Time-lapse ultrasonic measurements of laboratory hydraulic-fracture growth: Tip behavior and width profile

We performed acoustic measurements in a time-lapse sequence in scaled labor atory tests. The advantage of time-lapse measurements is that the fracture response can be separated from the background signal. As a consequence, not only can the hydraulic fracture be detected, but its shape and geometry ca n be measured during its growth. This application requires the combined inf ormation of both compressional- and shear-wave measurements. We apply this technique to propagation, flowback tests, and reopening of hydraulic fractu res. Acoustic waves excite diffractions at the fracture tip. These diffractions are used to locate and to characterize the fracture tip. The acoustic measu rements indicate that we can distinguish between a dry tip and the fluid fr ont of the fracture. Shadowing of shear-wave transmissions allows estimation of the moment of fr acture initiation. The width profile of the fracture is determined with com pressional-transmission measurements. This application is based on the fact that the attenuation and time delay of compressional transmissions are pro portional to the fracture width. Analysis of a flowback test shows that the fracture closed at the wellbore but remained open farther away from the we llbore.