Interpretation of Resonance Frequencies Recorded during Hydraulic Fracturing

The frequency content of continuous passive seismic recordings of hydraulic fracturing are sometimes used to gain information on the numerous microseismic events occurring during the fluid injection. In addition, resonance frequencies are frequently recorded as well. Resonances can be used in many ways due to their multiple origins possible. In every circumstance, all possible sources have to be reviewed to define their respective influence. We present two different experiments showing resonances with presumably different origins. For the first experiment, the low-frequency resonances (5-50 Hz) are only recorded by downhole geophones and broadband stations on the surface that are close to the injection well. For the second experiment, four resonances at 17, 35, 51 and 60 Hz are detected. The fluid injection being at approximately the same depth as the receivers, the path effect influence will be limited and the resonances coming from receiver effects are anticipated to be outside the frequency range of the observed resonances. A possible source would be the resonance of fluid-filled cracks. The size of a crack corresponding to a resonance of 17 Hz is calculated to be 17 m. These resonances would then correspond to mesoscale deformation of the reservoir.