--> Discrete Fracture Network Construction Using Microseismic Observations

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Discrete Fracture Network Construction Using Microseismic Observations

Abstract

Over the past five years, geophysicists have made a collective push towards extracting more value out of microseismic data. This includes stimulated reservoir volume calculations, dominant fracture orientation and mechanism using amplitude ratio analysis and moment tensor inversion. Microseismicity is generated by sudden displacement on pre-existing fractures and therefore, it represents only one component of the geomechanical response to hydraulic fracturing. To understand the entire picture, a microseismic geomechanics framework has been developed (Chorney et al. (2015)). One critical input for a distinct element geomechanical model of hydraulic fracturing is the DFN. To generate a DFN, we need to extract statistics on the orientation, size, spacing, and density of fractures from microseismic data. Ideally, a moment tensor analysis would be used to generate a DFN from microseismic observations but in practice, it is only possible in a limited number of cases. Moment tensor inversion requires a multi-well configuration, which allows for more complete coverage of the focal sphere to constrain the inversion. Most downhole array configurations consist of a single borehole and therefore, moment tensor inversion is not possible. In this abstract, we propose a novel DFN construction method that does not use moment tensor inversion. Fracture size statistics are extracted from event source parameters. The corner frequencies of microseismic spectra are used to calculate source radii using the Brune source model (Brune 1970, 1971). Fracture orientation statistics are extracted from the analysis of amplitude ratios (P/Sh, P/Sv, Sh/Sv). This analysis involves a brute force optimization routine that scans through all possible slip orientations (strike, dip, and rake) and looks for global and local minima fracture orientations. Microseismic volume density and a clustering analysis are then used to extract fracture density and spacing statistics.