Experimental Investigation of the Effect of Bedding Planes on Hydraulic Fracturing Under True Triaxial Stress

2017 ◽  
Vol 50 (10) ◽  
pp. 2627-2643 ◽  
Author(s):  
Bingxiang Huang ◽  
Jiangwei Liu
Keyword(s):  
Experimental Investigation ◽  
Hydraulic Fracturing ◽  
Triaxial Stress ◽  
True Triaxial ◽  
Bedding Planes ◽  
True Triaxial Stress

scholarly journals Characteristics of Acoustic Emission Waveforms Induced by Hydraulic Fracturing of Coal under True Triaxial Stress in a Laboratory-Scale Experiment

Minerals ◽  
2022 ◽  
Vol 12 (1) ◽  
pp. 104
Author(s):  
Nan Li ◽  
Liulin Fang ◽  
Bingxiang Huang ◽  
Peng Chen ◽  
Chao Cai ◽  
...  
Keyword(s):  
Acoustic Emission ◽  
Hydraulic Fracturing ◽  
Coal Sample ◽  
Injection Pressure ◽  
Triaxial Stress ◽  
Pressure Curve ◽  
Secondary Fracture ◽  
True Triaxial ◽  
True Triaxial Stress ◽  
Study Laboratory

Hydraulic fracturing (HF) is an effective technology to prevent and control coal dynamic disaster. The process of coal hydraulic fracturing (HF) induces a large number of microseismic/acoustic emission (MS/AE) waveforms. Understanding the characteristic of AE waveforms’ parameters is essential for evaluating the fracturing effect and optimizing the HF strategy in coal formation. In this study, laboratory hydraulic fracturing under true triaxial stress was performed on a cubic coal sample combined with AE monitoring. The injection pressure curve and temporal variation of AE waveforms’ parameters in different stages were analyzed in detail. The experimental results show that the characteristics of the AE waveforms’ parameters well reflect the HF growth behavior in coal. The majority of AE waveforms’ dominant frequency is distributed between 145 and 160 kHz during HF. The sharp decrease of the injection pressure curve and the sharp increase of the AE waveforms’ amplitude show that the fracture already runs through the coal sample during the initial fracture stage. The “trapezoidal” rise pattern of cumulative energy and most AE waveforms with low amplitude may indicate the stage of liquid storage space expansion. The largest proportion of AE waveforms’ energy and higher overall level of AE waveforms’ amplitude occur during the secondary fracture stage, which indicates the most severe degree of coal fracture and complex activity of internal fracture. The phenomenon shows the difference in fracture mechanism between the initial and secondary fracture stage. We propose a window-number index of AE waveforms for better response to hydraulic fracture, which can improve the accuracy of the HF process division.

scholarly journals Influence of principal stress effect on deformation and permeability of coal containing beddings under true triaxial stress conditions

2019 ◽  
Vol 6 (2) ◽  
pp. 181438 ◽  
Author(s):  
Jiahui Dai ◽  
Chao Liu ◽  
Minghui Li ◽  
Zhenlong Song
Keyword(s):  
Mechanical Behaviour ◽  
Volumetric Strain ◽  
Bedding Plane ◽  
Stress Conditions ◽  
Triaxial Stress ◽  
Stress Effect ◽  
Direction Parallel ◽  
True Triaxial ◽  
Bedding Planes ◽  
True Triaxial Stress

In situ stress is generally an anisotropic/true triaxial stress ( σ 1 > σ 2 > σ 3 ). Bedding weakens the continuity and integrity of coal. It is critical to understand the mechanical behaviour and gas migration of coal under true triaxial stress conditions. We performed experiments of cubic coal samples to investigate the permeability evolution and mechanical behaviour of coal under true triaxial stress conditions by using newly developed true triaxial geophysical apparatus. We analysed the effect of principal stresses on deformation and permeability characteristics of coal containing bedding planes. The results show that volumetric strain, stress states and bedding directions determine the permeability comprehensively. The variable quantity of strain was the largest in the direction normal to the bedding plane. The expansion or compression degree was characterized by the difference between the major and minor principal strain ( ɛ 1 − ɛ 3 ). Essentially, this represents the difficulty degree with regard to coal being compressed at the initial stress state and the deformation degree in ɛ 1 and ɛ 3 direction. The variation of ( ɛ 1 − ɛ 3 ) was consistent with that of permeability. Under an identical true triaxial stress condition, permeability was smaller when larger stress was applied in the direction normal to the bedding plane. Additionally, stress level in the direction parallel to the bedding planes and the directions between stresses in the direction parallel to the bedding planes and the flow direction also affect the permeability and strain. By solving lateral expansion coefficient, coal also exhibited anisotropic properties.

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