Microstructural Evolution of Porosity and Stress During the Formation of Brittle Shear Fractures: A Discrete Element Model Study

Abstract

Brittle fracturing in rocks is a progressive process involving changes in stress, strain, and porosity. Changes in these properties occur heterogeneously within a rock and are manifest at the grain scale, which is difficult to observe directly in the laboratory or the field. This study uses the discrete element method to show that fractures correspond to zones of generally lower stresses, higher porosity, and highly localized dilation and distortional strain. Using the discrete element method, we conducted numerical biaxial experiments at different confining pressures to probe the internal conditions of a low cohesive sandy sediment numerical analog. When compression begins, the stresses within the sandy sediment are relatively homogeneous with anastomosing stress chains. At yield stress, when the confining pressure is relatively low, multiple dilational bands start to open. At peak stress, high‐magnitude stress chains localize adjacent to the developing shear band and distortion is evident. Postpeak stress, through‐going shear fractures are fully developed. High stresses are transmitted across the fracture where porosity is low through a network of particles in contact. With increasing confining pressure, distortion is favored over dilation during deformation. Also, the number of particles that define the width of a stress chain across a shear fracture, and the steepness of the fracture, increases. Our results can be applied to understanding stress conditions of field samples, and in constraining rock property changes during reservoir modeling of fractured reservoirs.

Microscale Characterization of Fracture Growth and Associated Energy in Granite and Sandstone Analogs: Insights Using the Discrete Element Method

“…Two‐dimensional bulk porosity ( ϕ ) is calculated as the average porosity of all gridblocks within the membranes [LongJohn et al, ]. The bulk porosity of all numerical samples after preconsolidation is 17.6% (Figure a).…”

“…RICEBAL resembles a numerical sandbox but offers added value by allowing material properties and mechanical states to be monitored throughout simulations and be correlated with deformation behavior and structure. RICEBAL has been employed to simulate rock deformation from laboratory‐scale experiments to large‐scale geodynamic processes, including exploring the effects of interparticle friction on shear zone mechanics and evolution (Morgan, ), analyzing changes in porosity and stress during biaxial experiments (LongJohn et al, ), mechanics and evolution of fault gouges (Guo & Morgan, ; Morgan & Boettcher, ), evolution of slope failure and landslide processes (Katz et al, ), and deformation of fold and thrust belts (Dean et al, ; Morgan, ). The interparticle mechanics of RICEBAL are described in supporting information Text S1 and Figure S1.…”

“…We define the fracture zone as the region around a fracture influenced by the stress changes during fracture initiation and nucleation, resulting in microcracking and porosity changes. During biaxial experiments, the changes in porosity are largely confined to the fracture zone of the shear fracture and its conjugate fractures (Figures and S2); the bulk volumetric changes of the sample are indicative of the fracture zone character (LongJohn et al, ; Renard et al, ). We use platen and membrane displacements to track the sample volume at each increment of axial strain, V (m 3 ).…”

“…To capture local changes in porosity, we impose a mesh of 4,000 μm × 4,000 μm and calculate the twodimensional porosity within each gridblock as (1) Two-dimensional bulk porosity (ϕ) is calculated as the average porosity of all gridblocks within the membranes [LongJohn et al, 2018].…”

Rupture Of Veined Granite In Polyaxial Compression: Insights From Three‐Dimensional Discrete Element Method Modeling

“…This phenomenon is mainly due to the intrinsic limitation of the parallel bond contact model, which cannot provide enough interlocking such as occurring with natural mineral grains, especially under high confinement (Shang, Duan, et al, 2018;Zhang et al, 2019). Despite the limitation of this approach, it is still attractive because it can reproduce many significant physical features and mechanical mechanisms that occur in rocks, such as fracturing process, dilation, time-dependent behavior, and strength increase with confinement (Lee & Jeon, 2011;LongJohn et al, 2018;Potyondy & Cundall, 2004;Potyondy, 2007;Virgo et al, 2013;Zhang et al, 2019). The study based on the particle-based DEM approach can still provide us with some new insights into the rupture characteristics of veined rocks under in situ confinement, which has not been extensively emphasized before.…”