Using seismic crosswell surveys to determine the aperture of partially water-saturated fractures

An air injection experiment in a shallow fractured limestone at Conoco's borehole test facility near Newkirk, Oklahoma, has shown large effects on the amplitude but small effects on the traveltime of the transmitted seismic waves. We have analyzed data from a seismic monitor survey in the kilohertz range performed during the experiment and have modeled the fracture zone as a single fracture. The large amplitude decrease during the experiment is mainly attributable to the impedance contrast between the small velocities of gas-water mixtures inside the fracture and the formation. The intrinsic attenuation of the fracture fluid seems to be a second-order effect for small apertures. During the experiment the seismic wavelengths inside the fracture become comparable to the aperture dimension, which allows an estimation of fracture apertures. We also have analyzed a crosswell survey acquired shortly after the experiment and computed aperture and gas concentration profiles. Our aperture estimates range from <1 mm to a few millimeters, which is comparable to previous tracer tests. The results of this study are generally consistent with prior hydrologic work and single-well surveys. This study demonstrates that crosswell surveys could be an effective tool for obtaining in situ estimates of fracture aperture and gas concentration.