A major operator in the North Sea had a requirement for a special fluid system to drill a series of HPHT exploration and appraisal wells. The operations had to overcome substantial technical challenges, with an expected bottom hole temperature near 400°F, anticipated reservoir pressures around 17,000 psi and narrow margins between formation pore pressure and fracture pressure. The oil-based mud had to fulfill a number of requirements, including thermal stability, consistent properties and predictable density behaviour, especially given the extensive wireline programme. The first part of this paper discusses the approach taken to the initial laboratory design work, with special consideration given to selection of weighting agent and HPHT testing regimes. With a suitable, environmentally acceptable fluid formulated, the second part of the paper deals with operational considerations given to wellsite maintenance and testing that allowed the three-well campaign to be completed successfully. Careful monitoring ensured that fluid properties remained stable throughout the drilling phase, and enabled a sustained, balanced rheological profile. This was necessary to minimize effective circulating density in the narrow drilling window, and to prevent sag during drilling and lengthy logging periods. The successful use of this special fluid contributed to the success of the wells. The fluid achieved excellent hole conditions, removing the need for costly wiper trips and delivering excellent wireline logs. Furthermore, the fluid's stability facilitated the resolution of well-control situations that often required slow, controlled circulation.