Consideration of Multiple Rock Failure Mechanisms in Predicting Hole Stability Conditions for the Nahr Umr Shale Layer in Southern Iraq

For the development of Nahr Umr sandstone reservoir, in the lower part of this formation in Ratawi field, South Iraq, the standard architecture involves drilling highly deviated intermediate section and setting the 9 5/8' casing shoe within the middle of this formation. This interval corresponds to the Nahr Umr shale, which is known to be a problematic shale across the Middle East region. To define an optimal MW strategy, we conducted an extensive analysis of instability cases experienced by TotalEnergies and those reported in published literature. The various aspects of Nahr Umr shale have been addressed: in-situ stress orientation and stress regime, physical- mechanical properties, strength classification, anisotropy of the strength. We employed a multi-mechanisms failure model to predict the hole stability condition in terms of the minimum mud weight (MMW), including: shear failure, strength anisotropy, shale lamination, mud pressure filtration and creeping for this shale. Azimuthal effects, i.e well orientation, due to the anisotropy of horizontal stresses appear less significant than those of hole deviation angle because of strength anisotropy of the Nahr Umr shale. The strength anisotropy is estimated to have three times more impact than the hole azimuth. The cumulative effect of these failure mechanisms explains the persistent instability in both vertical and deviated wells, with well azimuth playing a relatively minor role. From a strict geomechanical point of view, drilling the high deviated well in NU shale requires a mud weight from 1.40 ~ 1.45 sg. This mud weight range should also be applied to wells with moderate deviation, i.e., between 30° and 60°, essentially due to the anisotropy of the NU shale. Success in drilling through the Nahr Umr shale requires not only appropriate MW but also effective mud sealing, compatible mud chemistry, and rigorous hole cleaning and back reaming practices.