Article
Engineering, Geological
Bingxiang Huang, Xinglong Zhao, Luying Shao
Summary: Hard roof control is an important issue in coal mine production, with hydraulic fracturing being a effective approach for this. A physical model experiment using true triaxial stress loading in a laboratory was conducted to observe the initiation of radial and axial cracks during hydraulic fracturing. The experiment showed that axial cracks intersect with radial cracks, but with few intersections and without the axial cracks passing through the radial cracks. The initiation of radial cracks occurs earlier than that of axial cracks, and three conditions must be met for the development of radial and axial hydraulic cracks during hydraulic fracturing.
ROCK MECHANICS AND ROCK ENGINEERING
(2023)
Article
Engineering, Geological
Lei Hou, Jianhua Ren, Yi Fang, Yiyan Cheng
Summary: The evaluation of brittleness index (BI) is essential for hydraulic fracturing design. This study redefines fracability and proposes a data-driven workflow to optimize BIs. Machine learning algorithms are used to predict fracability, and the contribution of brittleness on pressure prediction is proposed as an optimization criterion. Six classic BI correlations are evaluated and optimized to derive a new BI. The reliability of the new BI is verified through error analysis using field data. The new brittleness index provides a more reliable option for evaluating the brittleness and fracability of fracturing formations.
INTERNATIONAL JOURNAL OF ROCK MECHANICS AND MINING SCIENCES
(2022)
Article
Engineering, Geological
Xin Liang, Peng Hou, Yi Xue, Yanan Gao, Feng Gao, Jia Liu, Faning Dang
Summary: The prediction of crack initiation angle is an important issue in hydraulic fracturing. This study utilizes fractal dimension to represent the geometric irregularity of fractures, and derives three fractal fracture criteria to judge the crack initiation angle. Experimental results show that the energy-based fractal criterion prediction is the most accurate, and the predicted initiation angle considering the crack fractal effect is more consistent with the experimental data. Parametric sensitivity analysis reveals that the initiation angle of hydraulic fractures increases with the crack fractal dimension, which can be amplified with the increase of in situ stress difference or decrease of the Biot coefficient.
ROCK MECHANICS AND ROCK ENGINEERING
(2022)
Article
Mechanics
Zhenghe Liu, Jun Yang, Lusheng Yang, Xiaokai Ren, Xuan Peng, Haojie Lian
Summary: Increasing horizontal stress difference decreases critical pre-crack angle, while increasing fluid viscosity increases critical pre-crack angle.
ENGINEERING FRACTURE MECHANICS
(2021)
Review
Engineering, Geological
Fanzhen Meng, Louis Ngai Yuen Wong, Hui Zhou
Summary: The research reviewed various definitions of rock brittleness and identified 80 brittleness indices, showing a lack of standardization in the measurement methods. It was found that different field-specific indices are not directly applicable to others, and some empirically-obtained indices may not truly reflect rock brittleness.
JOURNAL OF ROCK MECHANICS AND GEOTECHNICAL ENGINEERING
(2021)
Article
Engineering, Geological
Nghia Quoc Trinh, Simon Alexander Hagen, Helene Stromsvik, Trond Larsen, Eivind Grov
Summary: Hydraulic fracturing is commonly used to determine in situ rock stress, and the interpretation of shut-in pressure is a key aspect of this method. SINTEF has developed two practical methods, zero flow and water hammer, for defining shut-in pressure based on singular events in the pressure/flow development. In this study, 12 existing methods were compared with the two SINTEF methods through laboratory tests and a field test. The SINTEF methods are mainly applicable in hard rock environments with low permeability and have been used in hydroelectric power, tunneling, cavern projects, and mineral mining, but not in deep petroleum applications in porous rock.
ROCK MECHANICS AND ROCK ENGINEERING
(2023)
Article
Geosciences, Multidisciplinary
Yapei Ye, Shuheng Tang, Zhaodong Xi, Renqi Lu, Zhongyao Zhang, Jintao Li
Summary: Studying the microscopic characteristics of shale can provide a theoretical basis for evaluating its macroscopic mechanical properties and fracturability. Different types of quartz in shale have different effects on the quality of shale oil/gas reservoirs. This study used finite element numerical simulation to set up different models based on the microscopic occurrence states of quartz, and found that the size and occurrence mode of quartz have a significant impact on shale compressive strength, while quartz content has the most significant influence on shale brittleness. The presence of quartz particles can enhance the compressive strength of shale and form complex fracture networks.
