A Method for Characterizing Pore Structure of Gas-Bearing Sandstone Based on Nuclear Magnetic Resonance

Tight sandstone reservoirs are generally characterized by low porosity, low permeability, and complex pore structures, which pose significant challenges to reservoir evaluation. Nuclear Magnetic Resonance (NMR) T2 spectra can characterize pore structures and fluid states in porous media. Therefore, gas-displacing-water petrophysical experiments were carried out to investigate the variations of T2 spectra under different saturation states.The study finds that the T2 spectrum in the original state presents a multi-peak distribution dominated by long relaxation components. With the increase of gas saturation, the macropore peak value decreases and the porosity tends to decrease. Meanwhile, compared with the gas-bearing state, the macropore peak widens when the core is water-saturated. Analysis shows that this phenomenon is caused by the low hydrogen index of gas.Thus, a spectrum correction model for gas-bearing sandstone reservoirs was established based on the Gaussian distribution, which corrects the T2 spectrum morphology and porosity components to their original states. After gas saturation correction, the T2 spectrum mainly presents a bimodal distribution dominated by short relaxation components. The irreducible water saturation calculated from the corrected T2 spectrum is more consistent with the core measurement results.Combined with NMR experiments, this study clarifies the NMR response mechanism of reservoirs under different saturation states, establishes a Gaussian distribution model for T2 spectrum correction of gas-bearing sandstone, and ultimately achieves accurate characterization of pore structures.