Full rock anisotropy characterization using laser ultrasonics

Abstract

Reliable seismic imaging and the estimate of the distribution of subsurface stress depend on the accurate assessment of elastic anisotropy in shaly rock formation. Anisotropy was typically evaluated in the lab with contacting transducers. However, these measurements frequently reported significant uncertainty due to variations in mechanical coupling, limitations on the number of ray paths analyzed, and the relative sizes of transducers. We assessed the effectiveness of the contactless laser ultrasonic pulse transmission technique, which uses a source and probing laser and a cylindrical rock sample placed on a rotating stage, to reduce uncertainties in the estimation of Thomsen’s anisotropy parameters (TAPs). The ambiguity of propagation distance was eliminated from the smaller imprint of the source and receiver lasers on the core sample, suggesting that group velocity was effectively estimated, and the observed wave attenuation was solely indicative of the rock. Three cylindrical samples of multilayer manufactured material, namely, phenolic grade CE (Canvas Electrical), which were rather homogeneous and oriented differently (0, 45°, and 90° with respect to the bedding), were examined for P-wave velocity over approximately 630 separate ray pathways. The most precise estimation of TAPs in a vertical transverse isotropic medium without knowledge of the symmetry axis was achieved by using the laser ultrasonic method on a phenolic grade CE sample that is extracted horizontally to the bedding. Multiple dip angles, dense sampling, and multipath inversion of these datasets reduced Thomsen’s δ parameter uncertainty by 20%. Application of the same technique on an anisotropic and heterogeneous shale sample suggested that (i) the mineralogy-controlled density heterogeneity observed from the 3D X-ray computed tomography images could be detected and identified from high-density laser ultrasonic data using reservoir monitoring techniques imported from field-scale geophysics and (ii) TAPs in the homogeneous and anisotropic sub-volume of the shale sample could be reliably estimated once heterogeneity was accounted for.