There is an increased demand for the design and application of non-damaging fluids in reservoirs that are depleted and susceptible to deep invasion of damaging mud solids. To enable a reservoir drilling fluid to function correctly, it is necessary to very rapidly deposit an external filter cake and reduce filtrate loss to a minimum. The cake must be backflowed in the production phase with minimum liftoff pressure and leave little or no permeability reduction. To achieve this, an accurate description of the pore throats in the reservoir is required. If this cannot be obtained, or the reservoir is at risk to fracture, then another approach is required.

This paper illustrates the methodology of utilizing a variable calcium carbonate bridging distribution mixed with powdered graphite which overcomes the problems of drilling reservoirs with varying pore throats and fractures. The bridging materials will enable an efficient control of filter cake deposition, and also allow for easy disruption of the cake integrity, when the well is back flowed in the production phase.

Results proving the effectiveness of this approach can be shown from return permeametry work in the laboratory, and successful field trials of its application in the Southern North Sea and the Middle East.

The use of these non-damaging products, and the engineered size distribution, will greatly decrease the difficulty at which fractured and depleted reservoirs can be drilled and will not reduce the productivity of these wells, that have in the past experienced formation damage due to deep fluid and solid invasion.

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