Abstract
The influence of water on the mechanical properties of rocks has been observed by many researchers in rock engineering and laboratory tests, especially for sedimentary rocks. In order to investigate the effect of water saturation on the mechanical properties of low-permeability rocks, uniaxial compression tests were conducted on siltstone with different water contents. The effects of water on the strength, elastic moduli, crack initiation and damage thresholds were observed for different water saturation levels. It was found that 10% water saturation level caused more than half of the reductions in mechanical properties. A new approach is proposed to analyze the stress-strain relations at different stages of compression by dividing the axial and lateral stress-strain curves into five equal stress zones, where stress zones 1–5 refer to 0%–20%, 20%–40%, 40%–60%, 60%–80% and 80%–100% of the peak stress, respectively. Stress zone 2 represents the elastic range better than stress zone 3 which is at half of the peak stress. The normalized crack initiation and crack damage stress thresholds obtained from the stress-strain curves and acoustic emission activities averaged 31.5% and 83% of the peak strength respectively. Pore pressure is inferred to take part in the deformation of low-permeability siltstone samples, especially at full saturation levels. A change of failure pattern from multi-fracturing to single shear failure with the increase of water saturation level was also observed.
Highlights
The mechanical properties of rocks in the upper crust are important in underground and geotechnical engineering projects, including tunnelling, drilling and reservoir exploration
The existence of water can be classified into five types: structural water as a component of minerals like clay; bounded water on the surfaces of minerals, especially hydrophilic minerals like montmorillonite, due to electrochemical reactions; water physically trapped by capillaries; water in small pores with limited mobilization and free water in large pores and fractures
Because large numbers of small pores exist in clays and the the smallest pores which can be several nanometers due to the capillaries [30] and onto the mineral cement minerals and both are highly hydrophilic, the water is first dispersed onto their surfaces, as surfaces depending on the hydrophilia
Summary
The mechanical properties of rocks in the upper crust are important in underground and geotechnical engineering projects, including tunnelling, drilling and reservoir exploration. Rocks’ physical and mechanical properties are influenced by the type of pore fluid [1] and the saturation level [2,3]. Water is the most common saturation medium and can be present in rocks at any level. The water saturation level may change due to geological changes or human intervention, like earthquakes, excavation, injection or extraction of fluids from underground reservoirs. With the depletion of conventional oil reservoirs over their life cycles, water is used to enhance oil production by replacing the oil in place, which causes changes in the saturation medium and saturation level. Hydraulic fracturing is widely used to stimulate unconventional oil and gas reservoirs by introducing water into
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