Effects of extreme environmental conditions on the mesoscale and nanoscale properties of calcite-cemented sandstone

Abstract

Understanding the fracture mechanism of brittle materials has great importance in the safe design of geo- and infrastructure projects. Rock fracture mechanics has several practical applications in geological disposal of radioactive waste, carbon sequestration, efficient underground storage of oil or natural gas, enhanced recovery of hydrocarbons, geothermal energy extraction and construction of underground structures. Mechanical and fracture properties of rock can dramatically vary with changes in environmental and loading conditions to which it is exposed. Microstructure of sedimentary rock changes significantly under the influence of high temperatures and due to the presence of water. These microstructural changes induced by high temperature or water saturation are irreversible to some extent even after cooling the rock to room temperature or drying the rock. The mechanical and fracture properties of rock are dependent on the microstructural damages created inside the rock. In addition, the presence of water inside a rock at subfreezing temperatures has a significant effect on its mechanical and fracture properties. Thus, it is very important to understand the effects of different environmental conditions on the mechanical and fracture properties of rocks. The results from this research can be applied in areas like tunnel fire damage assessment, geothermal energy extraction, underground nuclear waste disposal, tunneling below ground water table, space mining and others. This thesis examines the effects of extreme environmental conditions on the mesoscale and nanoscale mechanical and fracture properties of calcite-cemented sandstone along different bedding orientations through a series of experimental and observational methods. In the first set of tests, specimens were subjected to oven drying (105°C) and furnace heating (500°C) then cooled to room temperature before testing for their mechanical and fracture properties. In the second set of tests, specimens were water saturated for 24h inside a vacuum desiccator before testing. The third set of tests were conducted by freezing both oven-dried and water-saturated specimens to -50°C. Uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) tests were used to investigate the effects of different environmental conditions on the mesoscale mechanical properties of calcite-cemented sandstone. The cracked chevron notched Brazilian disk (CCNBD) test was used to determine the mesoscale fracture properties such as mode I fracture toughness (KIC), consumed energy and crack propagation velocity. For CCNBD tests, specimens were divided into three groups based on the notch orientation with respect to the bedding plane — divider, arrester, and short transverse. In addition, digital image correlation (DIC) was used for both strain and crack propagation velocity measurements, and environmental scanning electron microscope (ESEM) was employed to investigate the microstructural damages produced in specimens before and after CCNBD tests. Test results reveal that both mesoscale mechanical and fracture properties of calcite-cemented sandstone specimens decreased significantly when subjected to furnace heating and water saturation compared to oven-dried specimens. The reduction in KIC and consumed energy due to furnace heating and water saturation were pronounced in divider specimens compared to arrester and short transverse specimens. In contrast, the reduction in crack propagation velocity due to furnace heating and water saturation was higher in short transverse specimens compared to arrester and divider specimens. The consumed energy during rock failure was found to have a good correlation with the fracture toughness (R2 = 0.89). The microcrack density in furnace-heated and water-saturated specimens was significantly higher compared to oven-dried specimens. The UCS, BTS, Young’s modulus (Et50) and KIC of the calcite-cemented sandstone specimens in saturated frozen condition increased significantly when compared with oven dry frozen condition. The dramatic increase in mechanical strength and fracture toughness of the saturated frozen specimens can be attributed to the presence of ice in the pores and cracks of the specimens, which fills them as a solid material and prevents their widening and spalling. This thesis also investigates the nanomechanical properties (reduced modulus, Er and hardness, H) of minerals and mineral-to-mineral contacts in calcite-cemented sandstone specimens subjected to oven drying and furnace heating using a combination of automated mineralogy, ESEM and nanoindentation tests. Results from the nanoindentation test revealed that Er and H of rock forming minerals and mineral-to-mineral contacts decreased significantly in furnace-heated specimens compared with oven-dried specimens. Anisotropic thermal expansions along different crystallographic axes, thermal dissociations of certain minerals, differential thermal expansion, the presence of impurities and inconsistencies in the orientation of minerals during deposition were found to be the major reasons behind the increased microcrack densities in the furnace-heated specimens. In addition, an attempt was also made to compare the effect of annealing on the nanoscale and mesoscale mechanical properties of calcite-cemented sandstone. The results proved that the decrease in Er and H of the individual minerals and mineral-to-mineral contacts due to furnace heating were consistent with the decrease in the mesoscale properties of calcite-cemented sandstone.