Characterization of pore structure in tight sandstone reservoirs based on NMR

Tight sandstone reservoirs are characterized by low porosity, low permeability and complex pore structure, and there are difficulties in the evaluating tight sandstone reservoirs. Nuclear magnetic resonance (NMR) T2 spectra can characterize the pore structure and the fluid state of the porous medium, so we combined the NMR experiments with the gas-driven water to study the influence of different gas saturation and saturation states on the T2 spectra of the tight sandstone reservoirs. It is found that the movable peaks of T2 spectra become wider in the gas-bearing state of the pores compared with the water-bearing state, and the movable peaks decrease with the increase of gas saturation, and the porosity tends to become smaller, which is due to the lower hydrogen index of the gas. Therefore, the T2 spectrum correction model of gas-bearing sandstone reservoirs is established by combining the Gaussian distribution function, and the T2 spectrum morphology and porosity components are corrected to the original state. The T2 spectra of the NMR logging data were multi-peaked and dominated by the long relaxation component, while the gas-corrected T2 spectra were mainly two-peaked and dominated by the short relaxation component. The bound water saturation obtained from the corrected T2 spectra was basically the same with the results of the core measurements. This study combines the NMR experiments to clarify the NMR response mechanism of gas-bearing sandstone reservoirs, and establishes a Gaussian distribution model for gas-corrected T2 spectra to realize the accurate characterization of pore structure.