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

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.