Integration of 3D Seismic, Structural Geology and Geomechanics to Model Naturally Fractured Reservoir A Case Study in Carbonate Reservoirs, UAE.

Abstract

Abstract This case study presents a systematic methodology applied for integrating tectonic history, image data, 3-D seismic data, geo-mechanical study to develop the Discrete Fracture Network (DFN) based 3D fracture model. Motivation for the present study was the limited understanding of permeability distribution in reservoirs. Dynamic behavior of the field from drilled wells indicated the possibility of presence of natural fractures in reservoirs. Fracture characterization study was embarked upon to build a reliable fracture model with the ultimate aim to improve on understanding of permeability distribution in reservoirs and to assist future dynamic flow simulation studies in the field. The structural history of the field was analyzed to tie fracture related observations to the known tectonic events affecting reservoirs. Data analysis was done by attempting a simple structural restoration with available data. This analysis indicated two main tectonic events responsible for evolution of the present structure and the likely stress direction. Based on the tectonic history analysis, assumptions of a simplified plate-bending model and the Stearns model of fracturing related to folding have been applied during modeling. Available micro resistivity formation images from wells were interpreted for fracture type, fracture orientation & computation of fracture attribute. 3D seismic data was used to pick mappable faults & to generate the geometric seismic attributes. Variance attribute was selected for edge detection and was used to extract the Seismic Discontinuity Planes (SDPs). Faults in the reservoir & SDPs were further used as inputs to develop DFN. Fault related fractures were modeled using boundary element method based geo-mechanical approach which aims at computing maps of both natural fracture orientation and density trends, from observed major faults and observed fracture data along wells. Predicted tectonic models with stress directions show good match with the carried out structural geological analysis. Geo-mechanical model was used to estimates the breakability of rocks in reservoirs by computing Poisson's Ratio and Young's Modulus logs in several wells. 3D volumes of Young's Modulus and Poisson's Ratio were generated using Pre-Stack inversion results. Fold related fractures parallel to fold axes were modeled using the rock breakability predicted from Young's Modulus and Poisson's Ratio models calibrated with fractures interpreted from the wells. These fractures were constrained to the crestal part of the structure. The 3D DFN based fracture model was up scaled into the 3D matrix model to generate the fracture porosity, fracture permeability tensors and the matrix to fracture communication factor. Multiple realizations of the fracture model were generated to capture the uncertainty associated with various aspects of this model. Fracture pore volume maps were generated for each reservoir and the specific recommendations were made about their use in dynamic history matching process.