In mature fields the difference between the pore pressure and fracture pressure, expressed as the hydraulic window, is reduced. Underbalanced drilling and the use of low-viscosity drilling fluids are but a few of the many approaches that have addressed this challenge. While low-viscosity fluids severely reduce frictional pressure loss when compared to more viscous fluids, there is a limit to how much viscosity can be reduced before conventional weight material begins to settle. This paper describes the development and application of novel technology that has resulted in a ten-fold reduction in the particle size of the weighting agent. With this development, invert emulsion drilling fluids can be designed with low viscosity with minimal settling potential of the weight material. The authors will explain in detail the first field applications of this polymer-coated, micron-sized weighting agent in an oil-based drilling fluid. The system was used to drill two wells offshore Norway. These reservoir wells included an 8 1/2-in. section and a 5 7/8-in. thru-tubing section. This novel drilling fluid technology delivers a low-rheology, low-sag fluid offering a number of performance benefits over conventional technology. Performance data from offset wells will be presented that show the benefits of this unique approach. Equivalent circulating densities (ECD) and torque values were significantly lower than comparable wells and no instances of particle settlement were observed. Introduction The Statfjord Field was discovered in 1973 and declared commercial in August 1974, with production startup in 1979. The field spans approximately 25 km by 4 km, and is the largest producing oil field in Europe. Statfjord is located in the Tampen Spur area, in the northern portion of the Viking Graben, and straddles the border between the Norwegian and UK sectors. The field is developed by three fully integrated Condeep concrete platforms. Production is from the Brent, Dunlin, and Statfjord reservoirs, with Brent and Statfjord being the main reservoirs. Cumulative oil production by end of 2003 is forecasted to 626 million Sm3, giving a recovery factor of 63%. The aggressive drilling campaign necessary to achieve such a high recovery is described by Hansen, et al.1 Development of the field has required drilling of long and complex wells, often in reservoir compartments with high pressure depletion. Good hole cleaning and ECD have been and still are main focus areas for achieving a successful drilling operation. Introducing annular pressure-while-drilling tools in the mid 90's highly increased the understanding of hydraulic behaviour in the drilling process.2 However, introduction of Through-Tubing-Reservoir-Drilling (TTRD), possible future ERD wells, and several lost circulation events in the later years, have pinpointed the need for a highly inhibitive drilling fluid system exhibiting a low ECD, low friction and low sag potential. A drilling fluid with these combined properties will also be of key importance if the planned pressure blowdown of the reservoirs in the field is being carried out, as this will narrow the hydraulic window further. In case of pressure blowdown, there will also be a need for openhole sand screens, and the drilling fluid must preferably be "screen friendly". The geological anomalies of Statfjord and similarly mature fields worldwide prompted the initiation of a weighting agent research project (WARP). Conceptually, the foundation of the program was to employ ultra-fine particles of a weighting agent that have been polymer coated to provide a number of technical benefits in drilling fluids. The weighting agent, in this case barite, is milled in an enhanced mineral oil using high-performance milling technology. In this manufacturing process as the barite particles are milled, the new surfaces that are continuously exposed are coated with the special polymer additive. This coating provides effective oil-wetting of the barite weighting agent to produce stable high-solids, high-density and less viscous slurries.

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWhen drilling wells in HTHP fields, the drilling fluid properties are of utmost importance. The difference between fracture and pore pressure, expressed as the hydraulic window, can be quite narrow. One of the remedies for making sure that one stays within this hydraulic window, is to design a drilling fluid with very low viscosity. This will reduce the Equivalent Circulating Density (ECD) effect from the fluid. While lowviscosity fluids have the ability to dramatically reduce frictional pressure loss compared to more viscous fluids, there is a limitation to the reduction in viscosity before conventional weight materials start to settle. In HTHP wells with extremely high pressures and temperatures, sag stability of the fluid is essential to ensure adequate well control. This paper describes the development and application of a novel technology that has resulted in a ten-fold reduction in the particle size of the weighting agent. With this development, invert emulsion drilling fluids can be designed with reduced viscosity in combination with minimal settling potential of the weighting agent.The paper further explains in detail the first field application of this specially treated micron-sized weighting agent used in a HTHP well. This successful field application has proven this technology to be a viable alternative to the use of high-density, high-cost brine-based drilling fluids.

fax 01-972-952-9435. AbstractIn mature fields the difference between the pore pressure and fracture pressure, expressed as the hydraulic window, is reduced. Underbalanced drilling and the use of low-viscosity drilling fluids are but a few of the many approaches that have addressed this challenge. While low-viscosity fluids severely reduce frictional pressure loss when compared to more viscous fluids, there is a limit to how much viscosity can be reduced before conventional weight material begins to settle. This paper describes the development and application of novel technology that has resulted in a ten-fold reduction in the particle size of the weighting agent. With this development, invert emulsion drilling fluids can be designed with low viscosity with minimal settling potential of the weight material.The authors will explain in detail the first field applications of this polymer-coated, micron-sized weighting agent in an oil-based drilling fluid. The system was used to drill two wells offshore Norway. These reservoir wells included an 8½-in. section and a 5⅞-in. thru-tubing section.This novel drilling fluid technology delivers a lowrheology, low-sag fluid offering a number of performance benefits over conventional technology. Performance data from offset wells will be presented that show the benefits of this unique approach. Equivalent circulating densities (ECD) and torque values were significantly lower than comparable wells and no instances of particle settlement were observed.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe technical demands placed on drilling fluids used in the deep waters of Norway differ dramatically from those encountered in any other deepwater basin. Unlike the deepwater of the Gulf of Mexico, West Africa or Brazil, seafloor temperatures here can dip as low as -2.5º C (27.5ºF), with low ambient surface temperatures the year round. These characteristics combined with constantly changing sea conditions and some of the world's most stringent environmental restrictions, makes the engineering of drilling fluid systems for Norwegian North Sea wells an arduous effort.This paper describes the innovative development and qualification of a water-base drilling fluid system engineered for a well to be drilled in 837 meters (2745 ft) of water off Norway. The pre-qualified fluid was required to inhibit hydrates under normal drilling conditions and possess excellent shale inhibition characteristics to support hole stability and avoid bit balling. Since the primary objective was to obtain a non-contaminated formation water sample, the system must possess superb fluid loss control so the operator could avoid a full drill stem test, thereby saving some US$8 million. Furthermore, the qualified system would have to meet local environmental regulations for offshore discharge of cuttings and excess fluids.The authors will discuss the development protocol, the qualification of the novel fluid system and its application in the technically demanding Norwegian well. During the development phases, two hydrate testers were qualified and employed in both designing the fluid and to monitor its hydrate inhibitive tendencies during drilling. In addition, the particle size distribution and polymer concentration were engineered to reduce the filtrate invasion, while polymers were specially selected for improved rheological parameters to cope with the cold temperatures. The behavior of a number of glycols and salts were characterized prior to the start of drilling.The practical approach to qualify the novel fluid system will be explained in detail, emphasizing its use as a guideline to help non-fluid specialists design drilling fluids for this unique deepwater environment