Source Location,Inversion And Fracture Characterization In Anisotropic Media Based On Acoustic Emission Experiment

In the fracturing monitoring of unconventional oil and gas represented by shale oil and gas,accurate source event location is very important for analyzing fracture strike and evaluating fracture volume.Due to the significant inherent anisotropy of shale,in addition to the influence of noise,first arrival picking,velocity model and other factors,the source location accuracy of shale fracturing process also needs to consider the influence of anisotropy.In addition,the fracture characterization based on the microseismic location results is uncertain,so it is necessary to carry out targeted basic experimental and theoretical research to clarify the applicability and limitation of existing first arrival picking,source location and focal mechanism inversion in fracture characterization.In this paper,a systematic experimental study has been carried out on the Lower Silurian Longmaxi formation shale in Sichuan Basin(the main exploration and development target of shale gas in the South China),including ultrasonic pulse test,“three-point bending” fracture acoustic emission test,and CT scanning before and after sample fracture.The purpose is to explore the correlation between source location results and fractures during shale fracturing,and to clarify the reason why isotropic location results are inconsistent with fracture morphology.In the aspect of signal first arrival time picking,the conventional first arrival picking algorithm based on Akaike information criterion(AIC)is usually affected by the signal-to-noise ratio and the size of time window,and the global minimum value and first arrival time mismatch often occur in low signal-to-noise ratio data.Based on acoustic emission experimental data,an automatic first arrival picking method combining empirical mode decomposition(EMD)and Akaike information criterion(AIC)is developed in this paper,which better solves the problem of inaccurate first arrival picking for low signal-to-noise ratio data.Compared with conventional methods,it can pick up first arrivals more accurately for low signal-to-noise ratio seismic data,and the accuracy of first arrival picking is significantly improved by testing actual low signal-to-noise ratio microseismic data.In the aspect of source event location,there is a significant difference between the acoustic emission location results of strong anisotropic shale samples by conventional methods and the distribution of actual fractures.A new source location method is proposed by introducing anisotropic event matching conditions and anisotropic velocity field,the number of invalid events after matching is reduced,and the source event location accuracy of anisotropic shale is greatly improved.A new understanding was acquired that it is more important to consider anisotropy in event matching phase than in location phase.In the aspect of fracture characterization,the source location and focal mechanism inversion are combined.Through comparative analysis,it is found that the results of anisotropic source location and focal mechanism inversion can better describe the process of rock fracture,and clarify the fracture mechanism of shale acoustic emission experiment.At the initial stage of fracturing,the located source events show the scattered distribution characteristics of “multi-point blooming”,which has little correlation with the fractures.At the late stage of fracturing,a large number of acoustic emission events related to fracture fracture will occur.The located results and focal mechanism solutions at this stage can better describe the fracture.