Micro-structural Damage to Coal Induced by Liquid CO 2 Phase Change Fracturing
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Original Paper
Micro-structural Damage to Coal Induced by Liquid CO2 Phase Change Fracturing Zhiwei Liao,1 Xianfeng Liu,1,2,7 Dazhao Song,3 Xueqiu He,3 Baisheng Nie,4 Tao Yang,5 and Longkang Wang6 Received 4 July 2020; accepted 8 November 2020
The technology of liquid carbon dioxide phase change fracturing (LCPCF) was used to enhance the permeability of coal seams. The combination of mechanical tests, scanning electron microscopy (SEM) and high-pressure mercury intrusion porosimetry was adopted to study the damage characteristics of coal micro-structures. LCPCF had mechanical damage effects on coal micro-structures to varying degrees, and the maximum reduction in compressive strength reached approximately 25%. SEM results confirmed that surface morphology of coal was remarkably altered after conducting LCPCF. The fractal dimension (D) of coal subjected to LCPCF ranged from 1.5186 to 1.8794, demonstrating the three-stage changing trends. HP-MIP results showed that LCPCF mainly affected pores of > 100 nm within coal, and pores < 100 nm were hardly influenced. When 1.26 L of liquid CO2 was used to conduct physical blasting, at distance of < 1.5 m, the influence of LCPCF was strengthened. Affected by the high-energy gas and shock wave generated by LCPCF, mesopores within coal were damaged and shifted to the larger pores, resulting in the increase in the number of macro-pores and micro-fractures. When distance was > 1.5 m, the obvious reduction in macro-pore and micro-fracture volumes implied that the fracturing effect was attenuated with the increase in distance. Once distance was > 6.0 m, pore and fracture structures within coal tended to be stable. Thus, in this study, the influence scope of LCPCF was around 6.0 m for a single fracturing borehole. KEY WORDS: Liquid carbon dioxide, Phase change fracturing, Mechanical damage, Pore structure, Fracture morphology.
1
State Key Laboratory of Coal Mine Disaster Dynamics and Control, School of Resources and Safety Engineering, Chongqing University, Chongqing 400044, China. 2 State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University), Jiaozuo 454000, China. 3 School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100091, China. 4 School of Resource and Safety Engineering, China University of Mining and Technology, Beijing 100083, China. 5 School of Safety Engineering, North China Institute of Science and Technology, Beijing 101601, China. 6 Institute of Civil-Military Integration, CCID, Beijing 100048, China. 7 To whom correspondence should be addressed; e-mail: [email protected]
INTRODUCTION Despite the increasing proportion of renewable energy, it is predicted that traditional fossil fuels will continue to play the dominant role in the energy market in the coming decades (Busch et al. 2007; Service 2016; Behera et al. 2018; Yu et al. 2019; Li et al. 2020). With the growing demand for fossil energy, unconventional natural gas has attracted worldwide attention (Ma et al. 2018; Liu et al. 201
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