Numerical Simulation Investigation On Acoustic Transmission Characteristics In Logging While Drilling

The modern drilling is the high level of drilling which included geosteering drilling,logging while drilling and high-speed data transmission technique. Study on the transmission characteristics of acoustic along drillstrings can help to exploit the high speed data transmission techniques using drillstrings,research the acoustic logging instrument while drilling. All these have practical application values for upgrading the intelligent drilling technique in our conutry.In order to eliminate the collar wave's effect on compresional wave of formation, the groove's drill collar were used to reduce the collar wave based on the principle of wave isolating with breaking structure section, the relationship between groove parameters with acoustic characteristics and mechanical characteristics of isolator made up of periodic ring groove structure were calculated. The results show that groove's structure periodic decided the position and width of stop bands. The time-delayed and the sound insulation increased with the number and depth of the grooves; maximum Von Mises stress increase with deepening the depth of the groove but decrease with increasing the radius of the transition arc. Isolator can influence the stonely wave, interfering signal appeares pre-and-after stonely wave, the high amplitude and the"pre narrow and after wide"characters no more clearly.Avoiding the drill string fatigue failure caused by the resonance, this paper use the equivalent network method to derive the drillstring's equivalent mechanical impedance of longitudinal vibration and torsional vibration, and then the principles and methods for determining the mechanical resonance frequency of torsional and longitudinal vibration using the mechanical equivalent impedance have been given. The conclusions indicate that using the equivalent network method to calculate the resonant frequency of drill string system is an effective method, it is easy to use and its calculating results are reliable.It's very useful for LWD data transmission using drillstrings as the channel for carrier propgation. The low frequency acoustic propagation characteristics in periodic drillstring,non periodic drillstring and real drillstring were calculated using finite elements method and theory computation method in this paper. The results show that periodic drillstrings have pass band and stop band just like the comb filter, but the nonperioic drillstrings pass band become narrow while stop band become wide. The position and width of pass band and stop band were determined by the length of the drill pipe and coupling. It also excites wave propagate in fluid and formation when acoustic wave propagate along the drillstrings in borehole filled with liquid, this will increase the wave attenuation along the drillstrings.Based on the acoustic characteristics of drill string, after modulating with three different methods such as amplitude shift keying,frequency shift keying and phase shift keying, respectively the binary date, which produced by coding the LWD data, were tansmited along drillstrings without noise. After modulating,propagation,receiving and demodulating, the demodulated signals were in good with original signal.This paper is financed by the Shandong Province Natural Science Foundation of China and China National Petroleum Corporation (CNPC).