煤层水力压裂过程中的微震活动规律及其时频特征

Investigation on microseismic activity and time frequency characteristics of coal seam hydraulic fracturing

为了研究煤层水力压裂过程中的微震活动规律及特征,以山东某深部矿井煤层水力压裂试验数据为基础,提出基于奇异值分解SVD法的微震信号去噪方法,并结合煤层应力监测、管内压力监测等手段,利用短时傅里叶变换STFT法研究了典型水力压裂微震信号的时频特征,最后初步尝试了基于微震指标的水力压裂效果表征。结果表明,伴随水力压裂"注水-起裂-扩展-结束"过程的开展,微震信号特征呈现出规律性变化:信号的振幅值先增后减(由6.64 m V迅速跃升至97.51 m V,后回落至30.61 m V);信号频率普遍分布在30~250 Hz,呈现出缓增趋势(由64.0 Hz升高至109.5 Hz,并短暂维持在105.3 Hz)。最后利用微震事件的空间分布和能量分布,初步表征了水力压裂影响范围及煤层应力迁移规律。上述研究成果为后期利用微震数据解译、评价水力压裂效果提供了参考。

In order to study the microseismic activity and time frequency characteristics of coal seam hydraulic fracturing,we developed a series of research about staged fracturing technology including theoretical research and field test in Shandong Province. The main works were carried out as follows： in the filed tests,the small shear failures were triggered by high response microseismic monitoring system.And we proposed a denoising method Singular Value Decomposition（ SVD） for those microseismic signals. Short-Time Fourier Transform（ STFT） was proposed in this paper to extract feature from microseismic signals in different stages.Microseismic event frequency and energy contours were plotted to characterize the fracture development and propagation process.The interpretation method was initially and successfully applied in the coal seam hydraulic fracturing tests.The test of on-site application shows that the method described above serve as a description method for hydraulic fracturing very well,signal amplitude increases firstly and then decreases（ increase from 6.64 mV to 97.51 mV,and decrease to 30.61 mV）,signal frequency shows a slow rise tendency（ increase from 64 Hz to 109.5 Hz,and keep at 105.3 Hz）and in the range of 30 ~ 250 Hz.Based on the events number and energy distribution,the initial extent of hydraulic fracturing and regular of stress migration is evaluated.The conclusions above can provide some useful idea to quantitative evaluation of hydraulic fracturing in coal mines.