The loading rate has a significant effect on the mechanical properties of shale and its influence degree is related to the angle of weak plane. Tri-axial compression tests were performed on shale samples from Longmaxi reservoir formation with GCTS RTR1500 mechanical testing system. The failure modes and mechanical parameter sensitivity characteristics of shale under different loading rates were investigated. The relationship between compressive strength and loading rate was generally linear under different conditions of weak plane angles (β), meanwhile, elastic modulus manifested obviously nonlinear. With the increase of loading rate, the compressive strength and elastic modulus measured by the experiment were higher. The mechanical sensitivity of rock will be different due to the different weak angle. When the loading rate was increased from 0.05mm/min to 0.2mm/min, specimens with the angle β of 30° had the largest growth amplitude, reaching to 17.16%, while the largest amplification in elastic modulus belonged to the β =70° group. Moreover, elastic modulus was most sensitive to the loading rate at 0.12mm/min. Since then, this trend was beginning to stabilize. Specially, shear failure was in charge under the rate of 12mm/min, while tensing fractures started to increase as loading rates going up.
The mechanical characteristics of shale are not only one of the significant topics in the field of shale rock mechanics but the theoretical basis for stimulating shale oil and gas reservoir successfully. The influence of loading rate change induced by engineering construction on the mechanical properties of rock has always been the focus of academic research. Many scholars at home and abroad have conducted many studies on the mechanical characteristics and failure modes of rocks.
Chen et al. (2009) conducted acoustic emission experiments on coarse sandstone and found that for plastic rock, the increase of the loading rate would cause the stress value corresponding to the Kaiser point to increase. Wu et al. (1982) and Zhu et al. (1984) demonstrated respectively that the fracture strength of granite increases significantly with the increase of loading rate and the ratio of compressive strength and tensile strength also increases slightly with the increase of loading rate. M. S. Paterson and T. F. Wong (2005) made a very detailed summary of the studies on rock brittle failure and considered that rock failure has various forms such as single shear failure, double shear failure and split fracture. By means of experiments, Huang D and Huang R.Q et al. (2012) pointed that with the increase of the loading strain rate, the fracture initiation and critical expansion stress are closer to the peak stress and the fracture mode of the rock sample transitions from extensional shear type to extensional chapping or even cleavage ejection. Artemov V G and Lykhin P A (1968) theoretically established the relationship between external static loading and dynamic rock failure. Huang et al. (2012) and Yin et al. (2010) analyzed the effect of loading rate on rock failure modes from the aspects of damage morphology, fractures space shape and location. Saurav Rakhaiyar and Narendra K S (2017) found that rock compressive strength decreased logarithmically with the increase of loading rate when the limestone was subjected to cyclic loading experiments. From the energy point of view, Guo (2013), based on the experimental results of acoustic emission experiments tri-axial experiments, concluded that the faster the loading rate, the higher the acoustic emission energy rate and the rock destruction time, While there are no discrepancy in the rock acoustic emission energy rate. In terms of energy distribution, from related experimental results, Zhang and Kou (2000) obtained that the increase of the loading rate would reduce the energy utilization rate. Zhang and Zhao (2013) pointed out that the fracture toughness and dimensionality of granite were depended on the loading rate and experimentally observed the influence of the micro-damage mechanism of the loading rate granite.
Zdeněk P. Baɢant (1993) further pointed out that although the toughness and nominal strength of cracks decreased with the decrease of the loading rate, the length of fractures and the failure surface brittleness were not affected by them. Zhang and Kou et al. (1999) found static joint toughness as a constant, while dynamic joint toughness was positively correlated with loading rate. Xu and Chen et al. (2017) found that the peak strength and elastic modulus of rock samples increased with the increase of the loading rate, moreover, the logarithm of the loading rate showed a cubic polynomial fitting relationship. The peak strain decreased with the increase of loading rate and performed a linear fitting relationship with the logarithm of the loading rate. Considering the influence of temperature, Xu and Liu (2017) concluded the marble stamping test and it was verified that in condition of the high temperature, the peak stress and peak strain of marble showed a significant loading rate enhancement effect, which is similar to increased linearly with the increase of loading rate. Cai and Zhao (2001) considered the loading rate basing on the static BB model and proposed the normal behavior pattern of nonlinear dynamic joints.