Abstract
At present, the detection methods for influence range of coal seam hydraulic fracturing mostly have some problems, such as complex implementation, large amount of engineering volume, high cost and low accuracy, which are difficult to meet requirements of effective guiding of optimization of fracturing construction scheme and construction quality control. And single frequency data analysis in conventional electromagnetic wave perspective detection may lead to low detection resolution and accuracy. To solve the above problems, a detection method of coal seam hydraulic fracturing range based on multi-frequency synchronous electromagnetic wave computerized tomography (CT) technology was put forward. The method is based on principle of electromagnetic wave perspective, the detection data of multiple frequencies can be obtained at one time, and the comprehensive analysis of fracturing effect from different resolutions can greatly improve the detection accuracy and efficiency. Taking the M6 coal seam of a gas outburst mine as the research object, the detection test of hydraulic fracturing range in coal seam was carried out. The response characteristics of three different frequency electromagnetic waves of 0.3, 0.5, 1.5 MHz on coal seam hydraulic fracturing were studied, and the influence range and effect of hydraulic fracturing were analyzed. The results show that after hydraulic fracturing, the fractured zone is mainly characterized by high attenuation of electromagnetic field strength. The attenuation coefficient of electromagnetic wave in the fracturing area increases obviously, and the attenuation coefficient range widens. The energy absorption gap of electromagnetic wave in fractured area and non-fractured area is widened due to hydraulic fracturing. Therefore, the fracture range of coal seam can be divided. The abnormal detection areas of 0.3 and 0.5 MHz electromagnetic wave are basically in "sheet" distribution, while the response of 1.5 MHz electromagnetic wave is more sensitive to the hydraulic fracturing area with longitudinal "strip" distribution, more obvious attenuation amplitude, and better lateral resolution. There is a positive correlation between the water injection rate and the attenuation coefficient of electromagnetic field strength, and the attenuation coefficient increases with the increase of water injection volume. The distribution range of high attenuation abnormal area has a certain correspondence. The fracturing range of the test area is mainly distributed in the abscissa of 40-90, 100-140, 210-350 m section, corresponding to the radius of surrounding rock of No.2, No.3, No.6-8 boreholes within 15-25 m, accounting for about 65% of the detection range.
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