Abstract: The percolation of oil and gas in tight sandstone reservoirs is different from the conventional reservoirs because of its tight lithology,tiny pore and throat,Jamin effect and strong stress sensitivity.In order to understand the influences of pore structures on the percolation of oil and gas,the structural characteristics of micro pores of the Chang 8 reservoir were analyzed based on core and flake observation,SEM (scanning electron microscope) and mercury intrusion porosimetry.The results indicate that the reservoir has low surface porosity,complicated pore types and strong heterogeneity.The percentage of nanopore throats and submicron pore throats in the total pore throats is high(30%～55%)in the cores with permeability lower than 1×10^-3 μm²,while the percentage of micron pore throats in the total pore throats is higher in the cores with permeability higher than 1×10^-3 μm².The contributions of the pores with different throat sizes to the permeability were analyzed with the capillary's model.The result shows that the submicron size pore throats play a major role in the permeability of reservoir in this study.The result of core displacement experiments shows that the minimum starting pressure gradient of oil(S_wc)is negatively correlated with the largest pore sizes.The minimum starting pressure gradient of oil(S_wc)increases rapidly along with the decreasing of pore throat size when the largest pore throat radius is less than 1.0 μm.As the core permeability decreases,the throat distribution curve is shifted to the left,the pore throat radius lowers,and the nonlinearity section of the velocity-pressure gradient curve becomes longer.

Key words: percolation, minimum starting pressure gradient, structural characteristics of pores, tight sandstone, Maling oilfield, Ordos Basin