Multiscale Seismic Models of Complex Fracture Networks
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2017, Unconventional Resources Technology Conference (URTeC). Seismic modeling of fractures can aid in the characterization of fractured reservoirs by providing insights into the effects of fracture properties such as length, spacing, orientation and compliance. We apply new seismic models that explicitly represent fractures, unlike more common effective medium methods, and we can thus incorporate the complex geometry of realistic fracture systems. This also allows the prediction of the influence of a small number of major fractures that may control the reservoir flow behavior or that are generated by hydraulic fracturing. Specifically, we apply new, innovative generalized multiscale finite element methods (GMsFEM) to predict the effect of fracture compliances on scattered seismic waves from natural or hydraulic fractures. This numerical approach represents fractures on a finely discretized mesh; this fine mesh is used to capture fracture properties by generating quantities (basis functions) that are used for modeling wave propagation on a much coarser grid. This methodology reduces the size of the computational problem, allowing faster results while retaining the influence of the original fracture distribution on the fine grid. Another important feature is that the fractures are parameterized by their compliance, the variations of which are correlated with changes in fracture conductivity. Both compliance and conductivity will increase in a propped fracture, but will decrease when in situ stress increases as it closes the fracture. Thus, the inference of changes in compliance using seismic data will also help to identify changes in flow properties, guiding development of predictive models for reservoir management. We have applied the method to model seismic data from both natural fracture systems and hydraulic fracture networks. Simulation of a 2-D seismic survey in a model with multiple swarms of natural fractures illustrates the complex scattering of waves reflected between the fracture sets. Initial application of the method to 2-D models containing multiple hydraulic fracture stages shows how data acquired in microseismic monitoring can be affected by newly formed fracture networks. In particular, by varying fracture compliance, model results show the difference in seismic reflections from propped and unpropped portions of fractures. These models can allow the development of algorithms for using field data to provide new measures to verify hydraulic fracture location and flow properties.
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Proceedings of the 5th Unconventional Resources Technology Conference
