Method Research And Application Of The Fluid Substitution Based On Gassmann Equation

With the increasingly high levels of oil and gas exploration and development, the primary reservoir transfers from the stereotype of lithologic oil-gas reservoirs to the subtle oil-gas reservoirs. As one of the major petroleum geophysical technology, seismic exploration has made important contributions for oil and gas exploration. The petrophysics, which studies the relationship between the rock properties and the seismic response, comprehensively analyzes the synthesis of core data, logging data, seismic data to study the influence of the frequency, type of porosity, fluid, saturation, lithology and other parameters on the rock elastic parameters. As the bridge between seismic attributes and reservoir characteristics, petrophysics provides theoretical basis for quantitative reservoir prediction and four-dimensional seismic reservoir seismic monitoring. Fluid substitution analysis, as an important part of petrophysics, can be used in fluid detection and reservoir quantitative study.The basic theory of rock physics is introduced in this paper particularly on fluid parameters for Gassmann equation, including Hashin-Strikman boundaries, V-R-H model, Wood equation, Kuster-Toksoz model, equivalent moduli theory of self-consistent approximation, differential equivalent material model. Basic assumption of the Gassmann equation and the method to calculate the parameters for common fluid substitution is introduced specifically in this paper. Then, based on Gassmann equation fluid substitution method research, the influence of the water saturation on the velocity in gas-water model and oil-water model is studied. The result shows that in gas-water model, when the water saturation below80%, the density plays the major role in fluid substitution for the great variance of the density of gas and water, which is contrary to that in oil-water model.S-wave velocity as one of the important parameters in seismic exploration is often hard to get due to the actual problems, while the empirical formula gives a low accuracy and applicability but simple. The widely adopted method at present is the S-wave prediction method based on Gassmann equation. The Xu-White model is adopted in this paper to predict the S-wave velocity. The influence of the aspect ratio, water saturation, clay content on the model is analyzed on the base of numerical experiment. The result shows that the clay content holds the great influence indicator, the next is the aspect ratio in sandstone, the last is the water saturation. Considering the error influence factors on S-wave velocity prediction which is qualified on the bass of the Xu-White model, the test log data is measured and a good result is observed. Then the lithology analysis is conducted in the two target strata with the method above, mainly include sandstone and mudstone recognition and reservoir fluid recognition. The result shows good applicability of this method.If the reservoir contains fluid, amplitude will change with the offset. That means different angle gather contains different useful information. Particularly in the reservoir which contains fluid, the degree of change is evident. The AVO phenomenon is obvious, but the surrounding rock without fluid is opposite. Thus fluid identification with AVO is feasible. It has proven that using P-wave and S-wave for fluid identification is a more effective way. The fluid substitution technology is combined with the AVO technique in this paper. The AVO seismic response research and AVO attributes inversion study give out the AVO intercept, gradient and fluid factor parameters profile which provides the basis for reservoir prediction.