The Study On Pore Structure Characteristics And Seepage Law Of Low Permeability Carbonate Reservoir

Low permeability carbonate reservoir is complex pore structure and strong heterogeneity.There are many diverse types of pore space in carbonates such as fractures and vugs.Combined with X-CT images,carbonate cores are divided into three types: matrix,fracture and vugs.Based on the fractal theory and the High-Pressure Mercury Intrusion(HPMI)experiment,four fractal models were used to calculate the fractal dimension of carbonate rock samples.The relationship between the calculated fractal dimension and the physical properties of carbonate rocks is analyzed.By means of mathematical simulation,the single-phase flow intrinsic permeability model of fractal capillary bundle and the single-phase flow apparent gas permeability model of low permeability carbonate reservoir with slippage effect were established.The validity of the model was verified on the basis of the experimental data of gas permeability and intrinsic permeability.The fractal permeability model was used to predict the permeability of matrix,fractured and cavernous rock samples,and was compared with the Winland permeability model.The results show that the Winland permeability model is more suitable for predicting low permeability carbonate reservoir matrix type and karst cave type rock samples,but not for fracture type rock samples.Based on the tortuous capillary tube model and fractal theory,according to the fractal characteristics of pore size distribution and tortuosity,a novel and more generalized two-phase flow relative permeability prediction model was established.Based on the relative permeability experiment,the validity of the model was verified.The overall result shows that the fitting effect between the gas-water relative permeability obtained in the experiment and the relative permeability calculated by the model is very well.It is found that there is a strong consistency between the model prediction and the existing experimental data.At the same time,the factors affecting the relative permeability model parameters are analyzed.