Optimization And Acceleration Of Joint Inversion Algorithms For Microseismic Location And Focal Mechanism

In the shale gas exploitation,hydraulic fracturing is often conducted to produce artificial fracture to improve the exploitation efficiency of the low permeability and dense oil and shale gas.Evaluation of the hydraulic fracturing is generally achieved by monitoring induced microseisms.Microseismic monitoring is one of the best techniques to provide accurate,timely and abundant evaluation information for reservoir fracturing.Combining the events distribution of microseismic location and the inversion results of focal mechanisms,we can effectively explain the fracture development and stress distribution in the work area.Based the evaluation of the fracturing effect to guide the fracturing design and construction.The monitoring system can be deployed downhole or on the surface.Based on the advantages and disadvantages of the two methods,the latter one is suitable for our study.The data from surface monitoring system has some characteristics of weak signal energy and is difficult to pick up events.Therefore,we have developed the joint source scanning algorithm(JSSA)to determine the locations and focal mechanisms(FMs)of microseismic events simultaneously.This algorithm requires scanning of six parameters of source location(x,y,z)and focal mechanism(strike,dip,rake).Compared with the traditional Source Scanning Algorithm(SSA),the JSSA algorithm can get focal mechanism information,identify more microseismic events,and achieve better location accuracy.However,the JSSA algorithm needs six-parameter inversion.It's much time consuming.By comparing with different scanning methods,the multiple scanning acceleration method is finally used for joint inversion.Through the theoretical tests with different depths of sources,different levels of signal-to-noise ratios,different source positions and different focal mechanisms.The accuracy and stability of the optimized and accelerated JSSA algorithm are verified.The field data of the YSH1-1 well and the WeiH3-1 well are used for joint inversion.The YSH1-1well and the WeiH3-1 well are located in the Zhaotong shale gas demonstration work area and Changning-Weiyuan shale gas demonstration work area respectively.According to the distribution of microseismic events,the statistics of the P-axis calculated from focal mechanism,and other geological information,the triggering mechanisms of cracks are interpreted and analyzed.And then evaluating the fracturing effect and guiding the fracturing construction,the applicability of the optimized accelerated JSSA algorithm and the rationality of the inversion results are verified.In addition,in view of the complex stress state in WeiH3-1 well work area,using the method of regional stress inversion in natural earthquake study,gridding the region of the event distribution,and calculating the direction of the maximum principal stress in each sub-region,a better distribution of stress state can be obtained.Finally,the theoretical tests and field applications of the optimized acceleration JSSA algorithm are summarized and analyzed.It shows that the method itself has better accuracy,stability and applicability.And it can be widely used in microseismic monitoring of fracturing production.It also has important guiding significance for evaluation and analysis of fracturing effect and fracturing development.The optimized acceleration method of multiple scanning can significantly improve the computational efficiency on the premise of ensuring the inversion accuracy.And the method of multiple scanning is the core technology of this study.