Characterization and Simulation of a Complex Sub-Salt Fractured Carbonate Field Offshore Brazil

Abstract

The considered oilfield was recently discovered part of Petrobras effort in developing subsalt assets. As with many carbonate fractured reservoirs it has proved difficult to describe and model the fracture distribution and their consequence on the permeability especially in its current exploration and delineation phase where the wells are limited to three or four at most. Description of Paper: This integrated study used geophysical, geologic, and engineering data simultaneously to improve the reservoir description. The successful characterization of the fractures was mostly due to the extensive use of narrow azimuth 3D post and pre-stack seismic data. Using unique seismic technologies to enhance seismic resolution and using the derived enhanced seismic in post and pre-stack stochastic inversions, key seismic attributes were generated and were used to dramatically improve the characterization of the reservoir key rock properties including the fractures in the considered field. These key seismic attributes were used as input in the integrated fracture modeling process. High resolution seismic attributes were also used in a neural network algorithm as a constraint to derive geologic drivers such as lithology, porosity, water saturation, permeability and other key reservoir properties. Many geologic and seismic drivers were correlated, using artificial intelligence tools, against a fracture density computed from FMI interpretations available at very limited wells. The fracture models were built using the Continuous Fracture Modeling (CFM) technology and then calibrated to well test data to create an effective permeability model that is able to account simultaneously for the effects of the fractures and matrix. Results, Observations, and Conclusions: The derived 3D seismic attributes and 3D geologic and fracture models were used to better understand the factors affecting the distribution of the key rock properties including the fractures and their effect on the permeability. The resulting porosity, water saturation and permeability models were input into a reservoir simulator and allowed a good match of the individual well performances without the need for any extensive history matching thus validating the input reservoir models. This achievement was made possible by the use of key high resolution post and pre-stack seismic attributes as drivers in the CFM technology that provided the resulting rock properties. Applications: This study demonstrates the successful use of all available GG&E reservoir data in an integrated approach despite the very limited number of available wells. The use of high resolution post and pre-stack seismic attributes in the CFM technology is illustrated with a carbonate field as a viable solution for the characterization and simulation of such complex reservoirs during the exploration and delineation phases where the number of wells could be very limited. Equipped with these seismically driven reservoir models, validated with dynamic models, optimal field development strategies could be designed and implemented with a reduced risk.