A QUANTITATIVE EVALUATION METHOD OF LOW PERMEABLE SANDSTONE PORE STRUCTURE BASED ON NUCLEAR MAGNETIC RESONANCE (NMR) LOGGING: A CASE STUDY OF Es4 FORMATION IN THE SOUTH SLOPE OF DONGYING SAG

Low permeability sandstone has become an important target of exploration and development for increasing reserves and productions, while the complicated pore structure makes the reservoir and its effectiveness difficult to accurately identify. We used the physical property, mercury injection, and nuclear magnetic data to analyze the pore structure of low permeability sandstone of the Es₄ in the south slope of Dongying sag, and divided the pore structure into three different types. Nuclear magnetic T₂ spectrum and capillary pressure can reflect the distribution of pore throat to a certain extent. The pseudo capillary pressure curve reconstructed by T₂ spectrum with the routine method can be used to invert for the pore radius distribution, but there is a large error between the pore radius distribution and capillary pressure pore-throat radius distribution. The rock pore free fluid T₂ and mercury intrusion pore throat size distribution have a better corresponding relationship. Therefore, this relationship is used to construct the formula for different pore structure types and on different pore scales (large scale: the linear relationship; small scale: power function in different scale). By identifying the pore structure types along the wellbore profile, we can further determine the pore radius distribution using NML data without building the pseudo capillary curve. And this method not only supply a direct evidence for efficiency evaluation of low permeability sandstone reservoir, but also play an important role in the exploration of quantitative inversion of the pore radius distribution on a micro-scale.