Losses to the formation when drilling can lead to non-productive time (NPT), altering the wellbore construction due to unplanned casing points, and well control incidents due to the loss of overbalance pressure. The karstified and fractured carbonate reservoir formations sometimes encountered in the Barents Sea represent a high lost circulation risk. The large vugs (or caves) in these formations can be as large as 50 cm in diameter and cause severe to total losses while drilling. Dynamic loss rates are typically in the range of 30-60 m3/hr and can be greater. A unique lost circulation mitigation (LCM) solution is required for these conditions. An operator which plans to drill through this type of formation is wise to plan effective LCM solutions in order to rapidly cure any losses. The presented method is setting a reactive cementing fluid train downhole that creates an effective blockage in the loss zone. The cementing fluid train is composed of three fluids: sodium silicate, calcium chloride, and an innovative thixotropic and low compressive strength cement slurry designed to rapidly cure loss zones. When the first two fluids come into contact, they instantly form a viscous slush-like precipitate of calcium silicate. The fluid train is designed so the ‘slush’ forms inside the loss zone and creates a base for the cement following behind, allowing the cement to set up inside the loss zone. Together these fluids create the ideal LCM solution to large vugs and cavernous zones. The fluids are pumped through the drill string and the bottom hole assembly after it has been pulled above the loss zone. After the fluids are pumped, a short wait on cement time allows the operator to drill ahead through the cement and continue with the planned well path. The content presents a description of the procedures and successful results from two operations performed in the Barents Sea. The operator cured the loss rate to the necessary level to continue drilling, negating the need for a sidetrack. Furthermore, multiple LCM solutions had failed prior to pumping this fluid train, proving the effectiveness of the solution.
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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