微地震双差定位成像技术及煤层水力压裂的应用

Hydraulic fracture in coalbed methane drilling using microseism double-difference tomography method

在煤层气水力压裂工程中,微地震传感器记录的数据不仅包含了震源位置信息,同时还携带了弹性波旅行途经的地层速度结构信息等。利用双差定位成像方法对煤层气开采水力压裂微地震监测数据进行反演处理,可在确定地层裂缝位置的同时获得地层岩石弹性速度空间分布特征信息。首先,基于地震波射线理论,介绍了经典双差震源定位的基本算法可获得消除地层速度结构扰动影响的、具有较高精度的震源位置解,并以此阐述了双差定位成像算法;其次,根据煤层气的赋存条件构建了4层水平层状介质地质模型,模拟符合微地震震源空间分布和激发时间的弹性波场,采用以钻孔井口为中心的“五星”加“两环”观测系统记录数据,分析表明双差定位成像方法的微地震事件定位和地层速度层析成像结果较优于传统方法;最后,对实际煤层水力压裂微地震监测数据进行应用实践,获得微地震事件的空间和时间分布规律等,结合水力压裂施工参数不仅可以获得煤层水力压裂裂缝空间延伸扩张展布特征和局部地层速度结构,也可以推测煤层天然存在的裂缝发育方向和地应力的分布特征等。由于双差定位成像不受弹性波射线路径速度非均匀性的影响,获得微地震事件周围地层速度结构成像,较高的定位计算精度有利于对微地震波旅行路径的地层速度结构较好成像。

The data which are recorded by micro-seismic sensors in the coal-bed methane hydraulic fracturing project not only contain focal mechanism information,but also carry the information of stratum velocity structure through which elastic wave travels.Applying the processing and inversion method to micro-seismic monitoring data of hydraulic fracturing in coalbed methane exploitation can reveal the abnormal velocity of strata while determining the locations of rock fracture which generate micro-seismic events.Firstly,this paper introduced the basic algorithm of double-difference locations based on seismic ray theory,which can obtain high-precision source location solution because of eliminating the disturbance of stratum velocity structure,and then expounded the double-difference tomography algorithm.Secondly,the geological model of four horizontally layered media was constructed according to the occurrence conditions of coalbed methane,and the elastic wave fields which conform to the spatial distribution and excitation time of micro-seismic sources were simulated related to rock body.The authors had recorded a significant amount of data by the special monitoring layout around the borehole.The processing results show that the accuracy of micro-seismic event location and formation velocity images by using the dual-difference tomography method are better than those by using the conventional methods.Finally,the application of the actual coal seam hydraulic fracturing micro-seismic monitoring data to obtain the spatial and temporal distribution of micro-seismic events,combined with the hydraulic fracturing construction parameters,could not only obtain the spatial extension and expansion characteristics of hydraulic fracturing fractures and local formation velocity structure,but also infer the natural fracture development direction and the distribution of ground stress in coal seam.Because the double-difference tomography method is not affected by the non-uniformity of elastic wave ray path velocity,the higher locations accuracy is conducive to the better imaging of the formation velocity structure of micro-seismic wave travel path.