NATURAL RESOURCES RESEARCH
(2023)
Article
Chemistry, Multidisciplinary
Nikita Dubinya, Irina Bayuk, Milana Bakhmach
Summary: This study focuses on the problem of estimating the brittleness of rock masses using geophysical data for the petroleum industry. Three main ways of estimating brittleness-mineral-based, log-based, and elastic-based brittleness indices-are discussed from the perspective of scaling factor. The study highlights the contradictions in brittleness indices calculated using different methods of introducing brittleness. These contradictions are explained by the scaling factor, as geophysical data used for brittleness estimation are typically obtained at different spatial and temporal scales. A model based on the effective medium theory is used to understand the relationships between the inner structure of inhomogeneous rocks and their brittleness indices estimated from laboratory tests on core samples and log data analysis.
APPLIED SCIENCES-BASEL
(2022)
Article
Engineering, Geological
Matej Petruzalek, Z. Jechumtalova, T. Lokajicek, P. Kolar, J. Sileny
Summary: Laboratory loading experiments and acoustic emission (AE) monitoring were conducted on different structured granites to observe the fracturing and study the AE source mechanisms. The results showed that rock structure and grain size heterogeneity play crucial roles in controlling the mechanical properties. AE analysis effectively observed the fracturing process and indicated that microcracking was localized in the circumferential portion of the specimens, with sub-vertical cracks dominating the fracturing.
ROCK MECHANICS AND ROCK ENGINEERING
(2023)
Article
Mathematics, Interdisciplinary Applications
Xin Liang, Peng Hou, Yi Xue, Xiaojun Yang, Feng Gao, Jia Liu
Summary: This study investigated the fractal characteristics of crack propagation in hydraulic fracturing and the impact of fractal dimension of induced fractures on rock breakdown pressure. The results showed that N-2 fracturing can lower the rock breakdown pressure and induce more tortuous macro-fractures. The study suggests that crack fractal dimension is an important factor that should be considered in designing fracturing operations.
FRACTALS-COMPLEX GEOMETRY PATTERNS AND SCALING IN NATURE AND SOCIETY
(2021)
Article
Energy & Fuels
Xiaojie Fang, Caifang Wu, Hewei Zhang, Jiang Han, Geng Li, Bin Gao, Xiuming Jiang
Summary: The continuous injection of fracturing fluid redistributes stress, causing deformation and failure of coal. A fracture stress distribution model for cylindrical coal samples is developed based on triaxial compression experiments and hydraulic fracturing simulations. The concept of strain ratio (S) is proposed and its physical significance is explained. The deformation and failure process of coal samples during hydraulic fracturing is characterized by strain, fracture propagation, and AE energy.
Article
Engineering, Geological
Hui Wu, Randolph R. Settgast, Pengcheng Fu, Joseph P. Morris
Summary: The study introduces an enhanced VCCT for calculating energy release rates and stress intensity factors for cracks with arbitrary shapes in 3D domains on structured grids. It was implemented into a multiphysics simulation environment capable of simulating crack propagation in the framework of linear elastic fracture mechanics. The enhanced VCCT was validated through comparisons with existing analytical/numerical solutions and other VCCT approaches for SIF calculation along nonsmoothed crack fronts.
ROCK MECHANICS AND ROCK ENGINEERING
(2021)
Article
Engineering, Geological
Jun Peng, Ming Cai, Zhijun Wu, Quansheng Liu, Chuanhua Xu
Summary: This study examines the crack initiation stress (CI) as an important indicator for thermally damaged rock under uniaxial compression. Different methods for determining CI are reviewed and compared. The results show that the CVS method and CAEH method provide comparable CI values, while the LSR method generally gives lower values. A modified LSR method is proposed to accurately determine CI for thermally damaged rocks. The study also reveals that CI generally decreases with increasing thermal damage.
ENGINEERING GEOLOGY
(2023)
Article
Engineering, Geological
Dongsheng Wen, Xinzhi Wang, Haozhen Ding, Zhibin Fu
Summary: This paper presents an error analysis of various crack initiation stress determination methods and proposes a new method for the identification of crack initiation stress by the expansion rate of axial crack strain. The results show that the proposed method can accurately identify crack initiation stress.
ROCK MECHANICS AND ROCK ENGINEERING
(2023)
Article
Engineering, Mechanical
Xin Lv
Summary: In classical fracture mechanics theory, it is believed that crack surface is open under tension and closed under compression. However, under small angle uniaxial tension, the stress field at crack tip is small because the crack surface may also be closed. This paper considers both the far-field stress and the possible stress acting on the crack surface, and derives the stress field at crack tip. The crack initiation angles of open or closed cracks under uniaxial tension or compression are studied based on the generalized maximum tangential stress criterion.
THEORETICAL AND APPLIED FRACTURE MECHANICS
(2022)