Estimating Stimulation-Zone Anisotropy Effects During Microseismic Monitoring
Hydraulic fracture operations can be optimized using knowledge about the stress regime, flow permeability, and fracture networks in the subsurface. Surface seismic data and nearby well data give a first assessment of the properties present in the hydraulic fracture treatment zone, though on vastly different scales and at different resolutions. When hydraulic stimulation operations are monitored using downhole seismic sensors, microseismic events can be used to derive valuable velocity anisotropy estimates in-situ throughout the entire fracturing operation. These estimates can be both time and space variant. We show a real data example where microseismic events were recorded using a 40 level array of 3-component downhole seismic sensors in an observation well straddling the treatment zone in depth. The observed microseismic event gathers exhibit shear wave splitting even at the onset of treatment operations due to anisotropy which was not readily predicted by either surface seismic or sonic scanner logs. We demonstrate that this apparent contradiction can be explained by proper upscaling of sonic scanner data using equivalent medium theory. Using a group-theoretical approach, we calculate an anisotropic replacement medium that explains the measurements from surface seismic, VSP and microseismic data while also preserving the well-log defined properties. Although the fine-scale log velocities are isotropic, this averaging shows that some layers exhibit anisotropic behavior at larger seismic scales. We extract anisotropy parameters epsilon, delta, and gamma (Thomsen parameters) from the upscaled layers. These anisotropy estimates explain the shear-wave splitting observed in the data. Estimating anisotropy parameters in the fracture zone both before and during the treatment opens up the possibility of time-lapse characterization of the fracture zone anisotropy on a stage-by-stage basis. Reconciliation of microseismic observations, surface and sonic scanner data through upscaling using an equivalent medium approach bridges the resolution gap and allows further detailed frac zone analysis.
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