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Hydraulically Induced Fracturing Response of the Vaca Muerta Formation From Borehole-Based Microseismic Monitoring and Subsequent Single-Well Moment Tensor Analysis

Abstract

We present the results of a borehole-based hydraulic fracture monitoring campaign and subsequent single-well-based moment tensor analysis of the mapped microseismic events of the completion of a deviated well within the Vaca Muerta formation, Neuquen basin, Argentina. The Vaca Muerta formation, a Jurassic-Cretaceous lithostratigraphic unit, is an organic-rich source rock. Evaluating the geologic and geomechanical properties of the formation and its microseismic response to stimulation is key in validating and planning the exploration and production effort of this unconventional play. Microseismic event locations and derived attributes allow for the determination of fracture azimuth, height and length, location and complexity, and estimated stimulated volume. Moment tensor solutions are used to characterize the source and failure mechanism (fracture plane orientation and slip) and provide information to evaluate the geomechanical response of a reservoir to stimulation. Eight completion stages were monitored from a single vertical borehole array. Microseismic events were first mapped, and then focal mechanism solutions were obtained using the multi-event moment tensor inversion technique, which allows one to determine moment tensor solutions for ill-conditioned geometries. The results show a fracture geometry that varies with depth. Early stages, located deeper, show less vertical growth than later stages at shallower depths. Later stages also show a bimodal distribution with significant vertical growth and groups of microseismic events occurring shallowly along the top of the middle Vaca Muerta, an interval with higher carbonate content. This bimodal distribution of microseismic events is consistent with previous work showing the preferential distribution of events within low-impedance shales and higher-impedance carbonates. Moment tensor solutions for the deeper wellbore-centric events predominately show strike-slip motion oriented orthogonally to the treatment well. The solutions for the shallower events at the top of the middle Vaca Muerta formation show two groups of conjugate, strike-slip displacement, with the majority being left-lateral. The widespread distribution of uniform source mechanisms at or near the top middle Vaca Muerta formation suggests activation of a well-developed natural fracture population closely aligned with the maximum stress direction.