Controlling lost circulation during drilling operations in a reservoir prone to losses is typically remedied by cement squeezing or plug setting. The aim being to minimize or stop drilling fluid losses and to regain full returns at surface, and to affect wellbore integrity. Different placement methods of cement plugs have been discussed in detail in the literature, except for the ‘level-off’ method, which is particularly effective for curing complete loss circulation cases. Following the mathematical modeling of this cement plug placement method, its design, procedure and execution are discussed, together with some successful field cases, both drilling and workover situations in highly-fractured carbonate reservoirs in the Middle East.
Using drill pipe and an installed a Retrievable-Test-Treat-Squeeze (RTTS) packer, with some spacing from the loss zone, the method entails that the cement slurry is allowed to drop by gravity in order to cure lost circulation. As the column of fluid, mud and slurry in the well exceeds formation pore pressure, i.e. overbalanced conditions, a volume of acid-soluble cement slurry is allowed to slowly drop and freely penetrate the formation, i.e. through its fractures or caverns. During the penetration of this viscous slurry into the loss zone, the cement can set and the fracture or fissure openings are plugged. In addition, to minimize or prevent further losses, this cementing technique may also mitigate formation damage of adjacent zones.
Presented are detailed design calculations for the level-off placement technique: determination of required cement slurry and displacement volumes, and recommended displacement and RTTS packer setting depths. The expected depth of the top of cement is estimated. The design parameters are compared with field cases and explanations are given for possible discrepancies. Detailed field procedures and execution are also presented.
This is the first known, detailed description and modelling of the level-off cement placement technique in the literature.