Measuring changes in fracture properties from temporal variations in anisotropic attenuation of microseismic waveforms

We investigate fracture-induced attenuation anisotropy in a cluster of events from a microseismic dataset acquired during hydraulic fracture stimulation. The dataset contains 888 events of magnitude −3.0 to 0.0. We use a log-spectral-amplitude-ratio method to estimate change in t* over a half-hour time period where fluid is being injected and an increase in fracturing from S-wave splitting analysis has been previously inferred. A Pearson's correlation analysis is used to assess whether or not changes in attenuation with time are statistically significant. P-waves show no systematic change in t* during this time. In contrast, S-waves polarised perpendicular to the fractures show a clear and statistically significant increase with time, whereas S-waves polarised parallel to the fractures show a weak negative trend. We also compare t* between the two S-waves, finding an increase in t* with time. A poroelastic rock physics model of fracture-induced attenuation anisotropy is used to interpret the results. This model suggests that the observed changes in t* are related to an increase in fracture density of up to 0.04. This is much higher than previous estimates of 0.025 ± 0.002 based on S-wave velocity anisotropy, but there is considerably more scatter in the attenuation measurements. This could be due to the added sensitivity of attenuation measurement to non-aligned fractures, fracture shape, and fluid properties. Nevertheless, this pilot study shows that attenuation measurements are sensitive to fracture properties such as fracture density and aspect ratio