Introduction
Microseismic mapping of hydraulic fractures has gained importance recently. The success of a fracturing job depends on the fracture dimensions. Microseismicity monitoring offers an efficient means of real time analysis of hydraulic fracture propagation. The recorded seismic waveforms are processed and analyzed to locate the source of an event, to extract information about the fracture dimensions and fracture mechanisms. Fracturing shale introduces a complication because of the elastic anisotropy inherent in shale. This complicates field analysis of closure stress calculations and the velocity model required for accurate hypocenter locations. The purpose of our study is to understand the processes governing fracture propagation through acoustic emission studies. Focal mechanism analysis indicates that hydraulic fracturing is dominated by shear failure mechanism. Spatial distributions of hypocenters vary from the expected planar features to diffuse clouds. Spectral analysis shows considerable variation from event to event but a general consistency among similar lithologies.
Fracture dimensions provide a measure of the effectiveness of a stimulation treatment. Hypocenter location accuracy affects the resolution of these dimensions. Field implementation of monitor programs involve surface or downhole arrays of sensor and use perforation shots for calibration. Our laboratory experiments allow us to surround the sample with an array of sensors. Orientation of the hydraulic fracture is controlled by in-situ principal stresses. We duplicate this by applying a horizontal stress. Height containment is controlled by treatment parameter, rock moduli and stresses. The injection of fluid to create a hydraulic fracture in the subsurface induces changes in effective stress which creates new fractures or re-opens existing ones. A portion of the energy released during this process is radiated as seismic waves (P-waves and Swaves) (Reyes et al., 2009), and these are detected by an array of sensors located at surface or in an observation well. Analysis of field events indicates that shear failures dominate the fracturing (Ishida et al., 1997; Baria and Green, 1986; Talebi and Cornet, 1987; Talebi and Boone, 1998; Urbancic et al., 1999). Sample preparation The cylindrical samples used in this study are Indiana limestone and pyrophyllite samples which are of 5 inches high and 4 inches in diameter. Circumferential Velocity Analysis (CVA) was used to determine azimuthal velocity variation in each sample. CVA is a pulse transmission technique where the velocity is measured as a function of azimuth along the circumference of the sample. The velocity deviation across the limestone samples was less than 4%; we treated these samples as homogeneous and isotropic whereas the pyrophyllite had a measured P-wave anisotropy of 25%. Previous measurements on pyrophyllite (Sachse and Ruoff, 1981) suggest it is transversely isotropic. The measured vertical and horizontal p-wave velocities are 4187 m/s and 5137 m/s, respectively at 4000 psi, and the calculated anisotropic parameters e, ? and d are 0.25, 0.75 and 0.40, respectively. The sensor arrangements for the two sets of samples are presented in the figures below. Samples were uniaxially stressed at 4000 psi in a radial direction (see Fig. 1).
