In this study we present a geomechanical analysis workflow using microseismic focal mechanisms to investigate the dynamic response of the reservoir during and after stimulation. Focal mechanisms are derived using full waveform fitting techniques, and the ambiguity in identifying the true fracture plane is resolved by simply choosing the nodal plane that aligns with the developing hydraulic fractures. A global stress inversion of the fracture plane solutions is done to estimate the orientations and relative magnitudes of the principle stresses. Friction laws are then used to constrain for each event a suite of geomechanical parameters (failure potential, dilation tendency, and excess pore pressure) in order to identify fracture populations likely to control fluid flow, those that required different stimulation pressures in order to contribute to flow, and the mechanical conditions that favored out-of-zone growth and reactivation of geohazards. Additional observations, such as net wellbore pressure measurements and geophysical logs, are used to calibrate the model as well as to further understand the geological, geomechanical and treatment-related variables affecting the overall stimulated rock volume. The method is applied and discussed in the case of a microseismic event catalogue obtained during the stimulation of two horizontal wells landed in the Eagle Ford, where large variations in fracture patterns as well as the reactivation of a large macroscopic fault zone was observed.
The state of stress of the reservoir is one of the dominant factors controlling the reservoirs response to stimulation as well as the effectiveness of the treatment design. For instance, the orientation and magnitude of the maximum horizontal stress (SHmax) strongly affects the stimulated range of fracture orientations and in turn the geometry of the stimulated zone (i.e. localized versus distributed fracturing). The hydraulic horsepower, which takes into account the reservoir stress states and pressures, may be sufficient to stimulate parts of the reservoir with a specific state of stress, but any variations in the stress state can result in adverse effects such as damaging nearby wells ("frac hits"), out-of-zone growth, and large-magnitude earthquakes. Furthermore, the reservoir stress state can also impact the hydraulic conductivity of stimulated fractures (Barton et al., 1995).
