Investigation of wellbore stability in a North Sea field development

Planning drilling operations is key to the development of oil and natural gas fields. As part of well design, wellbore stability and pore pressure analysis are main factors in determining the safe operating window for selecting mud density. The motivation for this research was to conduct wellbore stability based on exploration wells as basis for planning new offshore field developments with deviated and horizontal wells. A second objective was to investigate the use of a probabilistic approach to wellbore stability, and compare it to the conventional deterministic methods, in order to evaluate the value of including a probabilistic approach to wellbore stability analysis. The field to be developed is located in the Southern North Sea, and includes a reservoir consisting of conglomerate and sandstone. Data from multiple exploration and appraisal wells were analyzed to create the geomechanical model used as the basis for developing safe mudweight window for the deviated and horizontal injection and production wells in the field. The wellbore failure analysis shows the necessity of triaxial testing, mini-frac tests, and other advanced data collection techniques to improve the geomechanical model when studying new lithologies, not included in existing rock strength correlations. A probabilistic approach to wellbore stability analysis was implemented using Monte-Carlo analysis. It was determined that this approach to wellbore stability resulted in an unrealistically high minimum mudweight for an acceptable risk level. Based on the level of uncertainty of the input parameters, with current geomechanical modeling techniques, the probabilistic approach to wellbore stability analysis was determined to be ineffective as a tool for planning new field developments. As such, it is concluded that the best method for conducting wellbore stability analysis is deterministically, based on the best estimate of each input parameter for this analysis --Abstract, page iii