Search

• Key MFCC factors for limestone and coal distinction was identified. • Mechanics behind MFCC's differentiation of AE in rocks was clarified. • Rock fracture mechanics was revealed by AE parameters. Fractures in the rock surrounding karst caves can result in substantial surface subsidence, especially in mining areas with abundant carbonate rocks. Acoustic emission (AE) is a crucial signal associated with rock fracture mechanics and lithology. This study aimed to distinguish the AE features between coal and limestone using the MFCC-based method and universal AE analysis method in conjunction with uniaxial compression tests. The results indicated that the MFCC-4–6 of AE could distinguish between coal and limestone prior to rock failure. The MFCC-1–6 of limestone exhibited a tail-like pattern change on the stress sawtooth and a step-like pattern change after reaching peak stress. The fluctuation degree of MFCC-2–5 exhibited an initial transition from small to large, followed by a subsequent transition from large to small, reaching its maximum value during the elastic deformation stage. Additionally, the power law distribution index of AE energy in the magnitude range of 102–104 for coal was larger than that for limestone. The AE events in coal were mainly characterized by shear fractures, while limestone exhibited more tension fractures than coal, as indicated by the AF/RA analysis. Furthermore, the lower the AE peak frequency, the larger the scale of fracturing in coal and limestone. These findings are valuable for ground control and early warning of rock failure. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image • Multi acoustic emission characteristics were combined to explain the rock failure of hydraulic fracturing. • A stratified formation was particularly studied comparing with the single stratum. • The multi-frequency-response index was proposed to indicated the macrohydraulic crack formation. • The type of ruptured stratum was diagnosed with the statistics of acoustic emission energy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]