A Wastewater Disposal Reservoir Sensitive to Teleseismic Waves

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

Faults in areas of injection-induced seismicity, like central Oklahoma, may have an enhanced sensitivity to dynamic triggering because only relatively small changes in stress or pore pressure are needed (van der Elst, et al., 2013). An understanding of how fluid-pressures in the disposal reservoir and the underlying basement rock respond to dynamic strains from seismic waves would help confirm or disprove this idea. While it is now common to find observations of seismically induced water-level oscillations in aquifers or reservoirs across the globe, in Oklahoma we have a limited understanding of the response of the Arbuckle Group – the principal wastewater disposal reservoir. Since April 2017 the USGS has been monitoring fluid-levels in a deep well with high frequency sampling (1/4 Hz), giving a continuous record of fluid-pressure changes in the Arbuckle. We observe a clear response to dynamic strains from the passage of long period surface waves, with peak dynamic fluid-levels appearing to be proportional to the magnitude and distance from the source; there is always a response to Rayleigh waves, as we expect. Radial flow is the expected flow regime because the Arbuckle is thought to be a confined, laterally extensive reservoir, but the seismic response shows evidence of a mixed flow regime modified by hydraulically conductive features. In particular, the transfer function between Arbuckle fluid pressures and broadband velocity seismograms at the wellhead can be modeled at short periods with radial flow models (e.g., Cooper, et al., 1965), but diverges from this at much longer periods, where poroelastic coupling between deformation and fluid flow is strongest. There is also a response to Love waves in some cases, which is unexpected because isotropic, poroelastic media are insensitive to shear stresses. These data suggest an influential role for hydraulically conductive fractures in controlling the hydraulic diffusivity of the reservoir, though it is presently unclear how reservoir permeability responds to strong shaking from local induced earthquakes.

Presenting Author: Andrew J. Barbour

Authors