Detection test of coal seam hydraulic fracturing range based on multi-frequency synchronous electromagnetic wave CT technology
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摘要: 当前煤层水力压裂影响范围的探测方法多存在实施复杂、工程量大、成本高及准确率偏低的问题,难以满足高效指导压裂施工方案优化和质量控制的需求,且常规电磁波透视探测中单一频率数据分析可能导致探测分辨率和准确性偏低。针对上述问题,提出了一种基于多频同步电磁波层析成像(CT)技术的煤层水力压裂范围探测方法,该方法基于电磁波透视法原理,一次探测可获取多个频率的数据,从不同分辨率综合分析压裂效果,提升探测准确性和效率。以某瓦斯突出矿井的M6煤层为研究对象进行了煤层水力压裂范围探测试验,研究了0.3,0.5,1.5 MHz三种不同频率电磁波对煤层水力压裂的响应特征,分析了水力压裂的影响范围和效果。结果表明:水力压裂后,压裂区主要表现为电磁波场强高衰减特征,电磁波衰减系数明显增大,衰减系数区间变宽,水力压裂导致压裂区和非压裂区对电磁波的能量吸收差距变大,据此可对煤层压裂范围进行划分;0.3,0.5 MHz电磁波探测的异常区基本呈现片状分布,而1.5 MHz电磁波对水力压裂区的响应更灵敏,呈纵向条带状分布,衰减幅度更明显,横向分辨率较好;注水量与电磁波场强衰减系数呈现正相关变化,衰减系数随注水量增加而增大;高衰减异常区分布范围有一定的对应性,试验区压裂范围主要分布在横坐标为40~90,100~140,210~350 m段,对应于2号,3号,6号—8号钻孔周边围岩半径15~25 m,约占探测范围的65%。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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