Inferring rock fracture evolution during reservoir stimulation from seismic anisotropy

Abstract

We have analyzed seismic anisotropy using shear-wavesplitting measurements made on microseismic events recorded during a hydraulic fracture experiment in a tight gas reservoir in Carthage, east Texas. Microseismic events were recorded on two downhole arrays of three-component sensors, the geometry of which provided good ray coverage for anisotropy analysis. A total of 16,633 seismograms from 888 located events yielded 1545 well-constrained shear-wave-splitting measurements. Manual analysis of splitting from a subset of this data set reveals temporal changes in splitting during fracturing. Inversion using the full data set allows the identification of fracture strike and density, which is observed to vary during fracturing. The recovered fracture strike in the rock mass is parallel to directions of regional borehole breakout, but oblique to the hydraulic fracture corridor as mapped by the microseismic event. We relate this to en-echelon fracturing of preexisting cracks. The magnitude of shear-wave splitting shows a clear temporal increase during each pumping stage, indicating the generation of cracks and fissures in a halo around the fracture corridor, which thus increase the overall permeability of the rock mass. Our results show that shear-wave-splitting analysis can provide a useful tool for monitoring spatial and temporal variations in fracture networks generated by hydraulic stimulation.

Microseismic Monitoring of Fracture Networks During Hydraulic Stimulation: Beyond Event Locations

“…The inversion gives a dominant crack strike of N68E, an average crack density of 0.024, and a " of 0.12. For further detail, see Wuestefeld et al (2012).…”

“…Temporal variations in shear-wave splitting have been observed in a number of datasets, including those acquired in volcanic settings (Gerst and Savage, 2004), mining settings (Wuestefeld et al, 2011), producing oil reservoirs (Teanby et al, 2004b), and during hydraulic stimulation of tight-gas reservoirs (Wuestefeld et al, 2012). The magnitude of the splitting generally rises as the fracture density increases.…”

“…These include: North Sea chalk reservoirs (Valhall (Teanby et al, 2004b) and Ekofisk (Jones et al, in prep, 2012)); carbonate reservoirs in west-central Oman (Al-Anboori and Al-Harrasi et al, 2010) and southern Saskatchewan (Weyburn) (Verdon et al, 2010b;2011a); and siliciclastic reservoir of Cotton Valley in Carthage, Texas (Wuestefeld et al, 2012).…”

Crack Features and Shear-Wave Splitting Associated with Fracture Extension during Hydraulic Stimulation of the Geothermal Reservoir in Soultz-sous-Forêts

“…10) is associated with a stronger splitting, hence to the extension of preferential cracks. Wuestefeld et al (2011) also noted a temporal increase in splitting delays for steep arrivals during the stimulation of a gas reservoir. They explain this result as a "halo" of increased fracture density around the stimulated zone.…”

“…Shear wave splitting is typically used as a tool for detecting anisotropy, in particular at reservoir scale (Verdon et al, 2009;Wuestefeld et al, 2011). When a polarized shear wave enters an anisotropic homogeneous medium, it splits into two orthogonally polarized shear waves with distinct velocities.…”