Estimation of The Source Mechanisms of Microseismic Events from a Barnett Shale play
Osakwe, Emmanuel Olisejindu
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The waveforms generated during hydraulic fracturing in unconventional oil and gas reservoirs contain information about the nature of the failures that gave rise to the observed wave fields. Inverting for the moment tensor has become an important way of gaining understanding of the fracturing process. Microseismic events recorded off of a Barnett shale play were first subjected to power spectra density analysis in order to determine the frequency content of the events. The analysis revealed that the peak energy of the first arrivals was in range 100 – 800 Hz, and no signal of significant energy was found beyond 900 Hz. The inversion for the moment tensor of about 100 of these events, which were of good enough quality, was then carried out. The moment tensor revealed that the source mechanisms of most of the events were predominantly shear failures, with a few of the events being of non-shear-type failure like compensated linear vector dipoles (CLVD). The condition number of the sensitivity matrix used for the inversion was somewhat high, even for a constrained inversion. The fact that the data were recorded from an array of geophones in a single borehole might be the reason for the unstable inversion, since data recorded from a single borehole cannot resolve all the six independent elements of the moment tensor.