Research On The Method Of Seismic Gas Detection Based On Time-frequency Analysis

In the process of the exploration and development of oil and gas, the problemabout gas detection of reservoir has always been a difficult one for geophysicalworkers. The key of gas detection is how to show the information of reservedreservoir fluid in seismic data. This thesis provides a more efficient and rapid methodof gas detection and applies the technology in the real seismic data by the research oftime-frequency analysis technology and the characteristics of the earthquake responseof the gas reservoir.The author first started from the time-frequency analysis technology method andstudied several commonly used methods of frequency analysis in the current seismicdata：Short-time Fourier transform, Hilbert-Huang transform, Wavelet transform and Stransform. The author further studied the theories and algorithms of four commontime-frequency analysis technology and summarized the study results of thepredecessors, then analyzed their respective strengths and weaknesses and applicablesignals. By the comparative analysis of analog signal and actual seismic signal, Stransform was selected as the main time-frequency analysis technology in this study. Stransform can not only transfer seismic signals into the two-dimensional histogramtime-frequency but also analyze the signals both from the aspects of time andfrequency. The design of window function is more suitable for seismic signalsbecause it has high frequency resolution in low-frequency parts and high temporalresolution in high frequency as well as its strong convergence of signal energy andhigh resolution, which can process the seismic signals by frequency division, theextraction of single frequency, instantaneous frequency and instantaneous phase andso on.Gas detection technique is the essential link of reservoir exquisite description.While seismic wave passing through reservoir, fluids filled in the pore of reservoirgenerate the attenuation of seismic wave, which includes amplitude, frequency andphase. Various fluids have different amplitudes of attenuation of seismic wave,different fluids, therefore, can be discerned by analyzing the attenuation of seismicwave in reservoir. This thesis provides detailed information on time-frequency features of the reservoir reflection seismic wave by the use of S transform andanalyzes the research on reservoir cap, the amplitude and frequency of bottomreflection wave through instantaneous frequency attenuation so as to find theory andmethod of detection in the feature of gas bearing. Three geological models aredesigned: water cut, containing air and dry rocks.The record of seismic was acquired by putting forward the viscoelastic waveequation. And then the gas detection was conducted by the respective analysis ofreservoir cap and the attenuation of bottom seismic wave and made a conclusionabout the difference between containing gas and water cut.The author combines the technique of S transform time-frequency analysis andgas detection of frequency attenuation,applying which in the real seismic data inTarim and Beibu Gulf, and then proceeds the instantaneous frequency attenuationhydrocarbon detection of reservoir, finding a relatively high goodness of fit betweentesting result and real-time drilling.This thesis further studies the technique of S transform time-frequency analysisand gas detection of frequency attenuation and proves the feasibility of the method byforward modeling, then finally obtains distinct improvement in approving theadaptability and reliability of this method by applying the real seismic data associatedwith drilling data. Any single technique, however, has some defects. Therefore, onlyby combing various gas detection techniques can a more precise and quantifiedprediction about the reservoir fluid be achieved.