Study Of Discriminating Fluids In Tight Gas Reservoir By Using Seismic Ray-path Elastic Impedance

The porosity and permeability of tight gas reservoir are very low and the fluids discrimination by using seismic methods is very difficult.Exploring new methods to discriminate fluids in tight reservoir has been one of the important elements of geophysical research.In reservoir of Mesozoic and Paleozoic which is in the north of Ordos Basin,the distribution of reservoir fluids is complicated.Therefore the deployment of drilling location is very difficult in Sulige gas field.The existing seismic prediction methods could not identify the fluids effectively.In this paper,we established a new method to finish fluid detection in tight gas reservoir with AVO and ray-path elastic impedance based on petrophysical theory in porous media.Firstly,by using known dipole sonic logs,an improved Xu-White method has been proposed to predict shear wave velocity logs of those wells without shear wave velocities in gas formation.Secondly,under the premise of reasonable and accurate rock physical model,we analyzed the influence degree and sensitivity of petrophysical parameters on elastic parameters.Furthermore,we built ray-path elastic impedance and AVO forward model in both angle and ray parameter domain in two layer model as well as well log model.Meanwhile,we studied the changes of reflection amplitude caused by gas saturation and compared the accuracy of REI and El with exact one from Zoeppritz equation.Finally,according to the above theoretical research and the analysis of crossplot,we found the sensitive factor to discriminate fluids and then made the identification of lithology and fluids in ray-path elastic impedance inversion data.My study shows that:1)When calculating the elastic parameters of the mixed fluids,the Brie empirical formula is more sensitive to the changes of gas saturation than the Wood equation.It can better simulate the elastic parameters under different gas saturation;2)Porosity influences elastic parameters much more than that of gas saturation.While gas saturation has greater impact on elastic parameters than the clay content;3)By improving the calculation method of shale pore aspect ratio in Xu-White model,we propose the nonlinear fitting formula of shale pore aspect ratio and sandy content.Then we use this new method to predict shear-wave velocity in gas formation.The average error between predicted and measured shear-wave velocity is 5.2%;4)Through forward models from both two layer and well log models in angle and ray parameter domain,we analyzed the influence of gas saturation on the ray-path elastic impedance and reflection amplitude.The accuracy of REI and EI is studied by AVO analysis,and we can conclude that the REI has higher accuracy than El;5)Compared with angle domain,the changed of reflection coefficient is larger and obvious in ray parameter domain with the changes of ray parameter.So that the study of ray parameter domain can reflects the change rule and trend of reflection amplitude more clearly and intuitively;6)the crossplot of ray-path elastic impedance(the ray parameter is equal to 0.10)with P-wave velocity is a good way to distinguish the gas reservoir.We can set up the sensitive factor to identify fluid in the crossplot;7)Using the sensitive factor to the ray-path elastic impedance inversion profile in which the ray parameter is equal to 0.10,we made the discrimination of reservoir lithology and fluids.Our prediction and discriminating of gas reservoir is consistent with daily gas production of wells.