Search

A series of undrained cyclic axial-torsional shear tests on saturated coral sand and quartz sand under linear, circular, and heart stress paths are performed. The test results show significant differences in liquefaction characteristics for coral sand and quartz sand. A unique relationship between deviatoric strain amplitude and excess pore water pressure ratio (Ru) can be established regardless of cyclic stress paths. The termed cyclic strain path is introduced to describe the generation features of strains, and different failure patterns for various cyclic stress paths can be easily identified. Moreover, the dissipative energy required to liquefaction triggering (Ea1) of coral sand is much larger than that of quartz sand. The generation patterns of dissipative energy with Ru for both sands are almost independent of cyclic stress paths, but the correlation between the cyclic stress ratio (CSR) and the number of cycles required to liquefaction triggering (Nf) is dependent on cyclic stress paths. Using the proxy of the unit cyclic stress ratio (USR) proposed by Chen et al. (2020) instead of the CSR, the correlation between USR and Nf is independent on cyclic stress paths. A remarkable finding is that, a unique correlation between USR15 and Ea1 exists for all sands considered. [ABSTRACT FROM AUTHOR]

Pulsating fracturing, as a new technology for unconventional oil and gas exploration, enables to enhance stimulated reservoir volume (SRV) effectively, which further helps to improve well performance. It is of great importance for the evaluation of fracturing performance and optimization of fracturing parameters by accurately calculating variables from formation impairment during pulsating fracturing treatment. Based on the principle of conservation of energy, a novel theoretical model describing the evolution of rock damage was proposed by analyzing energy evolutionary characteristics from the hysteresis loop of rock stress‐strain curve while the treatment. This model was in a good agreement with experimental data. In this paper, our study indicates that the rock damage is caused by the accumulation of damage in each cyclic loading and unloading process during pulsating fracturing. Moreover, the cumulative rock damage would increase with the increment of the number of cyclic loading and unloading. Conversely, the rock strength is negatively correlated with cyclic number. The cumulative damage variable approximates to one as the rock breaks. Additionally, the frequency and the stress level of pulsating fracturing have an obvious impact on the evolution of rock damage. The optimization of these parameters can help to accelerate the rock damage and reduce the corresponding rock strength. The speed of rock damage can be accelerated with the enhancement of the stress level at the later period of the step loading, which facilitates the increment of cumulative damage variable. Our new model provides a guideline for predicting the initial rock damage during pulsating treatment. [ABSTRACT FROM AUTHOR]