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Stress- and direction-dependent analysis of elastic and petrophysical properties, and texture in unconventional rocks
Firdaus, Gama
Firdaus, Gama
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2023
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Abstract
In the past decade, the study of geophysical anisotropy has gained increasing attention due to its significance in unconventional rock characterization and production. Anisotropy can be characterized as (a) intrinsic anisotropy, such as thin laminations, clay layers, and organic matter, and (b) extrinsic anisotropy, such as induced microfractures due to coring. The spatial distribution of such features is an essential mechanism affecting stress- and direction-dependency of elastic and fluid transport properties. Accurately measuring seismic anisotropy and its relationships with petrophysical properties while investigating rock fabric evolution using high-resolution textural images under reservoir stress conditions is essential to enhance the reservoir development of unconventional rocks.
In this thesis, cores from multiple formations of the Bakken Petroleum System (BPS) were analyzed in the laboratory as examples of strongly and weakly anisotropic rocks. First, mineral compositions and total organic carbon of the rock samples were acquired. The next step was visually analyzing the rock texture using high-resolution images from micro-computed tomography scans and backscattered electrons images. Following these analyses, ultrasonic velocities (P- and S-wave) of tight rocks were obtained in multiple directions, that is, 0°, 45°, and 90° with respect to the bedding plane, under elevated confining stress. Finally, the findings were correlated to stress-dependent porosity and permeability to monitor pore structure deformation and the evolution of elastic and transport fluid properties during stress change.
The results indicate that the primary driving mechanism dictating the stress dependency of velocity and permeability is the compaction of the pore space and compression of aligned compliant components, such as clay, kerogen, and microcracks that are deposited in the bedding-parallel direction. Furthermore, the multiphysics measurements performed in this thesis suggest that anisotropy and stress-dependent elastic properties, especially in organic-rich mudrocks, cannot be neglected. Using isotropic model for anisotropic rocks may affect the accuracy of the mechanical earth model and horizontal stress calculations.
Obtaining anisotropic elastic properties from the lab are, however, time- and resource-consuming and complicated. On the other hand, seismic and wireline log data of the independent elastic parameters of the rock are rare. Furthermore, due to low resolution, geophysical borehole tools do not always capture anisotropic features in thinly laminated organic-rich mudrocks. Consequently, the prediction of in situ anisotropic seismic parameters is associated with significant uncertainties. Therefore, an anisotropy template was developed to assess and estimate the expected anisotropy or a specific elastic modulus from readily available elastic wave data.
The anisotropy template was constructed by integrating several rock physics models, such as the Backus averaging, Hudson’s crack model, and linear slip theory, which consider crack- and layer-induced anisotropy in the effective medium. Wireline log and lab-measured elastic data from the Berea, Bakken, Three Forks, and Mancos formations were analyzed using the anisotropy template. The anisotropy template allows the user to (a) understand the causal mechanisms for seismic anisotropy, (b) understand the occurrence of non-uniform deformation due to change in stress, (c) narrow the range of anisotropic parameters for known mineralogy and texture, and (d) predict the texture of the rock with known stress-dependent moduli changes.
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