The oil & gas industry has evolved a new class of drilling systems designated for the drilling of the pay-zone sections of wells. In addition to high angle drilling operations, these fluids are now being utilized for underreaming and workover type operations where formation protection is of major concern. As compared to typical drilling fluids, these fluids are unique in that they are brine based and designed to not only fulfill requirements desired for drilling operations but are also essentially non-damaging to the production zone.

Brine based drilling fluids have become one of the most widely utilized systems for the drilling of the productive interval due to their ability to protect the formation while drilling.1 These fluids are formulated with a brine as the liquid phase, selected bridging solids and polymers. The brine liquid phase may be composed of NH4Cl, KCl, KBr, HCOOK, NaCl, HCOONa, NaBr, CaCl2, CaBr2 and ZnBr2 as single or multiple salt solutions. Bridging solids can be water soluble or acid soluble. Selected polymers are added to provide proper suspension and desired Theological properties.

Criteria used to properly design brine based fluid for specific drilling operations will be discussed. Proper formulation of a selected drilling fluid will take into consideration well hydraulics, formation compatibility and completion strategy. Proper selection of the base brine, bridging solids and polymers should result with an optimum drilling fluid that minimizes formation damage.

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