Integrated Geomechanical Characterization of Complex Salt-Anhydrite Gotnia Lithology of North Kuwait: Enhancing Drilling Efficiency in HPHT Formations

Abstract Drilling through thick salt-anhydrite sequences in the study area poses significant drilling related challenges due to the overpressured nature (and also high temperature) of these intercalations as well as the associated limestone stingers. The limestone stingers often leads to frequent inflow and loss events, and thereby present significant drilling risks, if not anticipated and controlled. In many drilled wells in this area, unexpected inflow-loss situations have been often reported due to the very narrow drilling margin (pore and fracture pressure difference) leading to severe well control situations, resulting in significant overrun of drilling cost. Therefore, it is very important to understand the geomechanical behavior of such complex lithology. The paper discusses integrated geomechanical characterization of complex salt-anhydrite sequence. No direct pore pressure measurements or petrophysical log acquisition was attempted due to the High-Pressure High-Temperature (HPHT) nature of the formation as well as the complex drilling operations. Moreover, routine porosity-based pore pressure prediction methods are also not applicable in such complex geological setup. Therefore, an integrated approach was adopted using offset well drilling data (with special focus on the ones recorded during well control situations). This comprehensive workflow enabled correlation of critical drilling events and information to the subsurface model, and thereby helped in indirectly estimating the possible pressure state in such complex lithological sequences. The above integrated workflow is adapted to build pressure cell model for solving drilling challenges. This geomechanics-based integrated approach helped in field-scale estimation of the in-situ pore pressure state of this complex salt sequence, leading to a better estimation of the operational mud weight window to help new planned wells to be drilled to deeper targets. Integration of geomechanical understanding with drilling events and operational data, as well as with the detail subsurface model played a major role in significantly reducing drilling related Non-Productive Time (NPT). Moreover, adopting a 3D-modeling approach and integrating it with a detail risk-management planning further enabled risk ranking of future drilling plans, allowing adoption of best practices at every phase of the planned field development project: from well planning and construction, to drilling, evaluation and completion. The study results have been tested in a number of wells with positive results and have been incorporated in pre-drill well planning for a number of upcoming highly deviated wells. Planning of new wells with this knowledge has facilitated smoother drilling operations through this thick salt interval and enable the wells to reach the deeper prospective targets.