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Characterizing the mechanical behavior of rocks plays a crucial role to optimize the fracturing process in unconventional reservoirs. However, due to the intrinsic anisotropy and heterogeneity in unconventional resources, fracture process prediction remains the most significant challenge for sustainable and economic hydrocarbon production. During the deformation tracking under compression, deploying conventional methods (strain gauge, extensometer, etc.) is insufficient to measure the deformation since the physical attachment of the device is restricted to the size of the sample, monitoring limited point-wise deformation, producing difficulties in data retrieval, and a tendency to lose track in failure points, etc. Where conventional methods are limited, the application of digital image correlation (DIC) provides detailed and additional information of strain evolution and fracture patterns under loading. DIC is an image-based optical method that records an object with a camera and monitors the random contrast speckle pattern painted on the facing surface of the specimen. To overcome the existing limitations, this paper presents numerical modeling of Brazilian disc tests under quasi-static conditions to understand the full-field deformation behaviors and finally, it is validated by DIC. As the direct tensile test has limitations in sample preparation and test execution, the Brazilian testing principle is commonly used to evaluate indirectly the tensile strength of rocks. The two-dimensional numerical model was built to predict the stress distribution and full-field deformation on Brazilian disc under compression based on the assumptions of a homogenous, isotropic and linear elastic material. The uniaxial compression test was conducted using the DIC technique to determine the elastic properties of Spider Berea sandstone, which were used as inputs for the simulation model. The model was verified by the analytical solution and compared with the digital image correlation. The numerical simulation results showed that the solutions matched reasonably with the analytical solutions where the maximum deviation of stress distribution was obtained as 14.59%. The strain evolution (normal and shear strains) and displacements along the central horizontal and vertical planes were investigated in three distinguishable percentages of peak loads (20%, 40%, and 90%) to understand the deformation behaviors in rock. The simulation results demonstrated that the strain evolution contours consistently matched with DIC generated contours with a reasonable agreement. The changes in displacement along the central horizontal and vertical planes showed that numerical simulation and DIC generated experimental results were repeatable and matched closely. In terms of validation, Brazilian testing to measure the indirect tensile strength of rocks is still an issue of debate. The numerical model of fracture propagation supported by digital image correlation from this study can be used to explain the fracturing process in the homogeneous material and can be extended to non-homogeneous cases by incorporating heterogeneity, which is essential for rock mechanics field applications.

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Most shale and carbonate formations violate the isotropic assumption of rock media in classical fracture models due to the presence of extensive natural-fracture network. However, the current application of strain gauges cannot provide information about how natural fractures interact with induced fractures in such heterogeneous rock system. In this paper, we present the application of Digital Image Correlation (DIC) technique which can measure strain development over time, thus it can determine the fracture initiation and propagation. We apply indirect tensile experiment on Buda Limestone samples with various patterns of complex natural-fracture system while the strain development in sample is measured by Digital Image Correlation (DIC) technique. As a benchmark, DIC technique was first conducted on a homogeneous sandstone sample. The results verify that that the maximum tensile strain is developed at the central vertical line of sample as it was expected from theoretical solutions. This DIC measurement matches the post-failure fracture pattern of specimen. In contrast to failure pattern in homogenous sandstone specimen, very complex fracture pattern was observed on several Buda Limestone specimen. Fracture slippages along natural fracture, fracture deviation toward inclined and horizontal fracture was recorded. The results of this study can help us to better understand the complex failure mechanism in naturally fractured media. DIC is a powerful technique in providing the strain development over the surface of sample in real time, thus it can provide the fracture initiation and propagation in such heterogeneous rock media.

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