MatDEM应用论文|多孔砂岩中曲折压密带的形成机理：2.离散元法数值模拟

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论文题目

Mechanism of formation of wiggly compaction bands in porous sandstone: 2. Numerical simulation using discrete element method

Liu et al., 2015, JGR-Solid Earth

研究内容

Wiggly compaction bands in porous aeolian sandstone vary from chevron shape to wavy shape to nearly straight. In some outcrops these variations occur along a single band. A bonded close-packed discrete element model is used to investigate what mechanical properties control the formation of wiggly compaction bands (CBs). To simulate the volumetric yielding failure of porous sandstone, a discrete element shrinks when the force state of one of its bonds reaches the yielding cap deﬁned by the failure force and the aspect ratio (k) of the yielding ellipse. A Matlab code “MatDEM3D” has been developed on the basis of this enhanced discrete element method. Mechanical parameters of elements are chosen according to the elastic properties and the strengths of porous sandstone. In numerical simulations, the failure angle between the band segment and maximum principle stress decreases from 90° to approximately 45° as k increases from 0.5 to 2, and compaction bands vary from straight to chevron shape. With increasing strain, subsequent compaction occurs inside or beside compacted elements, which leads to further compaction and thickening of bands. The simulations indicate that a greater yielding stress promotes chevron CBs, and a greater cement strength promotes straight CBs. Combined with the microscopic analysis introduced in the companion paper, we conclude that the shape of wiggly CBs is controlled by the mechanical properties of sandstone, including the aspect ratio of the yielding ellipse, the critical yielding stress, and the cement strength, which are determined primarily by petrophysical attributes, e.g., grain sorting, porosity, and cementation.

Fig 1. Outcrop photographs taken in the area studied by Hill [1989]. The azimuth of the estimated maximum compressive stress (σ1)is approximately 104°. (a) Transition of band type from chevron to wavy to straight CBs, as failure angle (γ) increases from approximately 45° to 65° to 90°. (b) Wiggly CBs merged into tabular shear-enhanced CBs. Figure 1b was taken about 2 m from Figure 1a.

Fig 2. (a) A close-packed discrete element model. (b) Two elements are bonded by a breakable elastic spring along the normal direction and interact through a spring force (Fn). (c) Two elements also are bonded by a spring along the tangential direction to simulate the shear force (FS). Xn is the relative normal displacement, and Xs is the relative shear displacement.

Fig 3. (a) A discrete element represents an assemblage of grains in porous sandstone. When the assemblage is compacted, its porosity and volume reduce. (b) Failure envelope of discrete element includes an elliptical yielding cap. An element will be compacted (shrink) when the force state of one of its bonds reaches the cap. Ff is the failure force; k is the aspect ratio of the yielding ellipse; Fb is the breaking force; and FS0 is the initial shear resistance (i.e., cohesion).

Liu C., Shi B., Pollard D. D., and Gu K. 2015. Mechanism of formation of wiggly compaction bands in porous sandstone: 2. Numerical simulation using discrete element method. Journal of Geophysical Research- Solid Earth, 120, 8153-8168.

来源：矩阵离散元MatDEM

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首次发布时间：2023-12-30