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.
