Anisotropy Measurements in a Core Sample by Using Pulse Transmission Method

Recent interest in unconventional reservoirs motivates our work in laboratory measurements of seismic anisotropy. Seismic anisotropy is the variation in speed of a wave as a function of its direction of propagation. Analyzing anisotropy in unconventional reservoirs is important since anisotropy leads, for example, to differential stresses upon loading and could affect hydraulic stimulation (Reinicke et al., 2010, Zimmermann and Reinicke, 2009); in this sense, laboratory measurements are an important tool to study seismic anisotropy at small scales which can aid in the characterization of larger formations. The purpose of this work is to describe the laboratory methodology used to simultaneously measure ultrasonic P and S-waves in three different directions from a single core sample. Elastic anisotropy of a Proterozoic sedimentary rock from south-west Alberta is investigated. In metamorphic rocks seismic anisotropy is caused by preferred mineralogical alignment or foliation. Assuming a transversally isotropic medium, arrays of compressional and shear piezoelectric ceramic transducers were mounted on a sample trimmed from a core to measure travel times of ultrasonic P-waves and S-waves as a function of confining pressure at directions perpendicular, parallel, and oblique to the plane of foliation (bedding). Results show strong anisotropy agreeing with the assumption of a transversely isotropic medium. Dependence of travel times and hysteresis effects can be observed when pressurizing and depressurizing.