Acoustic Logging In Cased Boreholes And In The While Drilling Environment

Acoustic well logging is a necessary activity in the process of oil & gas exploration and development,for it can obtain the essential formation elastic wave velocity.Low cost,high efficiency and high security,are the everlasting pursuing goals of scientific researchers and engineers.At present,the open hole wireline acoustic logging technique has became a mature technique,but in some cases,sonic logging in cased hole is needed to obtain the compression wave velocity and shear wave velocity.The presence of the steel casing and cement can significantly affect acoustic logging data.In this paper,theoretical modeling is used to investigate the logging response of a monopole source in cased hole under different cementation conditions.The results show that when it is poorly bonded,a large amount of acoustic energy will propagate along the pipe as casing wave.Due to the strong influence of the casing wave,the formation signal is usually difficult to identify.In order to solve this problem,an acoustic logging technique using dual source of opposite polarity is proposed to enhance the Signal to Noise Ratio of the formation signal while reducing the casing wave in the data acquisition process.In what follows,theoretical modeling was used to calculate the wavefroms of cased wells.Then in the laboratory,a series of physical experiments were conducted in a 1:10 scaled cased hole with the first interface poorly bonded and in the 1:3 scaled free casing well.The results show that this technique can effectively suppress the casing wave and enhance the formation wave,even in the poor bond condition.By analyzing the measured waveforms,the casing wave amplitude is reduced to 12% of that of the single source and the compressional wave velocity can be easily obtained from the measurement technique.Both the theoretical modeling and experiment results demonstrate the feasibility of the proposed technique.In order to verify the validity of the technique in the actual logging environment,the acoustic parameters of the tool's sonde were determined through theoretical simulation,the dual source transmit circuit and multi-channel acquisition circuit were designed,and furthermore,the cased-hole dual source of opposite polarity acoustic logging tool was designed.Then experiments were conducted in Teaching well of China University of Petroleum and Weigu No.1 well of Zhongyuan Oilfield.Experimental results showed that in actual poorly bonded cased hole the dual source of opposite polarity technique has obvious suppression effect on casing wave,the casing wave amplitude is reduced to 10%,and the data processing results of the dual source of opposite polarity showed that the formation compressional correlations can be significantly improved.The actual logging experiment further verified the application value of dual source of opposite polarity technique in cased hole.But it is a difficulty if the formation wave velocity is close to that of the casing wave in the poorly bonded cased-hole.Similarly,during monopole acoustic logging while drilling(LWD),there are some strong tool waves propagating along the drill collar,which,when recorded by the receivers on the LWD tool,strongly interfere with the elastic waves from the surrounding formation and adversely affects the measurement of formation elastic wave velocity.Existing LWD acoustic technology suppresses the tool waves by building an isolator along the collar using groove cutting techniques,which often results in reducing the tool's mechanic integrity.The third chapter of this paper applies the dual source of opposite polarity technique to solve the tool wave problem in the LWD acoustic measurement.Numerical simulations show that the tool waves can be removed if the wave's dispersion effect is minimal.In the presence of significant wave dispersion,the tool waves after suppressing may still exist,but the much reduced tool-wave amplitude allows for obtaining the formation elastic wave velocity using a wave interference method in the subsequent data processing.To prove the concept of using dual source in LWD,an experimental LWD model was built and laboratory experiments were conducted.Various excitation frequencies,ranging from 5 to 16 kHz,were used and the waveforms excited by the dual source system were measured.The results showed that the collar wave can be suppressed for the different working frequencies.Both the theoretical modeling and experiment results demonstrated the feasibility and practicality of the proposed technique.Compared with the traditional LWD technique,the dual source technique removes the need for sound isolation and therefore maintains the mechanic integrity of the tool.Besides,by suppressing tool wave from low to high frequencies,the new technique can operate in a broad frequency range for the LWD acoustic measurement.