煤层水力压裂诱发微震信号特征及识别研究

Hydraulic fracturing in underground coal seam has been achieved good results in control and development of coal mine gas. However, the research on fractures monitoring hydraulic fracturing in coal seams is not enough at present. In order to monitor and assess the fracturing process and fracture spatial distribution during the hydraulic fracturing of underground coal seam by microseismic monitoring technique, this project will focus on the research of microseismic signal characteristics and recognition induced by hydraulic fracturing of coal seams by using theoretical analysis, laboratory physical experiment and field testing. We will carry out small scale hydraulic fracturing experiments in laboratory and large scale hydraulic fracturing tests in the field, research the microseismic signal characteristics and temporal attenuation characteristics during the hydraulic fracturing process in the coal material. After that, we will reveal the frequency bands energy distribution of microseismic signal induced by coal seam hydraulic fracturing, and build temporal attenuation quantitative solution model of microseismic signal by theoretical analysis. Based on the above study, we will propose and build a microseismic signal characteristic recognition library of hydraulic fracturing in underground coal seam. On this basis, we will finally establish the microseismic event recognition and source location method which is suitable for the hydraulic fracturing of underground coal seam. The research results are the theoretical foundation of microseismic monitoring the hydraulic fracturing of underground coal seam. The research results also have important theoretical and practical significance for understanding the fracture initiation and propagation mechanism of coal seam hydraulic fracturing, and driving the research of monitoring and effect evaluation for hydraulic fracturing in underground coal seam.

井下煤层水力压裂在煤矿瓦斯治理和开发中取得了较好的效果，然而目前在煤层水力压裂裂缝监测方面的研究不够深入，为了实现微震监测和评价井下煤层水力压裂裂缝空间展布形态和压裂效果，本项目拟围绕井下煤层水力压裂微震信号特征和识别展开研究，分别开展实验室小尺度煤岩体和现场大尺度煤层水力压裂实验，测试研究煤体介质水力压裂过程中的声发射和微震信号的频谱特征和时空衰减特性；通过深入的理论分析，揭示井下煤层水力压裂微震信号的频带能量分布特征，建立煤层水力压裂微震信号时空衰减量化求解模型；在上述研究工作的基础上，研究并提出井下煤层水力压裂诱发微震信号特征识别库，建立基于特征识别库的煤层水力压裂微震事件识别和震源定位方法，并进行现场验证和应用。研究成果是井下煤层水力压裂微震监测的理论基础，对进一步认识煤层水力压裂裂缝发生和扩展机理，促进煤矿井下煤层水力压裂监测和效果评价技术发展具有重要的理论和应用价值。

近年来，水力压裂在煤矿井下煤层卸压增透与瓦斯抽采中得到了广泛应用，取得了良好效果。水压裂缝空间形态监测及压裂效果评价直接影响煤层卸压增透范围与瓦斯抽采效率。煤层水压裂缝起裂扩展过程会诱发微震信号，可以采用微震技术对煤矿井下煤层水力压裂过程进行监测。煤层结构松散、节理裂隙发育、非均匀性强，且高压水注入导致煤岩性质发生改变。本项目分别开展了实验室煤样吸水全过程波速实验、煤样水力压裂声发射监测实验和煤矿井下煤层水力压裂微震监测试验；研发了震动波形信号特征分析软件；研究了不同含水饱和度煤样的纵波波速变化规律和波形特征；得到了水力压裂过程诱发的大量声发射/微震波形信号，研究揭示了煤层水力压裂诱发微弱微震波形的持续时间、幅频、时频、频带能量分布特征和衰减特性。研究表明：煤样纵波波速随含水饱和度的增加呈非线性增加趋势，随孔隙率的增加呈线性降低趋势。含水饱和度越大、孔隙率越大，波形最大幅值越小；随着含水饱和度的增加，波形高频成分所占比例逐渐降低。煤层水力压裂诱发微弱微震波形很难区分P波和S波，波形持续时间多数集中在0.2~1 s，振幅主要在0.02~0.2 mV；波形中频带在70~110 Hz之间的能量占总能量的55%左右，30~70 Hz之间的能量占总能量的25%左右，其余频带范围能量占20%左右。微震波形峰后衰减符合负乘幂函数关系，衰减系数主要在0.6以下。. 对比研究了5种不同的诱发微震波形信号差异特征，波形持续时间、主频和衰减系数是3种典型的差异特征参数，差异特征参数及其合理的阀值构成了煤层水力压裂诱发微震波形特征库。在此基础上，研究建立了基于波形互相关的煤层水力压裂诱发微弱微震事件自动识别方法和基于L1范数和单纯形算法的煤层水力压裂微震震源定位方法，并进行了现场试验验证。本项目研究成果对深刻认识煤层水压裂缝扩展机理、防治煤岩瓦斯动力灾害、提高瓦斯抽采效率具有重要的理论和应用价值。