Fluid Substitution And Its AVO Analysis In Carbonate Reservoir

Fluid substitution is an important method to identify lithology and fluid inreservoirs, and has been widely used in sandstone reservoirs. Compared with theclastic rock, the reservoir heterogeneities and complex pore structure are seriousproblem for oil exploration and development. Different pore types, such as solutionpores, intergranular pores, and cracks etc., may exist in carbonate rock. Due to thedifficulties caused by all the factors mentioned above, this paper mainly focuses onthe fluid substitution method of carbonate reservoir in Yuanba zone.The existing researches show that the response of pore fluid of carbonate rock inthe research area is very weak because of the tight lithology. This paper attempts tosolve this problem in three aspects. Firstly, fluid substitution technology is used toanalyze the influence mode and degree on the rock elastic parameters caused bydifferent fluid type, in which the dry rock calculation method and accuracy is the keyproblem. In the second place, a reasonable and efficient fluid factor is constructed tohighlight the response characteristics of pore fluid of rock physical parameters.Finally, a new model with three-laminar medium is built to analyze the variationcharacteristics of AVO response on lithology and pore fluid of reservoirs. Then thepore fluid characteristics is extracted from the AVO response.The dry rock moduli calculation method can be divided into two types: singleparameter method and equivalent medium model. Based on log data, and on thepremise of sole model parameter value standard, the reservoir dry rock moduli iscalculated. The results show that equivalent medium model is more suitable toperform moduli calculation. On the basis of rock physical model optimization, acalculation method of dry rock moduli by optimizing pore aspect ratio underconsideration of the influence of carbonate pore structure is put forward. In thismethod, the pore aspect ratio is achieved from velocity inversion, then dry rockmoduli is calculated. Calculation results show that this method can effectivelyimprove the calculation precision of the dry rock moduli.According to the fluid identification ability, the existing fluid factors can bedivided into conventional fluid factors and high sensitivity fluid factors. After acomprehensive analysis on fluid factor, a new fluid factor (F) is put forward whichemphasizes the Poisson’s ratio effects and has the advantage of fluid factor f. This new fluid factor has higher sensitivity on the fluid changes in carbonate reservoir,and with it the fluid response becomes easier to identify. The high sensitivity of newfluid factor is proved by fluid sensitive evaluation factor (R) and the fluid factorcross-plot.Constrained by actual geology condition, a new three-laminar medium modelis built to engage the seismic forward modeling and calculate AVO attributes.According to the forward modeling, how the P-P wave angle gathers and the AVOslope and gradient change with the variation of pore, lithology and pore structure isfully understood. On this basis, the fact that reflection coefficient and gradientbetween different fluid saturated reservoirs have very stable linear relationship and arenot affected by lithology and pore condition is proposed.. What’s more, different fluidin the reservoir can be identified directly on angle gathers if there develop cracks.Based on log data, synthetic seismogram is made and AVO attribute is analyzed. theresult shows that the fluid changes on the P-P wave angle gathers can hardly be seen.It can be challenging to distinguish the different attributes with different pore fluid onP-G cross-plots.Aiming at the fact that the response of pore fluid in tight carbonate reservoir isrelatively weak, this paper certified that dry rock calculation with optimization poreaspect ratio is the priority selection of carbonate reservoir fluid substitution. Thenew fluid factor F can stress the weak response before and after fluid substitution andis of great importance in carbonate reservoir fluid identification. Moreover, newunderstanding on AVO forward modeling is instructive to carbonate reservoiranalysis.