Methodology and Implications for NaturalFracture Network Modelling in a Carbonate Reservoir of Abu Dhabi
Authors: Sirat M. , Zhang X. , Xi G. , Ni Q. , Mohamad Hussein A. 
Published: March 2015

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, includingNW-SE trending related to R shear,NNW-SSE and NNE-SSW trending related to R' shear, andWWS-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.

Introduction

The studied field is located in South-East of Abu Dhabi. The production in this field is within Lower Cretaceous carbonate formations, including the upper, middle and lower reservoirs. Those three reservoirs have a very similar structural framework. They are elongated NE-SW trending anticline, with several domal hubs that form saddles in between. The whole structure has been affected by the Tertiary tilt to the NE direction. Repeated structural reactivation of basement has played a key role in present day trap development. Hydrocarbons discovered to date in Abu Dhabi are trapped in structure grown mostly during the Late Cretaceous associated with Oman stress (Sirat et al., 2012 a&b). This gently folded carbonate field is characterized by the presence of two major seismic faults: The N75W and the N45W, and few other subseismic fracture sets. These conjugate-like appearance seismic faults are regularly spaced and are segmented both vertically and laterally forming several compartments (Sirat, 2013a).