Search Results (3)

Here, laboratory tests were conducted to examine the effects of natural fracture density (NFD) on the propagation of hydraulic fracture (HF), HF and natural fracture (NF) interaction, and the formation of the stimulated reservoir volume (SRV). Laboratory methods were proposed to prepare samples with dense, medium and spare discrete orthogonal fracture networks. After conducting a true triaxial hydraulic fracturing experiment on the synthetic blocks, the experimental results were analyzed by qualitative failure morphology descriptions, and the quantitative analysis used two proposed new indices. On the pump pressure profiles, it reflected the non-linear interactions between HFs and NFs well. For rock blocks with a dense DFN density, pump pressure curves present fluctuation shape and the degree of interaction between HF and NF is strong; however, for model blocks with a sparse DFN density, the pump pressure curves present a sudden drop shape. In addition, different propagation behaviors of NFs—offset, divert, branch, and cross NF—can be observed from the fractured model blocks. By using a proposed index of “P-SRV”, the relationship between NFD and the fracturing effectiveness was further confirmed. Furthermore, the most striking finding is that mixed mode I–II and I–III fracture types can be formed in the naturally fractured model blocks. The experimental results are beneficial for grasping the influential mechanism of NFD on the propagation of HF and for developing more accurate and full 3D-coupled simulation models for unconventional oil and gas development. Full article

In the Huainan and Huaibei mining areas, a layer of calcareous clay is buried deep in the surface soil layer (at approximately 400 m). This layer is in a high-stress state and is prone to freezing pipe fractures in the freezing method. To obtain the true triaxial mechanical properties of this clay in its frozen state, this study conducted a cross test (L₁₆(4⁵)) to explore the change law of the strength of frozen calcareous clay under the influence of multiple factors. The results showed that the true triaxial stress–strain curve of frozen calcareous clay was divided into three stages: the strain within 0.5% showed linear elasticity. Under compressive stress, ice crystals and their cements were damaged or melted and shrank. At approximately 5%, they showed plastic hardening. The soil particles and ice crystals in the frozen soil recombined and became denser, resulting in irreversible deformation. As the compression progressed, cracks bred and swelled. The failure stage was manifested as strain hardening due to the test loading conditions. As the deformation increased, the stress also slightly increased. The consistent strength-influencing factors could be obtained through range and hierarchy analyses. The primary and secondary order of influence of σ₁ was the confining pressure σ₃, water content ω, temperature T, Bishop parameter b, and salt content ψ. The influence weight of each factor was quantitatively calculated. In the significance analysis, when the interaction was not considered, the effects of the pressure and moisture content on the strength were always significant. The effect of temperature was significant only when the significance level Ω > 0.05. The salt content and b value had no significant influence on the strength, and the significance of each factor followed the order of the results of the range analysis method and analytic hierarchy process; when considering the interaction, the interaction factors had different effects on the strength. When Ω > 0.01, the influence of factor A (temperature T) × B (water content ω) on the strength showed significance, even exceeding that of temperature. This demonstrated that when studying the strength characteristics of frozen soil, it is necessary to comprehensively consider the various factors and their interaction to more accurately characterize the mechanical behavior of frozen solids. Full article

To investigate the mechanical properties of frozen saline soil, the deep calcareous clay in the Huanghuai mining area was chosen as the research object, and an orthogonal test was carried out to study the true triaxial mechanical properties of frozen saline calcareous clay under the influence of multiple factors. The stress–strain characteristics, strength, and influence trend of salted frozen calcareous clay with different salt content, stress paths, and water content, and the relationships among the above-mentioned factors, were investigated. The analysis of the test results by an orthogonal test revealed the order of importance of the factors that influenced strength. The primary and secondary order of influence σ₁ is as follows: confining pressure σ₃, water content ω, temperature T, medium principal stress coefficient b, and salt content φ. Under the influence of multiple factors, the strength of frozen soil in the real state is not the result of single factors with independent influences that are then simply superimposed. Instead, the complex interactions of various factors affect strength. The stress path significantly influences the stress–strain relationship of frozen calcareous clay. As the coefficient of the intermediate principal stress b increases, its influence on the strength of salted frozen calcareous clay diminishes, and the optimal principal stress coefficient b = 0.33 maximizes the strength of the specimen. The influence of the moisture content on the strength of the sample is negatively correlated. The effect of different moisture contents on the deformation and failure characteristics of unsaturated saline calcareous clay is not significantly different, and this difference does not exceed 5%. Full article