There is a clear advantage to drilling if we could strengthen the wellbore and drill at higher mud weights without losing fluid. A major prize is accessing difficult reserves in depleted reservoirs. Another application is in deep water drilling where the drilling window between pore pressure and fracture gradient is often narrow. This paper describes the approach taken by BP to produce a ‘designer mud’ which effectively increases fracture resistance whilst drilling, and which can be applied in both shale and sandstone. It works by forming a stress cage using particle bridging and an ultra-low fluid loss mud system. The theory is described and laboratory data show how the fluid system was developed. Field data are shown which quantify the increase in fracture resistance and demonstrate the value of the system. Logistics issues are discussed.

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