Estimates of In Situ Stress and Faults/Fractures in Carbonate Reservoirs in Onshore Abu Dhabi Using Geomechanical Forward Modeling
Authors: Noufal Abdelwahab , Sirat Manhal , Steiner Stefan , Trubienko Olga , Minton James , Zhang Xing , Koutsabeloulis Nick , Herrera Adrian 
Published: November 2015

The paleo-strain estimation of the restored structures at each geological time step for carbonate reservoirs in a prospective area, Onshore Abu Dhabi was simulated using a forward geomechanical modelling technique. This technique integrates seismic information, well data, geological knowledge and decompacted horizons to 1) model the evolution of the formation deformation, 2) to predict the present-day strain, 3) to locate potential faults and 4) to predict fracture potentiality based on brittlness (?) and "fracture intensity" calculation.

The decompacted geometry of twelve surfaces over geological time in onshore Abu Dhabi was used as input in the forward finite element geomechanical modelling. Three types of geological events were considered: deposition, erosion and hiatus. Both elastic and plastic deformation has contributed to the total compaction. The plastic deformation is determined by a failure envelope, which was divided into three types of failure: tensile, shear, and cap failure (also known as pore collapse or consolidation). The elastic deformation is determined by the elastic properties of Young's modulus and Poisson's ratio.

Eighty (80) 1D-MEM (Mechanical Earth Models) were performed over the studied area, and the simulated present-day formation thickness at the 80 locations with an error less than 0.001 (0.1%?) was achieved. Sixteen (16) 2D forward geomechanical models along 16 selected sections were performed, and the simulated present-day formation geometry was consistent with the seismic interpretation. The predicted vertical, minimum and maximum horizontal stresses along the 16 sections were computed with the calibration of the stress profiles at selected wells, at which 1D mechanical earth models existed. The computed shear stresses were used to locate the location of potential faults or sub-seismic damage zones. Although the interpreted seismic faults were not the input in the forward geomechanical model, the fault locations predicted using this method were largely consistent with the faults location interpreted based on seismic data. In addition, a number of other areas that have no interpretation of faults were identified due to high shear stress, which indicates the existence of potential sub-seismic faults between the detectable major faults. The results show that faulting is likely to occur along the flanks of the structures (anticlines), rather than at the crest . We used the accumulated plastic strains to determine the areas with a normalized "intensity" of potential fractures, which are more likely to occur. Although the results were based on 16 2D sections, the 2D sections were assembled into 3D space, and the results were extrapolated to the whole 3D studied area. The predicted stresses can be used to optimize mud weight, well position and drilling direction for reducing non-production-time in drilling, and the predicted faults and fracture intensity can be used for the development of static geological and dynamic reservoir models for reservoir management. In addition the prediction of the fractures will enhance the production in the known reservoirs and may introduce a play concept for exploration.