Understanding the role of fractures in hydraulic stimulation from anisotropic attenuation measurements of microseismic waveforms.

Posted by pkm at Feb 05, 2015 06:10 PM |
Philip Usher & Mike Kendall - School of Earth Sciences, University of Bristol

Microseismic monitoring is increasingly used in the monitoring of: hydraulic fracture stimulations, reservoir depletions, enhanced geothermal systems, mining operations, and carbon capture and storage projects. These industries require monitoring to understand very complex processes that happen in the sub surface. We observe microseismicity caused by the interaction of rocks, pore fluids, injected fluids and fractures, both natural and anthropogenic. If we can understand these complex systems better we could make these energy industries more efficient and reduce there environmental impact.

Standard microseismic monitoring techniques focus on locating microseismic events or investigating the focal mechanisms of the micro earthquakes (Rutledge et al., 2004). Shear wave splitting analysis investigates the anisotropic velocity of the medium in which the seismic waves propogate (Wuestefeld et al., 2011). Attenuation can also be used to measure the seismic medium. Kelly et al 2013, used clusters of earthquakes to measure changes in attenuation with time. They showed how a large earthquake affected fluid within the natural fractures around the fault system. Modeling of squirt flow within partially filled fractures by Chapman, 2002 could explain this. Attenuation (and velocity anisotropy) can be explained by the interaction of the fluids and fractures. His model also predicts that attenuation would be anisotropic. The model predicts attenuation will be greater on the slow shear wave rather than the fast shear wave.

Our experiment looked at the Cotton Valley dataset from East Texas. We used the log spectral ratio method to look at temporal changes in attenuation in both the fast and slow S-wave. The log spectral ratio method measures Δt*: a relative measurement of attenuation with respect to the path length between two waveforms.

Our results show Δt* increases for the slow S-wave (Fig. 1) for a series of events that show an increase in shear wave splitting delay time. Whereas the fast shear wave for the same events shows no increase. This measurement is consistent for different reference events, and the increase is much larger than that caused by differences in path length.

We concluded that attenuation measurements show increases in fracture density for the Cotton Valley hydraulic fracture experiment. The results also show that attenuation is anisotropic because it is different for different S wave phases. Attenuation measurements could be used to understand the roles of fluids and fractures within a reservoir.

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Chapman, M., 2003. Frequency-dependent anisotropy due to meso-scale fractures in the presence of equant porosity, Geophysical Prospecting, 51(5), 369–379.

Kelly, C. M., Rietbrock, A., Faulkner, D. R., & Nadeau, R. M., 2012. Temporal Changes in Attenuation associated with the 2004 M6. 0 Parkfield Earthquake, Journal of Geophysical Research-Solid Earth, pp. 1–48.

Rutledge, J. T., Phillips, W. S., & Mayerhofer, M. J., 2004. Faulting induced by forced fluid injection and fluid flow forced by faulting: An interpretation of hydraulic-fracture microseismicity, Carthage Cotton Valley Gas Field, Texas, Bulletin of the Seismological Society of America, 94(5), 1817–1830.

Wuestefeld, A., Verdon, J. P., Kendall, J. M., Rutledge, J., Clarke, H., & Wookey, J., 2011. Inferring rock fracture evolution during reservoir stimulation from seismic anisotropy, Geophysics, 76(6), WC157.

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