Investigating On The Signal Processing Technique To Extract Formation Signal Based On The Interference Of Casing Wave And Formation Wave

The cased-hole acoustic logging has a large proportion in well logging.In order to transform the existing oil field,we need to do the acoustic logging in cased boreholes.The acoustic logging while drilling has developed rapidly in recent years.In the acoustic logging while drilling and cased borehole logging,the existence of extensional waves along the steel pipe(called pipe waves)causes strong interference in the recorded acoustic wave data,making it difficult to extract formation acoustic velocity especially when the pipe is poorly bonded or the effect of the isolator is poor.A signal processing technique has been developed to extract formation acoustic signal in above situation based on the wave interference phenomenon.The acoustic data is modified by the pipe waves to extract the formation information.The technique can effectively process the acoustic data obtained from the LWD and cased boreholes to provide the velocity in real time for the oilfield exploration and development,explore the gas missed by conventional methods and transform the existing oil field.To figure out the influence of the pipe to the acoustic field in the borehole,we have utilized the concentric layered structure model and Thomson-Hanskell propagator matrix method to theoretically analyze and numerically simulate the acoustic field in cased boreholes and LWD.According to the simulation results in free-pipe condition and LWD,it is difficult to extract the formation information as the acoustic field characteristics in the borehole are almost entirely controlled by the pipe and fluid.To solve the above problem,we have respectively utilized the slowness-time-coherence(STC)method,the signal energy threshold method and the frequency-wavenumber correlation filtering method to process the data in different formation.However,the above methods have some limitations,we can not obtain the formation information in some condition.We have introduced a new method(called the waveform interference method)to extract the formation signal based on the wave interference phenomenon.We have verified its validity and effectiveness by using it to process synthetic data simulated in free-pipe condition and LWD.To preliminarily verify its practical application effect,we have used the method to process experimental data obtained from two cased wells at free-pipe condition with smaller proportion,verifying the validity and effectiveness of processed results.To furtherly verify the practical application effect of the new technique,we have processed some field data examples.Firstly,we provided an example of cased well in medium formation and found that the effectiveness of the method can be directly verified when the pipe wave and formation wave have large separation.Then we showed another example of cased well in fast formation and found that the method can effectively suppress the pipe wave and significantly enhance the signal to noise ratio and the correlation of the formation signal.The effectiveness of the formation information can be improved thus we demonstrated the validity of the new method on the application of the field data.