Sensitivity Analysis of Seismic Velocity and Attenuation Variations for Longmaxi Shale during Hydraulic Fracturing Testing in Laboratory

During the hydraulic fracturing procedure in shale-gas exploitation, the poroelastic properties of shale formation can be altered significantly. However, it is difficult to evaluate these variations using microseismic field data. In this study, we conduct a hydro-fracturing experiment using Longmaxi shale, which is a major formation for shale-gas production in China, to simulate the water injection and rock fracturing procedure. The variation of the velocity and attenuation for primary/secondary (P/S) ultrasonic waves was investigated throughout the entire experimental procedure. The results show that the attenuation is more sensitive to sample rupture than the velocity. However, P-wave attenuation loses sensitivity to the water injection after the fractures are saturated with water. In that case, it is preferable to use S-wave attenuation to identify the opening/closing of the fractures. Based on the experimental results, we can conclude that the variation of the attenuation must be considered during microseismic data processing and interpretation.