Mechanism of formation of wiggly compaction bands in porous sandstone: 2. Numerical simulation using discrete element method. Issue 12 (15th December 2015) - Liu, Chun
Abstract: 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 "MatDEM 3D " 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. Key Points: Yielding failure of element is considered in the discrete element method Failure angle increases with the increasing aspect ratio of yielding ellipse Pattern of wiggly CBs is related to the yielding stress and cement strength