Anisotropic elastic moduli of carbonates and evaporites from the Weyburn‐Midale reservoir and seal rocks

Abstract

A B S T R A C TAssuming Vertical Transverse Isotropy (VTI) symmetry the elastic anisotropy as a function of confining pressure of four carbonates and one evaporite from the Williston sedimentary basin in Saskatchewan, Canada is investigated using the ultrasonic pulse transmission method. Ultrasonic P-and S-wave velocities are obtained from cylindrical plugs cut from a main sample along horizontal, vertical and 45 • orientations with respect to the sample's presumed vertical axis of symmetry. The elastic constants were then calculated from the measured velocities and densities. Anisotropy was quantified by estimating Thomsen parameters (Thomsen 1986) from elastic constants. The results show that the samples are at the best weakly anisotropic. The presence of microcracks and pores as well as the heterogeneity of the samples play an important role in defining the P-and S-wave velocities. The weak anisotropy found in these samples suggests that 'intrinsic' properties of these rocks negligibly contribute to the anisotropy observed at the seismic scale.

Characterisation and multifaceted anisotropy assessment of Corvio sandstone for geological CO2 storage studies

“…Based on these, we assumed that Corvio sandstone is TI, allowing the application of weak anisotropy formulas (Thomsen ). Accordingly, since V p is normally lower in the direction perpendicular to bedding (King ; Martínez and Schmitt ; Thomsen ; Wang ), the direction along the X‐plug (or vector V) would correspond to the symmetry axis, whereas the isotropic plane containing the Y‐ and Z‐plugs axial directions (with velocities about 10% higher) defines the bedding plane (i.e., two perpendicular directions to the anisotropy symmetry axis, i.e., H 1 and H 2 ). Then, taking Thomsen's () notation and equations, it is possible to evaluate the anisotropy for P‐wave velocity ( ε ) and S‐wave velocity ( γ ) from four of the five elastic constants of the material ( C 11 , C 33 , C 44 , and C 66 ) as follows: where C 11 = ρV p(H1,2) 2 ; C 33 = ρV p(V) 2 ; C 66 = ρV s(H1,2) 2 ; and C 44 = ρV p ( V ) 2 .…”

“…Additionally, the low hysteresis observed at the end of the unloading path (∼2%) would suggest that the reduction of anisotropy due to compression is largely controlled by the compliance of equant pores. In this regard, the non‐linear trends are likely indicating that microcrack closing is also playing a role (Martínez and Schmitt ). Nevertheless, our data suggest that all core plugs evolve to a more isotropic state when confining pressure is increased as ε and γ tend to decrease.…”

“…Sample anisotropy can be assessed from the contrast in P‐ and S‐ wave velocities propagating in directions orthogonal and parallel to bedding planes (Thomsen ; Wang ; Schubnel et al . ; Martínez and Schmitt ) and similarly for P‐ and S‐wave attenuation ( Q −1 , the reciprocal of the quality factor Q ; Zhu and Tsvankin ; Best, Sothcott and McCann ; Zhu et al . ; Chichinina et al .…”

“…Sample anisotropy can be assessed from the contrast in P-and S-wave velocities propagating in directions orthogonal and parallel to bedding planes (Thomsen 1986;Wang 2002;Schubnel et al 2006;Martínez and Schmitt 2013) and similarly for P-and S-wave attenuation (Q −1 , the reciprocal of the quality factor Q; Zhu and Tsvankin 2006;Best, Sothcott and McCann 2007;Zhu et al 2007;Chichinina et al 2009). Electrical resistivity (Ellis et al 2010) and magnetic susceptibility (e.g., Louis, Robion and David 2004) have also been employed for this purpose.…”

Measurements of elastic constants in anisotropic media

“…Material parameters from the literature were analyzed to determine if the Rayleigh-wave method would apply to rock. Elastic constants from the literature (Musgrave, 1970;Thomsen, 1986;Christensen, 1989;Martinez and Schmitt, 2013) were used in the Rayleigh-wave method (equations 6-10) to determine a possible Figure 14. Rayleigh velocity as a function of C 13 from equation 9 using the phenolic G10 parameters in Tables 1-3.…”