A unique oil-based drilling fluid system weighted with treated micronized barite (TMB) slurries has been developed and successfully introduced to the field. The utilization of this weight material provides the fluid system with low viscosity, reduced torque values, superior sag stability thus giving a fluid with low Equivalent Circulating Density (ECD) contribution and excellent hydraulics performance. These exceptional fluid characteristics make the fluid system an excellent solution for drilling sections with narrow mud-weight windows, coiled tubing operations, managed pressure drilling and extended reach drilling. Many of these drilling challenges are encountered in high-temperature, high-pressure (HTHP) and ultra-deepwater field developments and in depleted, mature fields.

Much of the early fluid system development focused on design, the system's physics and chemistry, and the optimization of mineralogy of the weighting agent. Also of concern was the variability of results seen both from return permeability as well as from standard fluid-loss experiments. On this basis a comprehensive study was undertaken to identify and understand the damage mechanisms operating in the formation and filter cake. During this period the fluid system was used in a number of operations in the North Sea such that the current available database includes 5 different types of field applications.

The paper presents the findings of the formation damage study including relevant productivity data from the various field applications. The results demonstrate that while invasion of the formation by treated micronized barite can occur, it does not necessarily lead to permanent productivity impairment. Furthermore, the micronized barite does not interfere with the added fluid-loss-control material over a wide range of fluid densities and formation permeabilities. The authors discuss the processes observed relating them to current field experience describing why the formation damage mechanisms do not concur with previous preconceptions and moreover describes where the limitations of the system occur.

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