Jeremie Dautriat

Typical ultrasonic laboratory measurements of rock physical properties are conducted with ultrasonic transducers that are relatively large compared to the rock sample. We have determined the impact of the transducer size on the resulting velocity estimations. To improve data interpretation, we explore two complementary avenues: (1) explicitly account for the finite size of the transducers as part of the data interpretation/correction workflow, rather than assuming point sources, and (2) reducing the effective size of the transducers at the hardware design level. Both approaches as well as their combination have been tested successfully on multiple data sets including artificial homogeneous and isotropic samples as well as natural anisotropic rocks such as shales.

Mudstones are often anisotropic, which complicates depth conversion in seismic exploration and monitoring subsurface reservoirs during injection or production. In addition, the physical and mechanical properties of mudstones are highly sensitive to their water content. The elastic anisotropy of mudstones is not well understood because of their complex nature and the lack of laboratory experiments performed on well-preserved samples. Triaxial deformation tests were performed on mudstone core plugs to investigate the impact of water saturation on the elastic anisotropy of the Whitby Mudstone (United Kingdom). The mechanical and physical properties of the Whitby Mudstone were estimated from stress-strain and ultrasonic wave velocity data obtained on core plugs with different water saturations under isotropic and anisotropic stress conditions. The Whitby Mudstone has extremely high intrinsic elastic anisotropy (0.3–0.4) due to its composition and lamination. This elastic anisotropy increases with decreasing water content. There are three competing mechanisms that play a key role in the anisotropy increase due to dehydration such as (1) density contrast in the pore space (i.e., the presence of purely brine or a mixture of brine and air in the pore space), (2) formation of dehydration fractures, and (3) frame stiffening. Increasing the mean effective stress leads to a decrease in Thomsen’s anisotropy parameters ɛ and γ because of the closure of defects, such as natural and dehydration fractures, and the formation of stress-induced fractures. The relationship between the wavefront anellipticity factor δ and the mean effective stress is nonmonotonic and can be related to the onset of inelastic deformation.

Despite the efforts reported in the literature to explain contrasting experimental observations, the evolution of seismic attributes (velocity and attenuation) of rocks across the saturation range remains ill understood. In a comparative study, we monitored the evolution of ultrasonic P wave attributes in a porous sandstone subjected to two experiments of moisture adsorption and water spontaneous imbibition. Both experiments highlighted a significant (i.e., by 1 order of magnitude) and similar drop in P waves amplitudes, although the maximum saturations reached in each experiment is very different (i.e., about 2% vs. 70%). However, only moisture adsorption leads to a dramatic elastic softening (velocity reduction). This difference might be explained by the coupling between two competing physical mechanisms taking place during water imbibition, namely, elastic softening driven by water adsorption at the grain contact and elastic stiffening driven by full saturation of the grain contacts, at the ultrasonic frequency of the measurement.

We have developed a new numerical algorithm for inversion of ultrasonic data in transversely isotropic media. This algorithm is able to determine from the measured P-wave velocities the orientation of the symmetry axis of a rock sample and the Thomsen’s parameters, only assuming transverse isotropy. The inversion of ultrasonic data acquired on natural and potentially heterogeneous shale samples produced reasonable results. In addition, the algorithm was successfully tested on ultrasonic data acquired on synthetic samples with predefined orientations of the symmetry axis. An additional outcome of the algorithm is a simple approximation of Thomsen’s formulation, which can be effectively used for interpretation of seismic data in transversely isotropic media.