Robust determination of rock anisotropy in the laboratory using laser
ultrasonics
Abstract
Robust characterization of rock anisotropy is the preferred laboratory
method to support seismic data interpretation in the field. Especially
in shale formations, accurate elastic anisotropy helps delineate
subsurface stress distribution, improve seismic imaging, and enhance
hydraulic fracturing design. The conventional technique for evaluating
rock elastic anisotropy involves ultrasonic pulse transmission between
source and receiver transducers attached to the rock surface. The size,
position and orientation of the source and receiver in relation to the
propagation distance and direction, and their coupling to the rock
surface introduce undesired uncertainties in Thomsen’s anisotropy
parameters and rock attenuation: effective propagation distance; group
or phase velocity; impact of the contact interface on measured wave
attenuation; impact of heterogeneity on wave velocity measurements. We
apply here the contactless laser ultrasonic method, involving a source
laser (short-pulse high-peak power), a probing laser (vibrometer), and a
cylindrical rock sample set on a rotating stage. The footprint of the
source laser beam is 2 mm, and that of the receiver beam is 0.1 mm,
which can conveniently be approximated by a point on a centimetric rock
sample. The propagation distance is hence unambiguously known, implying
that a group velocity is effectively estimated, and the observed
attenuation is solely due to the rock, not to the rock-transducer
interface (extrinsic). The technique also allows for a denser ultrasonic
probing. Four samples are probed, where the P-wave velocity along up to
630 independent ray paths is evaluated. Three samples are made of a
known, homogeneous, and layered synthetic material phenolic grade,
approximately transversely isotropic. These samples were cored along,
across and at 45° to the layers. The fourth sample is a heterogeneous
shale from the Goldwyer formation (Canning basin, Western Australia).
The measurements on the three known phenolic samples are used to
validate the method, and optimise the measurement protocol. Application
of the method to the unknown heterogeneous shale suggests that (i)
anisotropy can be reliably estimated in the homogeneous sub-volume of
the sample and that (ii) the mineralogical heterogeneity can be detected
and identified in other sub-volume.