Methods allowing to estimate the size and aperture of induced hydraulic fracture are essential for a proper design of unconventional reservoir well stimulation. In this paper, we focus on ultrasonic velocities monitoring during hydraulic fracturing of a tight shale. We report experimental results, where a sample was first loaded in a polyaxial loading frame and then fractured by injection of a viscous fluid at a constant flow. During the experiment, P-wave velocities were periodically measured in different directions. Our results show that ultrasonic measurements can be useful for understanding the mechanics of the hydraulic fracture growth. More precisely, from the evolution of the P-velocities and their amplitudes during the loading, we are able: (i) to estimate the velocity of the hydraulic fracture growth; (ii) to show that propagation of a liquid-free crack always precedes the liquid front; (iii) to estimate the aperture of the hydraulic fracture.
These results show that ultrasonic velocities monitoring can yield direct measurements of fracture width, length and dynamics of propagation. These inferred properties of the hydraulic fracture can also provide verification of the results of various theoretical models describing fracture propagation.
Detailed characterization of rock fracturing induced by fluid injection is very important task for oil and gas industry, where hydraulic fracturing is crucial to enhance hydrocarbon production, as well for microseismic monitoring of intrusive dykes in the earth crust. The conditions under which fluid-driven fracture propagates within rocks are usually not well defined. Several fracture-propagation theories have been developed. Pioneer contributions have been made by Barenblatt , and more recently by Detournay et al. [2-4]. However, there is a certain lack of direct field measurements of the hydraulic fracture dimensions, verifying results of these models.