To investigate how the characteristics of interbeds affect hydraulic fracture propagation in the continental shale formation, a series of 300 mm × 300 mm × 300 mm concrete blocks with varying interbeds, based on outcrop observation and core measurement of Chang 7-2 shale formation, were prepared to conduct the hydraulic fracturing experiments. The results reveal that the breakdown pressure increases with the rise of thickness and strength of interbeds under the same in-situ field stress and injection rate. In addition, for the model blocks with thick and high strength interbeds, the hydraulic fracture has difficulty crossing the interbeds and is prone to divert along the bedding faces, and the fracturing effectiveness is not good. However, for the model blocks with thin and low strength interbeds, more long branches are generated along the main fracture, which is beneficial to the formation of the fracture network. What is more, combining the macroscopic descriptions with microscopic observations, the blocks with thinner and lower strength interbeds tend to generate more micro-fractures, and the width of the fractures is relatively larger on the main fracture planes. Based on the experiments, it is indicated that the propagation of hydraulic fractures is strongly influenced by the characteristics of interbeds, and the results are instructive to the understanding and evaluation of the fracability in the continental shale formation.

Numerical simulation is very useful for understanding the hydraulic fracturing mechanism. In this paper, we simulate the hydraulic fracturing using the distinct element approach, to investigate the effect of some critical parameters on hydraulic fracturing characteristics. The breakdown pressure obtained by the distinct element approach is consistent with the analytical solution. This indicates that the distinct element approach is feasible on modeling the hydraulic fracturing. We independently examine the influence of in-situ stress ratio, injection rate and fluid viscosity on hydraulic fracturing. We further emphasize the relationship between these three factors and their contributions to the hydraulic fracturing. With the increase of stress ratio, the fracture aperture increases almost linearly; with the increase of injection rate and fluid viscosity, the fracture aperture and breakdown pressure increase obviously. A low value of product of injection rate and fluid viscosity (i.e., Qμ) will lead to narrow fracture aperture, low breakdown pressure, and complex or dispersional hydraulic fractures. A high value of Qμ would lead wide fracture aperture, high breakdown pressure, and simple hydraulic fractures (e.g., straight or wing shape). With low viscosity fluid, the hydraulic fracture geometry is not sensitive to stress ratio, and thus becomes a complex fracture network.

This paper aims at investigating the ultrasonic and mechanical properties of clay soil under uniaxial compression by axial and lateral ultrasonic tests. Cylindrical soil specimens (50 mm diameter and 100 mm height) were produced to perform a series of uniaxial compressive strength test. The ultrasonic waves were transmitted and received by P-wave piezoelectric transducers (130 and 500 kHz for axial and lateral ultrasonic test, respectively), and the ultrasonic travel time and amplitude were recorded by an ultrasonic detector during the whole deformation process. From the test results, the ultrasonic pulse velocity (UPV) and transmission ratio (TR) increased with the increasing axial stress by axial ultrasonic test. The statistical correlations between UPV and TR and axial stress are presented and discussed. Also, the change in porosity was related to the increase in stress, relationship between porosity variation and UPV was deduced to analyse the stress-related compaction effect. In addition, an equation was proposed to link the relationship between UPV and width of cracks by lateral ultrasonic test, the width of cracks increased steadily with the increase in stress. Crack evolution can be reflected by UPV variation. Moreover, based on the relationship of width of cracks and UPV, continuous damage evolution equation and constitutive model were established, and compared with the testing data. These results confirm that stress-related compaction effect and damage cracking characteristics of clay soil are closely related to the ultrasonic parameters. In this regard, the ultrasonic tests can be suitably exploited for the mechanical study of soil.