Lost circulation while drilling across vugular or naturally fractured limestone formations is a costly challenge and has financial impacts including nonproductive time and remedial operational expenses. Many fields in the UAE are encountering notorious lost circulation complications, which are difficult to control with conventional lost circulation solutions while drilling surface sections. Novel lightweight thixotropic cement has proven beneficial to take control of severe losses in these vugular and naturally fractured limestone formations. The main challenge while drilling across the surface section in UAE offshore field is the heavy or total loss of returns. Drilling performance is affected due to poor hole cleaning, a risk of stuck pipe, surface fluid handling problems, and well control risks. Conventional extended cement slurries have been widely used to cure losses while drilling but with limited success. A new lost circulation solution combines lightweight (10.5- lbm/galUS) high solids fraction cement (trimodal system) and a thixotropic agent, which develop fast gels with high compressive strength. Thus, it enables plugging of large voids and fractures to deliver the wellbore integrity required to continue drilling with enhanced performance and efficiency. Intensive laboratory qualification tests focusing on static gel strength and compressive strength development was performed to tailor the new solution. The results were promising with more than 100 lbf/100 ft2 of static gel strength in 10 minutes and compressive strength development of 1,000 psi within 24 hours at low surface temperature. In addition, a transition time (TT) on-off-on test demonstrated more rapid gel strength development when the shear is reduced and regained fluidity with reapplication of shear. In one of the wells, heavy losses were encountered while drilling across surface section. The lightweight thixotropic solution was pumped for the first time worldwide and it was shown that the innovative lost circulation solution was effective in significantly reducing the losses and enabled the operator to continue drilling to section TD. This case study demonstrates that this advanced system is effective in curing losses and reducing nonproductive time. The unique properties of faster rapid gel strength and high compressive strength make this solution effective for treating a wide range of lost circulation events while drilling. Furthermore, the advanced lightweight thixotropic cement lost circulation solution exhibits strong performance in curing heavy losses and establishing well integrity with reliability.
While drilling through the initial section of extended reach drill (ERD) wells in Abu Dhabi where the trajectory requires a high inclination across a recognized loss zone various options were required to be assessed to maximize efficiency while balancing risks. Factors such as loss rate, capability of mixing fluid, necessary density to help prevent flow from a shallow water-bearing zone, and rig time, where all necessary and key factors to consider in the design process. For this UAE field with common losses in the surface casing, brine capping was determined the best solution to continue drilling without generating nonproductive time or creating a possible wellbore instability issue when unable to keep up with building mud to offset mud losses. For wells with a higher inclination angle, when the loss rate reached the point where it was not possible to prepare the fluid to keep up with losses, it was necessary to identify a different solution to cure or significantly reduce the losses and enable the hole section to be drilled without potential operational risks. For vugular/fractured porosity formations, using tailored particle size materials was unsuccessful for curing the losses. Therefore, a unique solution was implemented by combining two different systems to battle the losses: a swelling polymer lost-circulation material (LCM) that hydrates and helps reduce flow velocity into the formation, followed by a shear-rate rheology-dependent cement system that is a tunable and tailored slurry with thixotropic properties, which stops losses and develops low compressive strength. With this combined solution, the drilling process was successfully resumed and completed. The usual loss rate for this particular vugular argillaceous limestone formation is between 600 and 800 bbl/hr while drilling. Once the solution was successfully implemented, losses were reduced to 15 bbl/hr. The technique was performed on a second well, applying the lessons learned from the first attempt, and the unique solution achieved a dramatic reduction of losses to 2 to 6 bbl/hr. The cost and effectiveness of the treatment demonstrated that this solution is best for optimizing the drilling process for this particular condition. Applying a swelling polymer LCM and the shear-rate rheology-dependent cement system cured losses for an argillaceous limestone formation with fractured/vugular porosity. It is the first global application of this combined solution.
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