A new method to establish NMR T-2 spectrum based on bimodal Gaussian density function: A case study of tight sandstone in East China Sea Basin

摘要

The identification and evaluation of tight sandstone reservoirs has been very difficult due to their low porosity, low permeability, complex pore structure and strong heterogeneity. In order to evaluate the quality of tight sandstone reservoir, priority should be given to the quantitative evaluation of pore size. Nuclear magnetic resonance (NMR) log is frequently employed in pore size evaluation; however, due to its high technical costs and other limitations, it can only be conducted in very few wells and the wide application can never be found. In this study, 12 core samples from the tight sandstone in East China Sea Basin have been collected to conduct centrifugal NMR experiments. Based on the analysis of data obtained from experiments, it is discovered that the bimodal Gaussian density function is well fitted to the NMR T-2 spectrum and can be applied to the fitting of NMR T-2 spectrum. Furthermore, in the analysis of physical property parameters of reservoir and each individual parameter of bimodal Gaussian density function, it is indicated that the porosity and permeability are best correlated with them. Therefore, the empirical formula between porosity, permeability and the parameters of bimodal Gaussian density function were established to construct pseudo NMR T-2 spectrum by using bimodal Gaussian density function and it has been verified by the comparison between the result and experimental data. In order to achieve the more accurate and wide application of this method, the NMR T-2 spectrum versus proportion frequency curve was transformed into the pore radius R versus porosity component P-phi curve before the fitting. When used in exploration field, the method can construct continuous pore size distribution curve which can be used for the quantitative evaluation of tight sandstone reservoir's quality. Compared with the method of NMR logging, this technique can be applied to the evaluation of pore structure of tight sandstone reservoirs more widely.