We carry out small-scale hydraulic fracture experiments to investigate the physics of hydraulic fracturing. The laboratory experiments are combined with time-lapse ultrasonic measurements with active sources using both compressional and shear-wave transducers. For the time-lapse measurements we focus on ultrasonic measurement changes during fracture growth. As a consequence we can detect the hydraulic fracture and characterize its shape and geometry during growth. Hence, this paper deals with fracture characterization using time-lapse acoustic data.
During fracture growth the acoustic waves generate diffractions at the tip of the fracture. The direct compressional and shear diffractions are used to locate the position of the tip of the fracture. More detailed analysis of these diffractions can be used to obtain information on the geometry and configuration of the fracture tip, including the creation of a zone that is not penetrated by fluid. Furthermore, it appears that the acoustic diffraction is generated mainly at the fluid front and only weakly at the dry tip.
In addition, the wavefield that has been transmitted through the hydraulic fracture is measured. Shear-wave transmissions are shadowed because the shear modulus vanishes inside the fluid-filled fracture. From this observation we conclude that the fracture is mechanically open. In other words, no friction occurs related to the movement of fracture faces that are in mechanical contact. Compressional transmissions show a distinctive dispersion relative to the measurement in the unfractured medium. This dispersion can be used to determine the width (or aperture) of the fracture by fitting the measured dispersion with the theoretical prediction as a function of the unknown fracture width. We show that the width profile of the fracture can be reconstructed by using a set of transmission records with different source and receiver locations. By performing a validation experiment, we show that the width determination method is reliable, although the estimated fracture width is only a few percent of the incident wavelength. The strength of the method relies on time-lapse measurements combined with fitting the changes in the measured waveforms during the experiment.
The combination of diffractions and transmissions helps us visualize the dynamic process of hydraulic fracture growth. Hence, acoustic measurements with active sources prove their usefulness for fracture characterization.