The dual issues of band-limited vertical resolution and nonuniqueness of deterministic inversion results has led to the development of methodologies known as geostatistical, or stochastic, inversion. In these approaches, seismic data are typically inverted directly into a high-resolution geological model. Compared to deterministic inversion, stochastic methods deliver multiple realizations that are consistent with the available well and seismic data. The seismic inversion process is inherently nonunique, meaning that there is an unbounded number of elastic property models that fit the seismic data equally well above some threshold misfit. We explore the notion of the equally large number of possible stress states that could be interpreted from same seismic observations. We make use of stochastic inversion results to incorporate the impact of subseismic uncertainty in seismic-driven geomechanical models. By taking multiple realizations from a prestack stochastic inversion-acoustic impedance, V p /V s , and density-we generate and feed a series of distributions of elastic constants into a finite element stress simulator. The multiple stress solutions allow us to account for uncertainties in the inversion results that can be ultimately captured in a suite of numerical models to predict a set of possible geomechanical states of a field. Therefore, beyond a unique geomechanical forecast for a field, we can now solve for the range of variability in geomechanically safe operational parameters within the field's development plan.
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.
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