Experimental Study On Relationship Between Nuclear Magnetic Resonance T₂ Spectra And The Pore Structure Of Tight Rock

In the conventional reservoir,nuclear magnetic resonance?NMR?is one of the commonly used techniques to characterize the pore structure.T₂ distribution could continuously characterize the pore size distribution,also could effectively characterize the complexity of the pore structure.In order to study whether there is a certain relationship between T₂ distribution and the pore structure in the tight reservoir,scanning electron microscope?SEM?,nitrogen adsorption,high pressure mercury intrusion and NMR were used on the tight limestone in Daanzhai member of Ziliujing Formation in Jurassic System in the central of Sichuan Basin.Firstly,the pore structure of the tight limestone was analysed with the combination of SEM,nitrogen adsorption,high pressure mercury intrusion.The results show that pores are dominated by organic matter pores,interparticle pores and microfractures,a little by intraparticle pores;the adsorption isotherms show that samples mainly develop slit mesopores;pore volume and specific surface are mainly contributed by mesopores and macropores,which provide pore space for oil and gas storage;the full-size pore size distribution was characterized with the combination of nitrogen adsorption and high-pressure mercury intrusion.Then,the pore structure was analysed by NMR,and T₂distribution was converted into the pore size distribution based on power function.The conversion results match better with the full-size pore size distribution,the geometric mean value of NMR T₂ has a certain correlation with the full-size pore structure parameters of samples.Based on comparison of the movable water saturation calculated by the NMR and the full pore permeability contribution,there is no correlation.Based on the experiments and analysis,we found that in the tight reservoir,the T₂time and the pore size are power function relationship,and the geometric mean value of T₂ could be used as the characteristic parameter to evaluate the pore structure of the reservoir.In the tight reservoir,there are a large number of fluids in the micropores and mesopores.