| Logging while drilling (LWD) is an important direction of the modern oil welllogging technology due to its evident advantages compared to the traditional cablelogging, such as the the ability of the geo-steering drilling in real-time and moreaccurate stratum information because the beds have not been polluted by mud whilelogging is going on. Along with the development of LWD, we need to achieve higherdata transmission rate. But the traditional signal transmission can not satisfied therequirement of transmission rate for LWD, which leads the signal transmission becomea critical bottleneck toward further development of the LWD. On the other hand, alongwith the increase of the demand for detecting thin-beds, longitudinal logging resolutionhas to be improved. However, the traditional high resolution logging instruments, forexample, high resolution array induction logging tool, can be hardly applied in the LWD.To solve the two difficulties which are above-mentioned, a broadband signaltransmission method and a broadband electromagnetic propagation logging method arestudied in this paper.Lack of efficiency in transmitting logging signals has long been one of the crucialproblems for the development of logging while drilling. This study aims to address thisissue by exploring into the advanced scheme for logging while drilling signaltransmission proposed by NovatekTM. The main focus of the study is on electromagneticcoupling between two adjacent pipes and the signal transmission in coaxial cablesimbedded in drilling pipes. According to the Axial Symmetry of the electromagneticcoupling structure, the numerical mode matching(NMM) is used to establish thenumerical model for it. Through simulation analysis which based on the numericalmodeling of the electromagnetic coupling structure, the paper analyze how variousparameters of the structure influence the coupling coefficient. The coupling structurebetween the drill-pipe was optimization designed and manufactured. In addition, therectangular transmission line whose characteristic impedance is50ohm is designed forthe cable imbedded in the drill pipe, and the attenuation is calculated. At last, simulation and experiment are performed for one unit of the pipeline. The results are agreed witheach other and show the good transmission performance.To improve the longitudinal logging resolution, a novel broadband electromagneticpropagation logging method with the linear frequency modulation continuous waves(LFMCW) as the well-logging signal has been proposed for logging while drilling(LWD). The logging response of the layered media to the LFMCW under the noiseenvironment has been modeled numerically by using numerical NMM and fast Fouriertransform (FFT). Based on the frequency spectrum of linear frequency modulation(LFM) signal, the NMM is used first to calculate both the amplitude and the phase ofthe signals in receiving coils for every frequency point, and then FFT is applied toconvert them to the logging response in time domain. After that, the matched filtermethod has been imposed to realize pulse compression of the received signals andimprove the peak signal to noise ratio (PSNR) significantly. As the logging response ofthe electromagnetic propagation logging tool, the amplitude ratio and the phasedifference between the compressed signals from two separated coils in the peak pointare obtained respectively. Then, the apparent conductivity based on the amplitude ratioand the phase difference are obtained to get the logging curves. It has been shown fromthe simulation results that the use of the LFMCW as the wideband logging signals canimprove the signal to noise ratio (SNR) and the logging resolution remarkably,especially under the condition with correlated noise which restrained the SNR andlogging resolution strongly in traditional logging with single frequency continuouswave. |
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