Geomechanical Solutions for Drilling Through Challenging Nahr Umr Shale at High Angle: A Case Study From Offshore Field, Abu Dhabi

Abstract

ADMA-OPCO has undertaken a prestigious campaign to drill a large number of wells from artificial island in ABC field, most of these wells are extended reach drilling (ERD) wells with step-out up to around 18,000 ft. Operational efficiency/costs for drilling ERD wells is highly dependent on the wellbore stability, especially while drilling through the problematic Nahr Umr shale at different deviations and azimuths. Nahr Umr shale has a known history of causing wellbore instability in UAE and the surrounding countries and therefore a geomechanical study was initiated to understand the geomechanical setting in ABC field as well as fluid-rock interaction between drilling fluid and Nahr Umr shale formation. The main objectives of this geomechanical study were to optimize well design and drilling fluids in order to drill through Nahr Umr shale interval efficiently, additionally estimation of sustainable pressure variation that major faults can take without being reactivated was also performed. An integrated geomechanical study including a 3D geomechanical modeling was carried out, in order to ensure drilling through Nahr Umr shale formation efficiently. This study covered formation petrophysical characterization, chemical tests on cuttings from Nahr Umr shale, chemo-poroelastic modeling, weak bedding analysis and also faults reactivation analysis. Based on the study mentioned above, both customized drilling fluids program and suitable mud weights were optimized to stabilize Nahr Umr shale, and mitigate different types of wellbore instability issues. In addition to mud fluid optimization, the sustainable pore pressure variation was also estimated for several major faults. A successful drilling campaign is in progress; so far many deviated wells have been completed without any noticeable troubles while drilling through Nahr Umr shale. This geomechanical model is helping to implement an effective drilling program for a smooth well placement. A learning curve has been building up continuously for handling more complex well trajectories successfully in the future. From this study, it was realized that, not only fluid-rock interaction and geomechanics related factors need to be taken into consideration for stabilizing a wellbore, but also special attention is needed for the existing micro-fractures within the formation, where increase in mud weight may make hole condition worse. A balanced approach has been adopted including drilling fluid optimization in order to avoid possible multiple failure mechanisms.