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.