One of the wellbore instability problems in vertical wells are breakouts in Zubair oilfield.Breakouts, if exceeds its critical limits will produce problems such as loss circulation which will add to the non-productive time (NPT) thus increasing loss in costs and in total revenues. In this paper, three of the available rock failure criteria (Mohr-Coulomb, Mogi-Coulomb and Modified-Lade) are used to study and predict the occurrence of the breakouts. It is found that there is an increase over the allowable breakout limit in breakout width in Tanuma shaly formation and it was predicted using Mohr-Coulomb criterion. An increase in the pore pressure was predicted in Tanuma shaly formation, thus; a new mud weight and casing programs are proposed to overcome such problems in the drilling operations in field developments plans.
Many problems were encountered during the drilling operations in Zubair oilfield. Stuckpipe, wellbore instability, breakouts and washouts, which increased the critical limits problems, were observed in many wells in this field, therefore an extra non-productive time added to the total drilling time, which will lead to an extra cost spent. A 1D Mechanical Earth Model (1D MEM) was built to suggest many solutions to such types of problems. An overpressured zone is noticed and an alternative mud weigh window is predicted depending on the results of the 1D MEM. Results of this study are diagnosed and wellbore instability problems are predicted in an efficient way using the 1D MEM. Suitable alternative solutions are presented ahead to the drilling process commences in the future operations.
During drilling operations, wellbore instability is one of the reasons that increase nonproductive time (NPT). Maintaining a stable wellbore is the primary importance during drilling by applying a proper geomechanical model to analyze and understand the distribution of stresses around the wellbore in order to minimize drilling risk and NPT. The lack of any geomechanical studies in Zubair oilfield in southern Iraq is one of the reasons that lead to wellbore stability-related problems and thus an increase in NPT. Petrel Geomechanics 2018 was used to show the distribution of the stresses and the mechanical properties. Showing the distribution of stresses in 3D MEM has big advantages in several ways, and one of them is to select the best path for any future directional wells. To mitigate wellbore stability problems and depending on the 1D mechanical earth models (MEM) results, a detailed 3D MEM was built for Zubair oilfield as the method to precisely pinpoint and diagnose those problems. The results of this study show two points; first is an abnormally pressured zone in Tanuma shale formation, and second, a narrow mud weight window in front of the Tanuma formation. As a conclusion, the mechanical earth model was an efficient tool in diagnosing and predicting geomechanics-related problems, thus suggesting an alternative mud weight program for the problematic zone, which is more important to reduce the most essential factor in the NPT.
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