Thousands of hydraulic fracture treatments have been monitored in the past ten years using microseismic mapping, providing a wealth of measurements that show a surprising degree of diversity in event patterns. Interpreting the microseismic data to determine the geometry and complexity of hydraulic fractures continues to be focused on visualization of the event patterns and qualitative estimates of the "stimulated volume", which has led to wide variations and inconsistencies in interpretations. Comparing the energy input during a hydraulic fracture treatment and resultant energy released by microseismic events demonstrates that the seismic deformation is a very small portion of the total deformation. The vast majority of the energy is consumed in aseismic deformation (tensile opening) and fluid friction (Maxwell et al. 2008). Proper interpretation of microseismic measurements should account for both seismic and aseismic deformation, coupling the geomechanics of fracture opening and propagation with the shear failures that generate microseisms.
Interpretation of microseismic measurements begins with an evaluation of location uncertainty, using signal-to-noise ratios and error ellipsoids, along with event moment magnitude. In some cases, microseismic event location uncertainty is erroneously interpreted as fracture complexity. The next step is to normalize the data and correct for observation well bias, both distance and azimuth, including use of seismic radiation patterns. Without these corrections fracture behavior from well to well or stage to stage (especially in horizontal wells) can easily be misinterpreted. Advanced geophysical processing that describes the failure mechanisms in more detail may also aid in the interpretation. The final step in the interpretation is to include the geomechanics of the overall process, accounting for the fracture treatment volumes injected, the net pressure in the hydraulic fracture(s) and the shear failures that generated the microseisms. This final, critical step is often overlooked when interpreting microseismic measurements. The paper provides a comprehensive, yet practical guide to the interpretation of microseismic measurements.