Modeling Fractures in a Heterogeneous Carbonate Reservoir Onshore UAE: A Case Study

Abstract

Abstract A brown field producing from an Upper Cretaceous carbonate reservoir in an elongated anticline is facing water management challenges (increasing water cut, premature water breakthrough), impacting production and recovery. The complexity of heterogeneities encountered in the reservoir (fractures, karst-related features, high permeability dolomitic drains), where depositional settings, diagenesis, and tectonics are intertwined, makes reservoir characterization critical to a successful field development plan. Evidence of dual medium behavior, combined with the presence of fractures from image logs, suggests that fractures may play an important role in reservoir dynamics, and hence need to be modeled appropriately. In this study, the scope of work was threefold: reviewing the conceptual fracture model in considering the latest data, creating updatable workflows to build discrete fracture networks in commercial geomodeling software, and, ultimately, comparing upscaled properties to well test interpretations representing "fracture" and "matrix" responses. Main fracture sets trend NE-SW, parallel to the anticline axis, and E-W. Two fracture scales have been modeled in the present case study: large features that relate to the deterministic component of the dataset (fracture clusters from image log interpretation and seismic lineaments), and small-scale fractures, stochastically distributed, to reflect the permeability contribution of diffuse fractures connecting deterministic objects. Calibration to dynamic data is performed by analytical estimation of fracture aperture per fracture orientation set, upscaling to equivalent grid permeability, and iterating until reaching a satisfactory match with the test-derived permeability. The calibration exercise gives a consistent fracture aperture for the two orientation sets tested (NE-SW and E-W). Fracture modeling was done in parallel to building reservoir matrix properties hence the dynamic calibration considers only fracture permeability. In some cores, measured permeability can reach several Darcies, leading to dual medium behavior without the contribution of fractures. Future work could integrate this analysis with existing sedimentology and reservoir dynamic syntheses to assess the relative impact of high permeability streaks on equivalent permeability and deconvolve it from the role of fractures.