Summary
Geomechanical modelling of hydraulic stimulations requires data to build and calibrate models to predict decline curves and other aspects of long-term reservoir performance. The initialization of these models requires knowledge of the pre-existing fractures and geological properties of the media. Orientations of different fracture sets, their intensities and spacing together with a characterization of their size scales (power law) critically impacts geomechanical predictions of both the stimulations in terms of proppant and fluid placement, but also the decline of reservoir productivity. Insight can be gained on the properties of the fracture network through analysis of the microseismic data generated during the stimulation. The frequency response of the waveforms depends on the size of the rupture and necessitates wide band recording in order to provide constraints at a variety of scales. Supplementing low frequency sensors with traditional monitoring high frequency sensors extends the resolvable range to larger scale lengths of fractures. For all these scales, with a sufficient sampling of azimuths around the stimulation, the mechanisms and associated fracture planes and stress/strain conditions can be reconstructed through seismic moment tensor inversion (SMTI) of the recorded waveforms. At smaller scale lengths, signal to noise ratios can be low, and events may not be observed at high enough quality to permit SMTI. To extend the characterization of these fractures to smaller scales, we use a stochastic optimization algorithm designed to search for optimally placed fractures in the reservoir that intersect with the event locations while constraining their orientations to be from the same distribution observed at the larger (SMTI resolvable) length scales. Effectively, this technique allows for the extension of the power law governing the fracture distribution to smaller scales by invoking observed trends in self-similar behavior. In turn, characterization of the wider band of fractures in the reservoir provides necessary inputs into geomechanical models to predict the fluid and proppant distributions from the full band of generated microseismicity as well as long-term behaviour of the reservoir though decline curve estimation.
