Drilling highly deviated or horizontal wells can be prone to instability problems. This paper describes a case in Libya on which significant difficulties were encountered during drilling the first horizontal development well in a field in Murzuq basin. The first two branches of the well were lost due to severe instability problems. A comprehensive geomechanics study was carried out to understand the causes of the wellbore failure and to improve drilling design and drilling performance on further development wells in the field. The study specifically included:Performing a systematic data audit and integration to identify specific uncertainties in the input data and identify data gaps.Performing a comprehensive drilling event review to investigate what happened during drilling and what were the major instability problems. All drilling events were organized, categorized and input into a database. A graphical presentation of the drilling events review was generated.Applying a fit-for-purpose stability model. This simple model is driven by easily accessible input parameters and provides a rapid answer.Validating a Mechanical Earth Model through history matching. This procedure reduced the uncertainty of the model and the resulting wellbore stability predictions.Applying innovative visualization and presentation of wellbore stability analysis. This improves understanding and awareness of the various drilling hazards and enables effective utilization of the wellbore stability information during drilling. The analysis identified the cause of wellbore instability, as being inadequate mud weight while drilling the overlying shale formation in the deviation build-up section. The design of the second horizontal well was optimized based on this study. The well was drilled successfully without problems and, in fact, ahead of drilling schedule. This case demonstrated that a comprehensive geomechanics analysis can greatly improve drilling performance and reduce drilling costs. Introduction Horizontal wells can increase production rates and ultimate recovery, and reduce the number of platforms or wells required to develop a reservoir. The geometry also helps to delay water or gas breakthrough, bypass environmentally sensitive areas and reduce stimulation costs.1 To achieve avoidance of water coning and delay of water breakthrough, Akakus Oil Operations started to drill the first horizontal development well H1 in a field in Libya in 2006. However, unexpected drilling difficulties were encountered and the first two branches of the well were lost. Prior to this project, quite a significant number of exploration and development vertical wells had been drilled in the same block without experiencing any major problem. A wellbore stability study was carried out to understand the cause of wellbore failure in the horizontal well, to optimize the drilling design and performance for the next horizontal development wells to be drilled in the same field.