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Acoustic emission (AE) is a popular technique to monitor the process of rock failure during hydraulic fracturing for unconventional natural gas production. It contains abundant information that will be useful to study in-depth the nature of hydraulic fracturing. In this study, we focused on the AE count, energy, peak frequency, crack classification, and location recorded from four rock specimens subjected to a specific triaxial stress condition. We found the multi-frequency-response phenomenon of AE, and proposed the multi-frequency-response index to indicate the moment of the macrohydraulic crack formation. Furthermore, it was found that the power law distribution index of AE energy of non-stratified specimen was bigger than that of stratified specimens during hydraulic fracturing. The tensile crack dominated in all hydraulic fracturing tests. Our results are of significance for understanding hydraulic fracturing in stratified rocks.

Hydraulic fracturing can evidently improve the coalbed methane production in underground coal mines, but it is difficult to delimit the stimulated area accurately. In order to evaluate the stimulated area, microseismic (MS) monitoring technique is proposed to investigate the seismic responses of the induced fractures of hydraulic fracturing. Three coal seams were targeted to be treated in a coal mine. An array of geophones was set along the underground roadway to detect the MS signals caused by HF. In order to verify the result of MS monitoring, water content of each coal seam has been measured before and after HF treatment. The results showed that a series of MS events were detected during the entire HF process, and a sharp MS event usually occurred during the first hour of HF process. The energy of the sharp MS event had higher magnitude than others. The MS distribution exhibited complex morphological features. The directionless MS response was distributed over a radius of less than 40 m but tended to be significantly conjugated with a radius of more than 40 m. HF could stimulate both the coal seam and the rock layers nearby. The achievable stimulated area in the coal seam was determined to be 50 m × 50 m according to the MS density and water content. The stimulated area in terms of MS density was easily found to be broader than the area of water direct intrusion. The present study indicated that MS monitoring technique could potentially be used for evaluation of HF in underground coal mine.

Hydraulic fracturing is an efficient technique for enhancing coal seam gas production. Induced stress (or stress shadow) of hydraulic fracturing has been popularly studied recent years. In this study, induced stress was measured in a field during the process of hydraulic fracturing. It was found that the induced stress evolution was closely related to the increase of injection volume. The relation between the average induced stress value and the injection volume was fit by the Michaelis-Menten equation well. When carried out the injection operation, we found that the induced stress increased significantly within the radius of 70 m around the hydraulic fracturing borehole. The induced stress also dropped occasionally during the injection process, which indicated some large fractures’ propagation. When the injection operation was finished, the induced stress began to decrease slowly. The subsequent drilling of gas extraction could release the induced stress. In addition, the induced stresses caused by adjacent fracturing boreholes (spacing was 50 m) substantially influenced each other. The induced stress acted earlier in the upper part of the coal seam than the lower part influenced by the steep incidence of the coal seam. Furthermore, the orientation of the induced stress distribution was eventually parallel to the maximum horizontal principal stress direction controlled by a nearby large fault, although the initial orientation was arbitrary. The results are helpful for hydraulic fracturing evaluation and optimization and numerical simulation research.

Hydraulic fracturing is an effective method to improve the permeability of coal seams and enhance extraction of coalbed methane (CBM). Evolution of cracks in coal seam becomes increasingly complex when one borehole is drilled through multiple coal seams. In this work, physical models with two coal seams having different uniaxial compressive strength (UCS) ratios and height ratios were prepared to study the competitive initiation and extension of cracks based on pump pressure, acoustic emission (AE) source location, and crack morphology. The results showed that, a competitive relationship exists between the two coal seams in terms of crack extension. The pump pressure curves of all specimens presented four stages: (I) fluid-filling stage, (II) pressure rise stage, (III) pressure drop stage, and (IV) pressure stabilization stage. Compared with the pure specimen, the crack initiation pressure of specimens with two coal seams reduced generally. The AE source locations were first generated and mainly distributed in the softer coal seam, resulting in the formation of a complex crack network, whereas only a few cracks were generated in the harder coal seam, as observed after cutting the specimens. In seams with different height ratios, the AE source locations and tracers indicated that cracks preferentially developed and extended in the thinner coal seam. The higher the height ratio of the two coal seams, the more difficult it was for cracks to extend simultaneously in two coal seams. The research results will help improve the permeability of CBM reservoirs when subjecting multiple coal seams to hydraulic fracturing.