一种利用双源反激的随钻声波测井方法

随钻单极子纵波测井时存在很强的钻铤波的干扰.针对这一问题,提出了一种双源反激随钻声波测井方法,从数据采集环节对钻铤波进行压制;在后续数据处理中结合声波干涉法,从而实现一种无需隔声装置的随钻声波测井方法.理论模拟表明双源反激方法可以有效压制钻铤波,地层波的信噪比也能得到相对增强.对钻铤波压制后的数据进一步用声波干涉法处理可以从中提取地层纵波信息.设计了实验样机,并在空气中进行了实验.结果表明不同工作频率下该方法对钻铤波均有明显的压制效果,即使在5kHz低频情况下钻铤波也能压制到原来的25%.验证了该方法的可行性.应用该方法的好处是可以避免对钻铤强度的破坏,并能扩展现有随钻声波测井仪纵波测量的工作频带范围.

An acoustic logging while drilling technique using the dual source of opposite polarity

During monopole acoustic logging while drilling(LWD), there is a strong tool wave propagating along the drill collar, which, when recorded by the receivers on the LWD tool, strongly interferes with the elastic wave from the surrounding formation and adversely affects the measurement of formation elastic wave velocity. Existing LWD acoustic technology suppresses the tool wave by building an isolator along the collar using groove cutting techniques, which often results in reduction of the tool's mechanic integrity. This study proposes a new technique to solve the tool wave problem in the LWD acoustic measurement. This technique uses the dual source of opposite polarity to enhance the Signal to Noise Ratio of the formation wave signal.Two identical acoustic sources having opposite excitation polarity are placed along the LWD tool, with respective source-receiver distances. The source with longer distance is first fired, exciting elastic wave propagating in the formation and tool wave propagating along the collar. When the tool wave arrives at the second source, the source is actuated, exciting another tool wave with opposite polarity. By this firing scheme, the tool wave along the drill collar can be largely suppressed. Numerical simulation shows that the tool wave can be removed if the wave's dispersion effect is minimal. In the presence of significant wave dispersion, the tool wave 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 a laboratory experiment was 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 show that the collar wave can be suppressed for different working frequencies. Even down to 5 kHz frequency, the collar wave can be reduced to 25% in the experiment condition. Both the theoretical modeling and experiment results demonstrate 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.