Fracture Characterization from Attenuation and Generation of Tube Waves

The Stoneley waves observed in Full Waveform Acoustic Logs (FWAL) is about two orders of magnitude higher in frequency than the BSP tube waves. Compared to other phases present in FWAL seismograms, relatively more of the strain energy of this phase is trapped in the borehole fluid, and its behavior is useful in examining fluid-fracture interaction. The Stoneley wave phase is typically attenuated when a significant fracture intervenes between the source and the receiver of the FWAL tool. If an attenuating fracture is considered an open, parallel-plate reservoir saturated with compressible fluid, the attenuation process may be modeled (Mathieu, 1984). Fractures may be discriminated on the basis of parallel-plate aperture, which is directly related to hydraulic transmissivity.

Field testing has been conducted in cooperation with the U.S. Geological Survey and Weston Geophysical to evaluate these models in northeastern New England. Conventional temperature, caliper, resistivity and televiewer logs show the presence of fractures and their orientation, and provide indirect evidence of associated flow. Tube waves are generated in hydrophone VSP surveys, and substantial attenuation of the FWAL Stoneley wave is observed in these wells. Transmissivity values predicted from VSP and FWAL analysis compare favorably with flow tests and direct observation of flow effects in the borehole. Orientation information from VSP analysis is in agreement with televiewer logs.

Transmissivity estimates from VSP interpretation using the open, parallel-plate model are much smaller than the FWAL attenuation or pump tests. The discrepancy is important even for such a widely ranging parameter as transmissivity. Hydraulic significant fractures must in reality be propped open by asperity contact with fracture walls, the fracture closure is partially resisted by asperity deformation. This resistance is modeled as a proportionality between incident stress and closure. A new model for predicting transmissivity from observed tube wave amplitude is formulated using the stiffness concept. Intrinsic stiffness factor can be calculated given independent determination of transmissivity. Stiffness magnitudes are obtained which are comparable to the stiffness of an undrained water layer with the thickness equivalent to the flow aperture. Transmissivity predictions using the water-layer stiffness approximation are in agreement with FWAL interpretation and pump test results.