Zhou, Yi (2025). Comparing magnetic pore fabrics, pore fabrics and permeability anisotropy in synthetic and natural sedimentary rocks. (Thesis). Universität Bern, Bern
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Abstract
Pore fabrics represent the geometric arrangement and network of pores in rocks, with a particular focus on the pore connectivity and orientation, which have a significant impact on the fluid flow within the pores. Accurately describing the pore fabric characteristics is critical in assessing the flow behaviour (connectivity, permeability) of underground fluid resources such as geothermal fluids, groundwater, and hydrocarbons. Thin section analysis is the most direct and fundamental method for observing and describing pore fabrics, but it usually only presents two-dimensional features. X-ray computed microtomography (XRCT) is certainly an effective way to non-destructively observe samples and describe their three-dimensional features, but it is constrained by the trade-off between resolution and sample size. The magnetic pore fabric (MPF) method theoretically can detect pore sizes as small as 10 nm, but MPF is only empirically associated with pore fabrics and permeability characteristics, leading to potential biases in practical applications. Direct measurement of permeability anisotropy on samples is indeed a relevant method for determining the optimal flow direction of fluids, but the determination of measurement direction often requires prior knowledge of the fabric characteristics. If MPF can establish a more quantitative relationship with pore fabrics and permeability anisotropy, considering that impregnated samples can be prepared in large quantities at once and that MPF measurements are relatively simple and efficient, the MPF method is expected to provide a more accurate and efficient quantitative prediction of fluid flow direction in porous rocks. In this study, typical sandstones (Berea, Berea Spider, Bentheimer, Castlegate, Molasse, Salt Wash North) and carbonates (Calcarenite, Indiana limestone), as well as artificial rocks (quartz sandstone bonded by liquid glass, and hot isostatically pressed (HIP) calcite-muscovite mixture) with various porosities and pore fabrics were selected. For HIP samples, calcite and muscovite were mixed to simulate impure carbonate rocks. Irregular calcite and sheet-like muscovite grains were combined in ratios of 3:7, 5:5, and 7:3 to produce pore fabrics with different anisotropies. The grains were uniformly mixed and subjected to 20 MPa cold pressing, followed by 160 MPa and 670 °C hot isostatic pressing for 3 hours, resulting in a homogeneous structure. Pore fabrics were extracted using a combination of three mutually perpendicular thin sections. XRCT scans of samples were conducted to establish digital rock models, obtain pore fabrics, and simulate permeability anisotropy and MPF. Multiple sets of cores were sampled in various directions to form multiple sets of full tensor permeability anisotropy measurements for cross-comparison. The appropriate ferrofluids were selected based on the surface wettability and charge properties of the mineral grains, and then impregnated into the rock samples. Afterward, the MPF of each sample was measured. The research results demonstrate that pore fabrics, permeability anisotropy, and MPF generally exhibit consistent orientations for most samples. However, there are significant differences in the anisotropy degree and anisotropy shape. Simulated permeability anisotropy and MPF model show more similarity with XRCT-derived digital pore fabrics which they are based on, compared to directly measured permeability anisotropy and MPF, which exhibit differences in some samples. For HIP samples, MPF maintains consistent orientations with permeability anisotropy. The MPF and permeability anisotropy also exhibit similar changes in anisotropy degree with varying muscovite ratios. This is because the shape of flaky muscovite has a higher anisotropy degree compared to irregular calcite. As the muscovite content in the sample increases, the anisotropy degree of both MPF and permeability also increases. Overall, MPF and permeability anisotropy show a strong correlation in terms of orientation and anisotropy degree. Therefore, the MPF method can serve as an effective prior method for assessing the sample heterogeneity and pore fabrics, providing a robust tool for predicting optimal flow directions in the exploration of underground liquid resources, including geothermal energy and hydrocarbons.
| Item Type: | Thesis |
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| Dissertation Type: | Cumulative |
| Date of Defense: | 30 April 2025 |
| Subjects: | 500 Science > 550 Earth sciences & geology |
| Institute / Center: | 08 Faculty of Science > Institute of Geological Sciences |
| Depositing User: | Sarah Stalder |
| Date Deposited: | 14 May 2025 08:21 |
| Last Modified: | 14 May 2025 08:21 |
| URI: | https://boristheses.unibe.ch/id/eprint/6154 |
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