Study on in-situ testing technology for microscopic pore damage in coal induced by ultrasonic cavitation
[Objective]The existing research on ultrasonic stimulation of coal seam gas extraction remains limited to the realms of thermal and vibrational effects,lacking a scientific understanding of ultrasonic cavitation damage to coal pores,leading to an incomplete theoretical framework.Therefore,there is a need to promote the practical application of ultrasonic-induced coal seam gas extraction technology and to conduct an in-depth exploration of the scientific connotation of ultrasonic cavitation damage to coal pores.A self-developed ultrasonic cavitation damage cracking experimental system for coal pores and in-situ X-ray nano-CT scanning technology were utilized in this experiment to investigate the changes in coal pore structure under varying ultrasonic cavitation conditions and explore the mechanism behind the ultrasonic cavitation damage induced on the microscopic pore system of coal.[Methods]This study utilizes Phoenix v|tome|x s,a multifunctional high-resolution nano-CT scanner,for the detection of coal pore structure damage.Equipped with state-of-the-art nano-X-ray tubes,this instrument demonstrates exceptional resolution capability,enabling clear visualization of microstructural defects and accurate acquisition of internal structure and topography information.Moreover,it facilitates precise,three-dimensional analysis.[Results]Regarding the comparison of the CT scanning results of the same sample before and after treatment,small scanning position errors lead to significant differences in the results,making it impossible to accurately compare and analyze the pore reconstruction results before and after treatment.To solve this experimental technical problem of structural comparison distortion caused by scanning position errors,the coal sample is firmly fixed on the carrier needle throughout the ultrasonic cavitation treatment process to ensure in-situ observation of the pore microstructure.Second,during the untreated CT scan of the coal sample using ultrasound,the fixed sample is loaded onto the rotating sample holder;subsequently,the distance between the nanofocus X-ray tube and the sample is repeatedly adjusted to ensure that it is as small as possible,and the optimal initial angle between the sample and the X-ray tube is marked,ensuring that the initial angle of the CT scan of the coal sample after processing remains consistent with this angle.The scanning voltage and current are continuously adjusted to the optimal value based on the initial scanning image to eliminate image noise,thereby reducing the error caused by variations in the scanning parameters across different processing samples.[Conclusions]The experimental results demonstrate the following:1)the ultrasonic cavitation effect significantly enhances the volume of interconnected microscopic pores in coal,expands the spatial configuration of connected clusters through pore depth damage,and extends the topological structure to encompass vacant regions.2)The ultrasonic cavitation effect enhances the microstructural characteristics of the pore space of coal,transforming it from a slit-type to a ball-type configuration.This facilitates communication between the initially isolated small-sized pores and interconnected pore networks.However,its impact on larger-sized pores is limited.3)The ultrasonic cavitation effect significantly contributes to the improvement of coal micropore topology,particularly by increasing the equivalent diameter of the throat.In addition,nonconnected micropores can be induced to connect,thereby enhancing coal micropore topology.The ultrasonic cavitation phenomenon observed in this study provides a theoretical foundation for the application of ultrasonic antireflection technology in coal and possesses significant practical value for enhancing coal permeability and promoting gas extraction.In addition,this study highlights the effectiveness of ultrasonic cavitation in high-pressure water environments,resulting in coal undergoing hydraulic fracturing.
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