Summary Some early high-inclination wells were impacted adversely by wellbore instability within Cretaceous mudstones overlying the Clair oil field. Detailed investigations of the possible reasons for borehole failure led to a decision to cut two cores within the unstable interval and carry out laboratory testing to investigate the failure mechanism. Analysis of rock failure based on a "planes of weakness" theory fitted previous drilling observations. This theory has been applied primarily for layered hard rocks (e.g., mining and tunneling) and is here extended to significantly weaker mudstones showing strength anisotropy. Minimum required mud weight predictions were made for a range of well configurations. The results of this study were used to plan an extended-reach-drilling (ERD) development well targeting a previously undeveloped part of the Clair field. The new well was successfully drilled, cased, and brought on stream within budget in 2009. This paper describes the basic geology of the field, rock properties of the Cretaceous mudstones, details of laboratory tests conducted, and analyses carried out. Cretaceous mudstones of the Clair field are characterized by low strength and minor anisotropy. Various rock tests were conducted on core to define the failure behavior: unconfined compressive strength (UCS), single-stage triaxial strength, and multistage triaxial strength tests were conducted, all at various angles to bedding. In addition, the effect of mud invasion was analyzed by performing a multistage triaxial test in drilling mud at high pressure. The analysis used commercial software to evaluate borehole instability in a rock medium that fails along weak planes. Minimum mud weight requirements derived from this analysis were compared with previous drilling experience and were used subsequently for predicting mud weight required to drill an ERD well. Lessons learned from the successful drilling of this well will be applied in future Clair development wells and may be of relevance elsewhere.