Lithologies under deepwater conditions usually show relatively reduced effective stress, due to the reduced lithostatic column. This translates into relatively narrow mud weight windows, driven mainly by shear failure or pore pressure in overpressured conditions, and by minimum horizontal stress gradients. Drilling operations should consider wellbore collapse, kick and losses as the primary geomechanics-related drilling hazards. These should be investigated and predicted during well planning, and should also be appropriately monitored during drilling, especially when an appraisal campaign will require highly deviated wells.
Real-time geomechanics is defined as a workflow that takes into consideration mud weight window planning, identification of geomechanics-related drilling hazards and possible mitigation actions, and, while drilling, operations monitoring by real-time data acquisition and interpretation, drilling occurrences detection, drilling practices revision, and the real-time update of mud weight window for further drilling. The authors present the case study of a drilling campaign in Chevron operated Rosebank Lochnagar Discovery, deepwater West Shetland, in almost 3,700-ft water depth. This campaign had the goal of proving the development concept of drilling horizontally in a field where the previous maximum inclination was only 35 degrees.
The planning phase consisted of mud window modeling using a mechanical earth model from offset wells. Potential drilling hazards were then identified and synthesised using a Drilling Roadmap as a drillling planning and management tool. The monitoring phase consisted of real-time detection, from analysis of logging-while-drilling and wireline data, of drilling hazards typical in the area, such as cavings, losses, and packoffs. Data interpretation required a multidisciplinary team of geologists, petrophysicists, geomechanics engineers, and drilling engineers.
The application of real-time geomechanics allowed an improvement in operations, safe drilling practices, and refined calibration of the 1D geomechanical model for further drilling campaigns.