A Field and Numerical Investigation on Wiggly Compaction Bands in High-Porosity Sandstone
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The Rock Fracture Project was an industrial affiliates project within the Structural Geology and Geomechanics Program in the School of Earth Sciences at Stanford University from 1990 to 2015. The project was co-directed by Professors David D. Pollard and Atilla Aydin. The Rock Fracture Project was one of the largest and most active research groups in the world devoted to conducting state-of-the-art research on problems of rock fracture and related crustal deformation and fluid flow, with special attention to the needs of the petroleum industry. Funding for the project came from more than twenty-five member companies. The 405 scientific reports from 25 annual workshops are archived here.
Field data, 3D geometrical modelling, quantification of micropores are combined with numerical simulation to investigate the mechanism of wiggly compaction bands in high-porosity aeolian sandstone. Field data show that the segments of wiggly compaction bands have similar orientations as that of preexisting shear-enhanced compaction bands H1 and H2. The wiggly bands are inferred to propagate and switch orientations between H1 and H2. Based on the geometry of band, the direction of greatest compression (ε3) is interpreted as perpendicular to the overall strike of wiggly compaction bands. And the band segments perpendicular to ε3 are the pure compaction bands. Analysis of micropores shows that the pure compaction bands have the greatest porosity in the host rock, and may have a different failure envelope. In discrete element modelling, a discrete particle is used to represent a pore structure that surrounded by several quartz grains. Similar to the collapse and compaction of a pore structure, the breakable particle may be compacted (shrink) when the force status exceeds the yielding cap determined by failure force (Ff) and aspect ratio (k). Discrete element model built by the breakable particles is compressed to simulate the formation of compaction bands. The direction of compaction bands is determined by the aspect ratio (k) of the cap. When k=0.5, compacted zone tends to propagate perpendicular to the greatest compression direction, which corresponds to pure compaction bands. When k=2, two 45-degree directions are the predominant directions of the compacted zones. Compacted zones propagate along one inclined direction, and may switch to the other direction when the stress state is changed. As a result, the wiggly compacted zone shows a chevron pattern. To conclude, the direction of compaction bands is determined by the failure envelope of the host rock and the local stress; the pure compaction bands formed at higher rock porosity; and the wiggly compaction bands composed of segments of shear-enhanced bands are perpendicular to the direction of greatest compression.
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- Liu, Chun and Pollard, David D. (2014). A Field and Numerical Investigation on Wiggly Compaction Bands in High-Porosity Sandstone. Stanford Digital Repository. Available at: http://purl.stanford.edu/sn871qn0452
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