A multiscale comprehensive study on pore structure of tight sandstone reservoir realized by nuclear magnetic resonance, high pressure mercury injection and constant-rate mercury injection penetration test

There is a large error in characterizing the pore structure of tight reservoirs in a single test. Based on this, the present study selected five samples of tight sandstone, and a multiscale comprehensive study was performed on the pore structure of tight sandstone reservoirs by scanning electron microscopy (SEM), casting thin sections (CTS), high-pressure mercury injection (HPMI), and constant-rate mercury injection (CMI) tests. The combined nuclear magnetic resonance (NMR) and HPMI test was performed to calculate the connectivity ratio of different samples, and evaluate the connectivity of tight sandstone reservoirs. Furthermore, the combined NMR and CMI test was performed to recalculate the pore throat radius of the sample, from quantitative characterization to qualitative analysis. This not only solves the shortcomings of the HPMI and NMR test, in which the throat could not be distinguished from the pore, but also solves the shortcomings of the CMI test, in which the pore radius calculated is larger than actual one and does not correspond to the CTS. On this basis, the lower limit of the movable fluid pore throat radius of tight sandstone reservoirs was investigated, and the results revealed that the lower limit of the movable fluid pore throat radius was not a constant value, in which the T-2 cutoff value corresponded to the pore throat radius, but a range that was determined through the saturation water curve and centrifugal curve. The upper and lower bounds of this range were r(1) and r(2), respectively. According to r(1) and r(2), the fluid state can be divided into three regions on the T-2 spectrum: irreducible region, transition region, and movable region. These results revealed that as the reservoir became tight, the r(1) and r(2) increased, and the proportion of irreducible fluid in the pores gradually increased.