Abstract When 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 low- viscosity 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.