Enhanced Reservoir Insight and Precise Well-Placement in a Karstic Carbonate Layer With Real-Time Ultra-Deep 3D Inversion: A Case in a Horizontal Appraisal Well, Offshore Abu Dhabi, UAE

Abstract

Abstract This paper details challenges in placement of an appraisal horizontal well. Challenges are associated with reservoir geology and well requirements. The top boundary of the fractured carbonate reservoir holds large uncertainties in depth and in structural dip and it also suffers from severe karsts. Well requirements are related to constraints on placing the horizontal well within a certain offset below the top reservoir to avoid the karst features while avoiding the potential for water or early water breakthrough. Preparation, execution and evaluation phases of well placement demonstrates the importance of Morzhov the geosteering approach for successful completion operations in complex geology. An Ultra-Deep Azimuthal Resistivity (UDAR) tool was deployed as the main geosteering tool, because the low resistivity contrast between the carbonate reservoir and above shale is too small for conventional deep-resistivity tools to deliver on the well objectives. An advanced 1D inversion algorithm provides high certainty on detecting boundaries around 100 ft from the wellbore, allowing early identification of changes in the karstic surface. Pre-well Modelling enabled optimal tool selection and simulated 1D inversion responses to the challenging geology. Resistivity image and Nuclear Magnetic Resonance (NMR) were also included to improve geological understanding. The horizontal section was targeted to stay within a certain offset below the top reservoir. At this depth, both top boundary of the reservoir and below water zone were mapped and identified by the real-time UDAR 1D/3D inversion. Moreover, a potential target zone was mapped at a deeper depth below the top boundary. While geosteering to avoid karsts, the Borehole Assembly (BHA) was stuck and a sidetrack was planned based on the new real-time info obtained from the inversion in the first horizontal. The new deeper target zone was drilled and the UDAR Inversion mapped further karstic features. Towards the TD, the inversion showed a sharp boundary that correlates with a sharp Bit-Gamma ray increase suggesting another karstic feature. Application of UDR-Image-NMR assembly provides critical insights and comprehensions of appraisal areas; Laffan-Mishrif boundary is positively shallower than expected and a new unit with excellent properties is mapped and produced. The appraisal well was successfully placed in the optimum zone, with oil saturation up to 69%. Mapping the upper boundary provided additional understanding of the geological uncertainty caused by the Karstic surface. Mapping of the aquifer allowed the assessment of the impact of changing the trajectory relative to the top boundary when required to increase distance from the upper karst boundary.