Methodology and Implications for NaturalFracture Network Modelling in a Carbonate Reservoir of Abu Dhabi

Abstract

To optimize the areal and vertical sweep efficiency in a fractured carbonate reservoir of Abu Dhabi, 3D coupled reservoir geomechanical modelling is carried out, investigating the impact of natural fractures on reservoir deformation and performance in future production. In this study, AntTracking, Neural Network and fracture simulation techniques were used to model natural fracture networks by integrating FMI image logs, petrophysical and seismic data. The seismic data were smoothed to improve signal-noise ratio by removing noise. Based on smoothed seismic cube, variance processing was performed to enhance the edge effect, followed by AntTracking processing to enhance the discontinuity. SDPs (Seismic Discontinuous Planes) were extracted from AntTracking for the carbonate reservoirs representing fractures including faults and joints.The extracted fractures were validated against FMI interpretations of 11 wells in terms of orientation and intensity. To further validate the developed fracture model, the identified fracture sets were calibrated against the geological structures in the field, such as the main anticline and major faults. Four fracture groups were identified. Three groups of fractures were created in an early tectonic event associated with Oman stress, during which the main folding structure was generated, including (a) NW-SE trending related to R shear, (b) NNW-SSE and NNE-SSW trending related to R' shear, and (c) WWS-EEN and WWN-EES related to P shear. Another fracture group identified from AntTracking volume is NE-SW trending, which was possibly associated with the same early event associated with folding and/or due to Zagros stress forming later tensile fractures.The fracture model was incorporated in 3D coupled geomechanics simulations and the impact of the presence of the natural fracture networks on the in-situ stress state, bulk Young's modulus, bulk permeability and wellbore stability were investigated. It is concluded that highly deformable zones of the upper and middle reservoirs are generally clustered in NE-SW (maximum horizontal stress) direction. Better productivity is expected to place horizontal wells in this direction. The deformability of the lower reservoir is very different from the shallow reservoirs and has high deformable corridors oriented in NW-SE (minimum horizontal stress) direction. Better productivity is expected to place horizontal wells in this direction.In addition to productivity, the results also suggest that drilling horizontal wells needs to consider the presence of fractures and stress direction. It is recommended to drill horizontal wells towards the minimum horizontal stress direction. Drilling in this direction is more stable and less wellbore failure is expected.