The Analysis Of Seismic Wave Attenuation And Dispersion In Dual-phase Media And Its Application

Oil and gas is the major source of energy in today’s society. The oil and gas is notonly related to the lifeblood of the national economy, but also closely related to thestrategic development of the country. Exploration and exploitation of fossil fuels, suchas oil and gas, are also essential. With the continuous improvement of the accuracy ofthe hydrocarbon exploration, the simple structural reservoir has basically found,lithological reservoirs and porous reservoirs are the main exploration direction. Basedon the theory of the single-phase elastic medium it has been difficult to solve theproblems faced by the exploration and exploitation of new oil reservoirs. Thehydrocarbon stratum is actually a two-phase media include rock matrix and porousfluid. The dual phase medium theory reflects a subsurface stratum filled with porefluid, has a more reasonable petrophysical assumptions, can be better used forcomplex porous reservoirs.The seismic wave attenuates and disperses when it is propagating through thesubsurface hydrocarbon area, so attenuation characteristics and dispersioncharacteristics can be used as a signs of underground oil and gas detection. At present,the researches of attenuation property got from the dual phase medium theory justenter the development stage, the researches on the dispersion property are much less.In this paper, the finite difference elastic wave equation is used for forward simulationof the two-phase medium, the attenuation and dispersion characteristics of the fastcompressional wave which got through wave field separation are analyzed. Thevariation of Attenuation and dispersion of seismic waves in dual phase medium withthe fluid parameters is researched. By means of time-frequency analysis methods, thetheoretical results are applied to the actual hydrocarbon detection, Combination ofdispersion and attenuation can be better for hydrocarbon detection.For the above objects, in this paper, the following researches will be done. First,the two-phase medium model with the same size as single-phase medium model was designed, By the staggered grid finite difference numerical simulation of elastic waveequation, snapshots of the wave field and a single shot seismogram was recorded. Fastcompressional wave was got by wave field separation, and analyzed for attenuationand dispersion information while propagating in the two-phase medium. Second, thedispersion characteristics of the seismic waves reflected by the phase velocitychanged with frequency, the attenuation characteristics of the seismic waves reflectedby the inverse quality factor. From the Biot equations, the phase velocity and inversequality factor analytical expression of the fast and slow compressional wave in BISQmodel were derived. In the case of different frequencies, the phase velocity andinverse quality factor influenced by permeability, viscosity, porosity, and the length ofthe squirt flow and other fluid parameters were studied. Meanwhile, mostly populartime-frequency analysis methods are summarized. The advantages and disadvantagesof short-time Fourier transform, wavelet transform, the S transform and matchingpursuit time-frequency analysis method were analyzed. Considering the efficiencyand effectiveness, the appropriate time frequency analysis method was chosenthrough the design of signal for the application. Finally, two methods weresummarized to extract the attenuation properties and dispersion properties from theactual seismic signal, the extracted results combined with the front theory and methodwere used for hydrocarbon detection.Through on the above studies, the dual-phase theory can describe undergroundmedia containing fluid very well. Fast compressional wave had significant attenuationand dispersion. The study of the relationship between the attenuation and dispersionand fluid parameters and frequency of seismic wave show that: The fluid parametersaffect the phase velocity of the seismic waves and the inverse quality factor. Seismicwave phase velocity dispersion and attenuation changing versus frequency can beused for seismic inversion that underground formations contain fluid. In the seismicfrequency band, hydrocarbon reservoir with middle and low porosity and lowpermeability has high attenuation and dispersion; on the contrary, the reservoir withhigh porosity and permeability has low attenuation and dispersion. The spectrums ofthe synthesized signal was calculated by short-time Fourier transform, wavelet transform, S-transform and matching pursuit methods, their resolution is progressively.As the best resolution method, the matching pursuit time frequency method is used forreal seismic profiles. The matching result is satisfactory, but the operation efficiencyis poor, and two orders of magnitude more than other transforms. Considering theeffectiveness and efficiency, the S transform is used for the following calculation. Theattenuation and dispersion of field data were extracted through S-transform to detectoil and gas. The results are in accordance with the well logging data. It proved thecorrectness of the theory of the dual-phase medium, and dispersion and attenuationcan be used as a sign of the presence of hydrocarbons and means to detect oil and gas.