fax 01-972-952-9435. AbstractA conventional oil-based drilling fluid was determined not to be applicable for drilling narrow hole annuli in a mature field offshore Norway. Drilling these wells with a very narrow hydraulic window required an oil-based drilling fluid with reduced viscosity and improved sag stability. A totally reformulated drilling fluid system was developed. The combination of base oil and selected products produced a fluid with a flatter rheological profile, resulting in reduced plastic viscosity (PV) and increased low shear rate viscosity (LSRV). This rheological profile reduced equivalent circulating density (ECD) and improved sag stability. This rheological profile also directly impacted pump pressure and hole cleaning capabilities, delivering lower pump pressure and allowing for higher pump rates and improved hole cleaning. This paper presents the experimental work performed when developing the system for this specific application. The authors also will present field data and compare the performance of the new drilling fluid system with that of conventional systems.

Open hole gravel packing of reservoir sections drilled with oil based fluid is traditionally performed with an aqueous carrier fluid. This typically involves displacing oil based fluid to aqueous fluid once the gravel pack screen is in place. In reservoirs with swelling or unstable shale this approach reduces the risk associated with open hole exposure to aqueous fluid over time. However experience has shown that instability can still occur resulting in an incomplete, or even an aborted gravel pack. In addition, mixing of incompatible oil and water based fluids downhole has the potential to generate very viscous emulsions that negatively impact gravel pack efficiency and well productivity. The objective of the new technology was to maintain borehole stability, eliminate fluid incompatibility and enable a complete gravel pack.An oil based carrier fluid has been developed and qualified using laboratory and yard scale testing. The fluid is a solids free invert emulsion that exhibits near Newtonian rheological behavior; thereby promoting settling of proppant during gravel packing. The density of the fluid is controlled by adjusting the volume fraction and density of the brine phase. The fluid has been qualified up to a density of 1.25 SG with further potential to achieve a density of 1.63 SG.The oil based carrier fluid has been introduced on a mature field, with a long history of gravel pack completions. Progressive reservoir depletion has created operational challenges, resulting in inconsistent gravel pack performance. Consequently, several procedural changes have been implemented over time. Reservoir inclination is typically up to 50°, open hole length up to 200 meters, and bottom hole static temperature around 90°C. Gravel packs were most recently performed with a 1.10 SG aqueous carrier fluid.The new carrier fluid has exhibited stable properties during implementation on multiple well completions. Gravel pack efficiencies have been consistently good, at 100% or higher. As a result, well productivity expectations have consistently been achieved or exceeded. The operational time for installing the lower completion compares well with the traditional approach with aqueous fluid.The implementation of oil based gravel packs in multiple wells, allows a comparison with brine based gravel packs in the same field. It is therefore considered to be an industry first.