This paper presents the experiences gained from mud formulation and hydraulics modelling to achieve best operational practices during the Shearwater high-pressure/high-temperature (HPHT) project in the Central Graben of the UK Continental Shelf (UKCS). The Shearwater project is located in Block22/30b of the UKCS with a reservoir pressure in excess of 15,000 psi and a maximum temperature of 380oF. The principal factor making this project most challenging is the extremely narrow drilling windows, in the worst case less than 480 psi, which is the difference between the pore and fracture pressures. Historically, wells in this area were drilled and completed in over 165 days. The six-well HPHT project was drilled and completed more than 230 days ahead of schedule and $48 million below budget due to the continued improvements in drilling performance, in which a robust mud system, optimum mud formulation, HPHT hydraulics modelling and best operational practices have played asignificant part. This paper discusses the following technical issues, which have contributed to the success of the project:*Mud system and formulation*Criteria for the optimum level of treatments with emulsifier and oil wetting agent*Specification of critical mud properties*Management of bottom-hole pressure*Hydraulics program versus downhole pressure tool*HPHT procedures*Operational practices The paper also presents the drilling performance on the project and some conclusions derived from our experiences. Introduction Drilling performance has a significant economic impact on the petroleum industry, particularly so for HPHT projects. Due to the complexity and technical challenges, HPHT wells were usually drilled at much longer rig time and higher expenditure than non-HPHT wells. As far as drilling is concerned, there are several major challenges from HPHT wells:*The mud must be stable under extreme pressures and temperatures as unstable mud systems can often lead to mud gellation, barite sag and other problems.*The effects of pressure and temperature on mud weight and on the equivalent circulating density (ECD) can not be ignored due to the potential impact.*Rheology must be optimised to minimise ECD without inducing barite sag.*The drilling margin or window between pore pressure and fracture gradient becomes significantly narrower. Loss and gain situations can be experienced with a slight error.*As the mud hydrostatic pressure is very close to the formation overburden pressures, the formation behaves abnormally. The terms used to describe this complex behaviour include formation ballooning, plastic formations or formation instability, which makes it more difficult to differentiate a kick from returns of previous down hole losses. Due to the small tolerance in terms of mud weight and ECD or rheology in HPHTwells, drilling is often problematic. Loss/gain and swabbing of formation fluids are often experienced from an imprecision in intended mud weights, pump rates and tripping speeds. As a result, these wells had hardly been drilled smoothly in the past. Mud related drilling problems such as losses, gains, and barite sag are typically associated with the problem wells. These problems can be costly and if not properly controlled can result in the loss of the well.