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Pre-salt layer carbonates are among the primary exploration targets in Brazil. However, their microstructural complexity presents significant challenges for geophysical characterization (Vasquez et al., 2019).
Elastic anisotropy is a critical property that influences the interpretation of seismic velocity, stress distribution, and fracture behavior. In pre-salt carbonates, complex pore geometries and diagenetic alterations lead to variable elastic responses, making laboratory characterization challenging (Martínez & Schmitt, 2013).
Digital rock physics (DRP), based on micro-computed tomography (µCT), provides a framework for connecting microstructural and elastic domains, allowing for direct numerical simulations under controlled conditions (Lima Neto et al., 2023).
This study leverages the GeoDict software to analyze carbonate samples from the Barra Velha Formation in the Santos Basin, Brazil.
The objectives are:
(a) Compute the effective stiffness matrix and directional VP and VS velocities from µCT data samples - F90V and F92H, under a confining pressure of 22.1 MPa, 12.14 μm voxel resolution, and 1.5";
(b) Extract the Thomsen anisotropy parameters (ε, δ, γ) to classify the magnitude of anisotropy (Thomsen, 1986);
(c) Quantify deviations from elliptical anisotropy using non-ellipticity indicators, providing insight into the anisotropic character (Thomsen, 1986; Alkhalifah & Tsvankin, 1995);
(d) Validate the applicability of VTI/HTI symmetry models and correlate the numerical results with laboratory data.
This work develops a digital workflow to analyze the elastic behavior of pre-carbonates, aiding in more precise reservoir characterization.
Figure 1 displays the deformation planes for samples F92H and F90V, illustrating the angular dependence of the elastic response obtained from GeoDict simulations. These diagrams show the magnitude of deformation as a function of propagation direction, providing a representation of the anisotropic behavior of each carbonate sample.
Sample F92H exhibits nearly circular contours at 70 GPa, indicating a weakly anisotropic that is consistent with a homogeneous pore distribution across the XY, XZ, and YZ planes. In contrast, F90V exhibits slightly elongated lobes along specific orientations in the XZ and YZ planes at pressures below 70 GPa. This pattern reveals directional mechanical anisotropy associated with preferential pore alignment and textural heterogeneity. In the XY plane, the pressure measurement reached 80 GPa.
The comparison of the two polar plots confirms that sample F90V displays a higher degree of elastic anisotropy. These results underscore the strong correlation between digital deformation fields and the microstructure that governs the elastic behavior of carbonate rocks.
Figure 2 illustrates the consistency between laboratory-measured and simulated wave velocities for F92H and F90V samples, demonstrating the reliability of the digital simulation results in reproducing elastic properties and anisotropic trends.
Table 1 shows that the simulation model produces higher velocities than those physically measured in the lab, with performance varying depending on the specific sample and wave type.
Figure 3 shows the Thomsen parameters for the analyzed carbonate samples under 22.1 MPa, measured in the laboratory. Thomsen parameters' digital values for F92H (ε = -0.0058, γ = -0.0052) and F90V (ε = -0.0320, γ = -0.0191) reveal weak and moderate anisotropies, confirming the laboratory results.
| References | Alkhalifah, T., & Tsvankin, I. (1995). Velocity analysis for transversely isotropic media. Geophysics, 60(5), 1550-1566. Lima Neto, I. , Ceia, M., Misságia, R., Oliveira, L., Santos, V., & Moreira, J. (2023). Anisotropic elastic moduli of presalt carbonates from Santos Basin, Brazil. In SEG International Exposition and Annual Meeting (pp. SEG-2023). SEG. Martínez, J. M., & Schmitt, D. R. (2013). Anisotropic elastic moduli of carbonates and evaporites from the Weyburn-Midale reservoir and seal rocks. Geophysics Prospect, 61(2), 363–379. Thomsen, L. (1986). Weak elastic anisotropy. Geophysics, 51(10), 1954–1966. Vasquez, G. F., Morschbacher, F. M., Anjos, C. W. D., Silva, M. P., Madrucci, V., & Justen, J. C. R. (2019). Petroacoustics and composition of presalt rocks from Santos Basin. The Leading Edge, 38(5), 322–416. |
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| Country | Brasil |
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