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

Abstract

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.

Changing Law of Permeability of Coal Reservoirs under Variable Pressure Conditions and Its Influence on Extraction Efficiency of Coalbed Methane

“…Fitting Equation HM Cp = 0.48095p −2.87622 WY 1 C p = 0.60954p −2.47003 WY 2 C p = 0.22574p −3.66777…”

“…Coal permeability is an important parameter that affects the flow of coalbed gas and the efficiency of coalbed gas 2 of 16 extraction. Many relevant experiments and theoretical and numerical simulations regarding the factors influencing coal permeability have reached important conclusions [2][3][4][5][6].…”

Numerical Simulation of Acoustic Emission Events in Supporting Roadways under Different Dynamic Loads

“…By capturing the AE energy events of loading and unloading specimens and infrared monitoring anomalies, the energy evolution characteristics of the model during the entire loading process were quantitatively analyzed. Li et al [9] conducted hydraulic fracturing (HF) tests under true triaxial stress on threedimensional coal samples in the laboratory and studied the time variation of injection pressure curves and AE waveform parameters at diferent stages through AE-monitoring technology. Te results showed that the characteristics of the AE waveform parameters well refect the growth behavior of coal HF.…”