Mechanical earth model to mitigate wellbore instability of Nahr Umr formation in Southern Iraq oilfield

Abstract

Borehole instability problems have caused non-productive time while drilling operations in the Nahr Umr shale formation. The drilling difficulties, including stuck pipe, caving, and tight holes, have been identified as significant problems in this formation. This study aims to comprehend the causes of wellbore failure and choose the most suitable drilling strategy. In this regard, a mechanical earth model (MEM) was performed using open-hole wireline logging measurements from the offset wells data. Those data included gamma ray, density log, compressional wave velocity, shear wave velocity, neutrons porosity, and image log. Moreover, there were also measured data such as leak-off tests and modular formation dynamics testers that were applied to validate the model’s accuracy.

The analysis exhibited that the leading cause of wellbore instability problems was improper mud weight (10.6 ppg). In addition, inappropriate drilling practices and the heterogeneity of the Nahr Umr formation have magnified the risk of drilling problems. The MEM outcome proved that the mud weight should be 11.6–13.5 ppg to safely drill a vertical well. Thus, this work can be applied as a cost-effective tool when designing future neighboring deviated wells.