Controlled Mud Level (CML) drilling in the Barents Sea. The CML process allows control of the mud level in the riser in order to mitigate and manage losses while drilling, coring, tripping, cementing casing/liner, wireline operations, and well abandonment operations. With a sub-sea pump module (SPM) docked to a modified riser joint, pressure sensors on the riser and mud return line connected to the top-side, the EC-Drill equipment was able to precisely control the fluid level in the riser and manage the bottom hole pressure while drilling through potential loss zones in naturally fractured and potentially karstified carbonate formations. This paper discusses the CML system used during the project, as well as the drilling, tripping, and cementing strategy, examples of how the system was used to reduce wellbore ballooning, reduce losses during drilling, cementing and well abandonment, and several key lessons learned over the the past seven exploration and appraisal well operations. The ability to raise and lower the riser level to manage the loss rate allowed the CML process to be used during drilling / coring operations, cementing of 7" liners and 9 5/8" casing, as well as wireline operations and well abandonment. Once the SPM and modified riser joint were deployed with the marine drilling riser and SS BOP, the system is ready for use. This allowed using the CML process in hole sections as large as 17 ½" with minimal modification of conventional drilling practices, or sacrificing ROP due to restricted flow rates. The accuracy of the riser pressure sensors on the modified riser joint allowed the system to be used while tripping to reduce surge pressures while running in hole. The system also added value during cement jobs by managing the wellbore pressure profile and return flow throughout the cement circulation and displacement. During plug and abandonment operations critical path time and cost have been reduced by managing losses to an acceptable rate during casing retrieval. The CML process has become an integral part of the operation from start to finish for Barents Sea operations for Lundin.
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