In this study, microseismic data acquired during a hydraulic fracturing operation in an Eagle Ford shalefield was reprocessed using a relative microseismic multiplet hypocenter determination technique.
The term microseismic multiplet means a group of microseismic events which have similar waveforms. Cross correlation processing of similar waveforms provides precise measurements of the relative arrival time and enables inverse analysis of precise relative hypocenter locations of multiplet events. In order to improve the applicability of the multiplet analysis to the signals with low S/N, the authors introduce an automatic migration stacking algorithm to the relative hypocenter determination. In this method, cross-correlation functions between a pair of microseismic multiplet events are migrated in the relative location space which has the origin at the hypocenter location of one of the multiplet events (master/reference event). The migrated cross-correlation functions are focused in theory on the relative hypocenter location of the other slave event.
The hydraulic fracturing jobs in the Eagle Ford shale field were monitored by a single downhole receiver array. In order to reduce the azimuthal uncertainty of the hypocenter location determination due to the spatially biased distribution of the receivers, both the P-and S-wave polarity angles were also utilized. Evaluation of cross-correlation coefficient extracted around 80% of all the 6,837 events as multiplet events. The multiplet events were classified into over 1,000 multiplet groups, suggesting complex fracture networks were created by the hydraulic fracturing jobs. The improved precision of hypocenter determination shrank the hypocenter distribution to two thirds of the original hypocenter distribution, suggesting that the stimulated reservoir volume (SRV)had been overestimated in the original hypocenter distribution evaluation. The hypocenters of the individual multiplet clusters, especially in the nearest 2~3 fracturing stages within around 1,500 feet from the monitoring well, tend to be concentrated in a small area, suggesting the events occurred at a single fracture or neighboring parallel fractures.
