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.
Highlights
The results show that the microseismic/acoustic emission (MS/acoustic emission (AE)) waveform produced by rock failure under high-pressure fluid has significant characteristics [6–8]
It is thought to be of great significance to carry out AE monitoring experiments of true triaxial Hydraulic fracturing (HF) of coal samples and systematically study AE waveform characteristics of coal samples in different stages of true triaxial HF for AE monitoring and the evaluation of coal
This study aimed to reveal the characteristics of the dynamic growth process of hydraulic fracture in coal through the analysis of the AE monitoring data
Summary
Hydraulic fracturing (HF) is a widely used rock failure and reservoir modification technology [1–3]. Under high-pressure fluid, a fracture occurs in coal and rock mass, and a hydraulic fracture network structure is formed. Many studies show that rock materials will release energy in the form of elastic waves in the process of deformation and failure. Microseismic (MS) monitoring/acoustic emission (AE) monitoring technology can continuously collect the elastic waveform generated in rock deformation and failure in real-time. The frequency range of MS monitoring is generally distributed in 10–1000 Hz, while the general monitoring range of AE monitoring system is 20 kHz–20 MHz. Acoustic emission monitoring systems are generally used in laboratory physical simulation tests to monitor coal rock fractures. The results show that the MS/AE waveform produced by rock failure under high-pressure fluid has significant characteristics [6–8]. It is thought to be of great significance to carry out AE monitoring experiments of true triaxial HF of coal samples and systematically study AE waveform characteristics of coal samples in different stages of true triaxial HF for AE monitoring and the evaluation of coal
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