基于离散元法的多孔砂岩压密破坏过程数值模拟

摘要

采用紧密堆积离散元模型研究了多孔砂岩压密带的产生和发展过程.模型中的一个离散单元代表一系列颗粒的集合体,通过屈服力和屈服椭圆的高宽比来定义单元的屈服包络面.当单元中一个连接的受力状态超过其屈服面时,单元被压密(收缩).基于这种改进的离散单元,用Matlab构建了相应的模拟程序MatDEM.结果表明:当屈服椭圆的高宽比由0.5增加到2时,压密带由平直形变为波浪形,表明砂岩的压密带形状受其屈服面的形态影响,压密作用引起的弹性波促进了压密带的形成.根据模拟结果推断,当压密带传播时,其顶端单元的受力状态发生变化,并在压密带传播几厘米时超过一定限度.最终压密带方向交替在两个主要的倾斜方向之间变化,并显示为V形或波浪形.%A bonded close-packed discrete element model is used to investigate the generation and development of compaction bands.A discrete element represents an assemblage of sand grains in the model.An element is compacted (shrinks) when the force state of bonds exceeds the yielding cap defined by the failure force and the aspect ratio of yielding cap.A Matlab code "MatDEM" is developed on the basis of the enhanced method.Simulation results show that compaction bands vary from planar shape to wavy shape when the aspect ratio of yielding ellipse increases from 0.5 to 2,suggesting shapes of compaction bands influenced by the shape of yielding cap of sandstone.Results also indicate that seismic wave induced by compaction promotes the formation of compaction bands.It is inferred that the force state of a tip element varies when it propagates,and exceeds a limit when the band segment propagates by several centimeters.As a result,the band direction alternates between two predominant inclined directions and shows a chevron or wavy shape.