Optimized Design And Field Data Processing Method Of Acoustic Logging-While-Drilling Tool

Logging-while-drilling(LWD) tecnology has undergone a rapid development recently, becoming a must have technology in land and deep-water drilling. The goal of LWD acoustic measurement is to determine formation compressional and shear velocities even while the well is being drilled. The two elastic wave velocities provide information important for the exploration and production of oil and gas. By providing detailed research of LWD multipole waves from the theory, experiment and the field data processing in the LWD environment, we analyze the measurement principle and the controlling factors affecting multipole acoustic wave propagation along a borehole with the presence of the drill collar. We then develop the measurement technology of the LWD acoustic waves. In particular, we present an innovative LWD acoustic isolation system by combining collars of variable sizes and its optimized design method. The major work of this paper consists of three parts as follows.In the first part, the propagation characteristics and acoustic isolation theory of the collar mode wave are studied. We first provide theoretical foundation and numerical simulation method for LWD multiple acoustic field and analyze the propagating characteristics and inherent stopband of the collar extensional wave. In the following, we theoretically analyze the acoustic isolation theory of periodic cut designs to broaden the width of the stopband using a finite-difference wave simulation method. Based on the analysis, we develop an LWD acoustic isolation technology by varying thickness of drill collars at a distance greater than wavelength. We carry out the experiment with the physical model of actual size, and the results of the experiment confirm that the isolation technique of two-collars combination can effectively broaden the stopband.In the second part, we focus on the optimized design method of the LWD acoustic system, and design an innovative LWD multipole acoustic system that does not use the groove-cutting method. First, we design an LWD isolator with periodically cutting grooves along the drill collar. The designed isolator has been applied successfully to the field test, obtaining a high-quality compressional velocity measurement data in the LWD environment. We then use the optimized method to design two LWD multipole acoustic systems of different sizes(the outside diameter is 6.75 in and 4.75in) with no groove cutting, based on the monopole isolation theory of two-collars combination and low-frequency quadrupole shear wave characteristics of a thick collar. Finally, we build a LWD multipole experimental prototype with the size 4.75 in. Using a compensating method of acoustic attenuation measurement, we measure the frequency range of the stopband(13.5~19k Hz) and largest attenuation value(-58 d B). Uing the glassteel pipe to simulate the borehole condition, high-quality tube wave of monopole and quadrupole are measured, and the collar wave are effectively eliminated by the new isolation method. The processed velocities are respectively close to the compressional velocity and shear velocity of the glassteel pipe.In the third part, the data processing methods of the LWD multipole acoustic logging are studied. First, we study the energy threshold coherence method for the LWD data processing, and apply it to the downhole real-time data processing. The field test of the LWD acoustic experimental prototype shows that the formation compressional slowness obtained from downhole real-time processing is very stable and reliable. The LWD field-data processing examples also present that this method can effectively suppress the interference from the coherence of the collar direct waves, which is helpful for extracting the slowness of monople-compressional wave and quardrupole-shear wave in slow formation. Then, we develop a method using the LWD quadrupole-wave dispersion data to estimate a radial shear-velocity profile near the borehole in this paper. The result shows that the results of the radial shear-velocity profile can be used to provide guidance for the rock brittleness of the tight reservoir. Finally, we use a high-frequency constrained inversion method to solve the nonuniqueness of the inverse problem. The inversion results of the field data validate the method, and clearly show that the near-borehole rock properties can be instantly changed by drilling in those tight reservoirs. This method provides a measure of rock brittleness from the LWD measurement.