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| Temporal, frequency and spatial characteristics of acoustic emission in specimens fractured with different fluid media |
| CHONG Zhaohui1, 2, 3, YU Chenshi1, YAO Qiangling1, 3, LI Xuehua1, SU Fengsheng1, WANG Shuo1 |
(1. School of Mines, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China; 2. Key laboratory of Deep Earth Science and Engineering, Ministry of Education, Sichuan University, Chengdu, Sichuan 610065, China; 3. Jiangsu Engineering Laboratory of Mine Earthquake Monitoring and Prevention, China University of Mining and Technology, Xuzhou,
Jiangsu 221116, China) |
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Abstract Hydraulic fracturing is predominantly utilized in coal and rock fracturing; however, it increasingly reveals various drawbacks. In recent years, scholarly attention has shifted towards incorporating additives into water or directly employing water-free fracturing fluids. Consequently, the comparative evaluation of fracturing effectiveness using different fluid media has become particularly crucial. This study utilized a self-developed true triaxial fracturing experimental apparatus for multi-field and multi-phase fluids to conduct fracturing experiments on specimens using guar gum aqueous solution, pure water, nitrogen (N?), and supercritical carbon dioxide (CO?) under varying confining pressures. The fracture propagation patterns in specimens fractured with different fluids were analyzed based on multi-dimensional acoustic emission (AE) data, including temporal, frequency, and spatial localization characteristics. The results indicate that the AE ring count in specimens fractured with liquid media exhibited cyclic fluctuations, whereas specimens fractured with non-liquid media showed a sharp surge in ring-down count near the fracture initiation pressure. The proportion of shear fractures in specimens fractured with guar gum solution consistently ranged from 50% to 60%, demonstrating little difference from tensile fractures. Conversely, specimens fractured with other fluids were predominantly characterized by tensile fractures, often coexisting with mixed tensile-shear fractures. The -value for specimens fractured with liquid media hovered around 1.0, even exhibiting a decreasing trend. In contrast, specimens fractured with nitrogen and CO? displayed an inflection point in the -value, rising sharply near the initiation pressure, indicating significant energy dissipation and specimen instability in the later experimental stages, which led to the formation of complex fracture networks. The dominant AE frequencies in the time domain for specimens fractured with guar gum solution and pure water were primarily distributed within the ranges of 200–400 kHz and ≥800 kHz. In contrast, the dominant frequencies for specimens fractured with nitrogen and CO? were concentrated entirely below 400 kHz, exhibiting a “multi-spectral-band” signal pattern. Following the achievement of initiation pressure, the occurrence of AE-located events began to increase rapidly, coinciding with the emergence of high-energy events for all fluids. The propagation and coalescence of micro-fractures ultimately resulted in the formation of macroscopic fractures. The fracturing process with liquid media was relatively moderate, whereas specimens fractured with nitrogen and CO? developed complex fracture networks. This phenomenon is attributed to the instantaneous fracturing resulting from the high-pressure accumulation of non-liquid media within the specimens.
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2025, Vol. 44 
