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A mechanical earth model was constructed for an oilfield in Kuwait that has a history of borehole related problems and prone to significant non-productive time in the highly deviated wells, the typical sidetrack wells took more than 100% of the time compared to the low deviated wells. The field is geologically complex, tectonically stressed with faults, fractures, unstable shales and anomalous pore pressures. Wells drilled within the structure are highly deviated with trajectories that almost parallel reservoir bedding planes, and that are aligned with the direction of minimum geomechanical stress. A comprehensive study was conducted to better understand the stability issues and to investigate the feasibility of drilling numerous additional high deviation wells. Data from selected offset wells was collated, analyzed and combined with field and regional information to construct a geological model which could then be used to predict and mitigate drilling related problems. The equivalent mud weight is arguably the most critical drilling variable and an element of the new plan would take account of stress dynamics and formation strengths to develop a mud program for vertical and horizontal sections. A geological model was developed using data from offset wells combined with field and regional knowledge. Well logs, mud logs and operational reports were analyzed, while unstable zones and failure mechanisms identified and incorporated into the earth model. A well plan was developed which included a comprehensive mud program and operational contingency actions for unplanned events. The planned well was drilled and monitored in real time, with emphasis on mud weight and mud rheology through the unstable and reactive shales. Multiple failure mechanisms such as stress induced wellbore instability, invasion of drilling fluids into weak bedding / micro-fractures and osmotic sensitivity, were found to be the root cause of wellbore instability across reactive shale formations especially during drilling of highly deviated wells. Correct mud weight and type prediction was one key factor during the drilling stage to keep the wellbore stable and deliver good borehole geometry, including the water phase salinity and fluid properties. The key objectives of the study were to define a safe MW program for the vertical and deviated sections of the planned well by conducting a wellbore stability study and to determine a real-time strategy to mitigate or manage wellbore instability problems as they arise. The paper describes the process of optimizing drilling practices and the application of real-time geomechanical monitoring for successful drilling. This application promises to open the prospect of drilling additional complex trajectory well while mitigating against non-productive time. |
