Grain fracturing and grain re-arrangement in sands and sandstone: Studying brittle deformation features in sugar glass as an analogue.

Abstract

In order to better understand the effect of extracting or injecting fluids can have on a sandstone reservoir, the knowledge on the mechanical behaviour should be improved. This study focusses on this aspect by using sugar glass as an analogue for quartz sand. This is done by performing experiments on a hexagonal and cubic packs of sugar glass discs, with or without flattened contact areas. The results were then compared to micrographs from experiments on sand aggregates in order to determine whether sugar glass is a good analogue for quartz sand. Aggregates build out of discs of sugar glass were deformed using an Instron 8862, which compressed the different samples in a vertical direction. This compaction caused the failure of multiple discs. The cracks formed in these failed discs were subdivided into different categories: meridional, straight, convergent, and spalling cracks. In the experiments on cubic packing, the majority of cracks formed in a vertical direction. In the rotated cubic and hexagonal experiments, the majority of cracks formed at an oblique angle. The packing of the aggregates caused a big difference in structures that formed in the aggregate. These structures could be compared to micrographs of sand aggregates. The structures formed in the hexagonal experiments were more similar than the structures formed in the cubic experiments. In the sand aggregates, most cracks recognizable in the quartz sand grains were at an oblique angle to the compaction direction. Also the type of cracks was very similar: a multitude of contact-to-contact meridional and convergent cracks. Another important feature arises in the rotated cubic experiments with flattened contacts: due to slip along the boundaries of the discs, cracks formed in a standard pattern. The cracks that form due to this grain boundary slip all form on the edges of the contact area and break off a side of the disc. These patterns can be used to predict structures in deforming sandstones or sand aggregates and to improve models on sandstone reservoir behaviour.