Geomechanics Recipes for Advanced Reservoir Stimulation — A Case Study for an ADNOC Field

Abstract

Abstract The recent surge of interest in developing tight resources in the United Arab Emirates (UAE) has been accompanied by a growing appreciation of geomechanics challenges with potentially far-reaching implications on successfully placing and producing from stimulation treatments. The challenges are mostly posed by regional tectonics and localised ‘stress regime’ changes from normal faulting to strike-slip. Therefore, understanding the in-situ geomechanical conditions plays an important role in optimising stimulation designs and recommending safe drilling mud weights. In this study, a 3D mechanical earth model (MEM) was built by comprehensively integrating data from multiple sources—including interpreted mechanics and lithology information such as stresses, stress direction, and core results. This 3D geomechanical model covered the target reservoir and extended from the surface to deeper formations to fully capture geological structures with high resolution. A set of faults was also incorporated to help examine their impact on well planning. Sensitivity analyses were conducted to account for uncertainties in mechanical properties and local tectonic stress state. The 3D MEM successfully captured the observed drilling events in the thin weak inter-layers between the target reservoir for multiple wells. Some faults were shown to be critically stressed at present day. Risk of fault reactivation would increase with future production, and horizontal drilling may become increasingly difficult. The horizontal stresses in the target reservoir were significantly affected by porosity. Specifically, the low-porosity zones may have strike-slip stress regime, while high-porosity zones are in a predominantly normal faulting stress regime. The geomechanical modelling approach helped qualitatively highlight the variations in fracture containment (for a range of ‘net pressure’ thresholds), thus demonstrating the risk of breaching in a reservoir seal. It is moreover an interesting side effect that this geomechanical modelling ‘recipe’ provided invaluable insights into delineating potential ‘sweet spots’ for stimulation—essentially characterised by having high-porosity and low breakdown pressures. The developed 3D MEM helped extend the understanding of subsurface behaviour beyond a few well locations and historic events. Moreover, the model demonstrated its predictive power in relation to identifying the potential location and pressure boundary for a future stimulation plan ultimately impacting the well placement strategies.