Multi-scale Analysis Of Acoustic Characteristics In Fractured Or Cavern Carbonate Rocks

Carbonate rocks account for nearly 20% of the total amount of sedimentary rocks,which contain abundant oil and gas resources.Compared with the clastic rock reservoir,the heterogeneity of carbonate reservoir is relatively strong.The distribution of pore-crack and caves in carbonate is very complex,which makes the elastic properties of carbonate reservoir more complex,and it brings great challenge to the fine exploration of oil and gas.In order to study the elastic properties of carbonate rocks,the comprehensive analysis of elastic properties from different scales has important theoretical significance.In this paper,the study of carbonate rocks was carried out on multi-scales,which means the analysis of elastic properties on the pore scale,the quantitative analysis of minerals and core experiments on the core scale,and the study of rock physical modeling on well-logging scale.First,the digital core of carbonate rock is established by X-ray CT scanning on the pore scale,the pore network model was extracted,and the fractured core is established by image processing.Based on the theory of elastic mechanics,the elastic response characteristics of the digital core of carbonate rock and digital core with fractures were calculated by Finite Element Method,and the effects of the micro-factors to the rock elasticity is studied.As the study shows,the bulk modulus and shear modulus decreases as the porosity or shale volume increase,and the velocity of vertical and horizontal waves decreases.The volume modulus of rock increases slowly as the water saturation increasing,however the shear modulus is unchanged;for the core with penny fractures,all the elements of the stiffness matrix of the rock gradually decrease and the wave velocity of rock decreases as the longitudinal and transverse ratio and the crack density increasing,and with the increase of the crack opening and length,the elements of the elastic stiffness matrix gradually decrease.Secondly,on the core scale,the main pore types of carbonate rocks were analyzed,the quantitative analysis of minerals in carbonate rocks was carried out,and the longitudinal and transverse wave velocity of the core is measured,the pore throat parameters of the digital core are extracted using the pore network model,and the results of the core measurement are compared with the numerical results of the calculation of the digital core.The results show that the velocity of the vertical and horizontal wave calculated by the finite element method is different from the measured velocity of the core.With the relative error of the core porosity between the digital core and the measured core,the error of the longitudinal and transverse wave velocity calculated by the digital core and the measured P-wave and S-wave velocity of the core is also greater.Finally,the method of rock physics models for carbonate is explored on the log scale,and the rock physical model is established.The model applied to the actual carbonate reservoir to predict the shear wave velocity.Results show that because of the complex pore-fracture system in carbonate reservoir-compared to the Critical Phi model,the Greenberg model and the Krief model-the Xu-Payne model based on the inverse pore ratio predicts the transverse wave velocity more accurately.In this paper,the analysis and study of carbonate rocks on pore scale,core scale and log scale are carried out,and the comprehensive analysis of the elastic properties of carbonate rocks on different scales has been studied.The research results provide a new perspective for the study of carbonate rock physics,and lay a foundation for the analysis of acoustic characteristics of carbonate rocks on multi-scales.