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Description
Carbonate rocks form some of the most complex and significant reservoirs globally, accounting for nearly half of the world’s hydrocarbon reserves. Understanding their physical properties is crucial for improving reservoir characterization and supporting the development of enhanced oil recovery strategies. In prolific carbonate reservoirs, rock characterization is challenging due to their complex textures, characterized by strong heterogeneity across both micro- and macroscales. These rocks contain pore systems of diverse types and sizes, resulting in pronounced variability and anisotropies in their physical properties. Elastic anisotropy is influenced by factors such as the spatial distribution of mineral phases, preferential pore orientation, and presence of fractures. To address these complexities, this study proposes an integrated rock physics model (RPM) that incorporates pore systems with both randomly and preferentially oriented pores to investigate the velocity-porosity relationships in carbonate rocks, with emphasis on the role of pore structure parameters relevant to seismic interpretation. The proposed approach is validated using a finite element procedure to simulate wave propagation in models with explicitly represented pores. Moreover, the methodology is applied to three core samples from a pre-salt reservoir of the Santos Basin, offshore Brazil, to assess anisotropic effects. Digital rock physics techniques are employed to construct digital models of the samples from X-ray micro-computed tomography (micro-CT) images. The cylindrical samples, measuring 50 mm in length and 38 mm in diameter, were scanned at a voxel size of 10 μm. Ultrasonic wave velocities were measured on dry core plugs with a central frequency of 1 MHz, while porosity was determined using a gas porosimeter. The mineralogical composition was determined through X-ray diffraction measurements, indicating that calcite and dolomite are the dominant mineral phases. The proposed RPM implementation requires detailed information on the pore structure to estimate wave velocities, including pore shapes, preferential orientations, and volume fractions. These parameters were extracted from the digital images by applying a watershed segmentation algorithm to separate the pore phase into individual objects, enabling quantitative measurements of their geometric properties. The approach allows for the incorporation of the full distribution of pore geometries into the model, rather than relying on a limited set of pore types. However, due to the resolution limitations inherent to micro-CT imaging, a significant portion of the pore space remains unresolved. To account for this, the proposed methodology estimates an equivalent pore aspect ratio (AR) for the unresolved pores by minimizing the mismatch between predicted and experimentally measured wave velocities. This effective geometric parameter provides a simplified representation that reproduces the elastic response of the actual rock. The results show that the estimated AR are larger than those obtained under isotropic assumptions, highlighting the influence of pore anisotropy on wave velocity propagation. Overall, this work demonstrates that the proposed method offers a robust framework for evaluating the elastic properties of heterogeneous carbonate reservoirs, supporting the development of advanced rock physics-based characterization methods.
| Country | Brazil |
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