Nuclear Magnetic Resonance T2 Spectrum and Porosity Correction Model of Shale Reservoir Based on Mineral Magnetic Susceptibility

With the large-scale exploration and development of shale oil and gas resources, fine stratigraphic evaluation plays an increasingly important role. Porosity and pore structure, as important parameters affecting shale reservoir development plan, productivity, production, and recovery, are the core contents of fluid quantitative evaluation. However, there are iron-bearing minerals and organic matter in shale reservoirs, and the pore structure is complex, which makes it difficult to estimate the nuclear magnetic T2 spectrum and porosity accurately. Therefore, it is of great significance to correct the T2 spectrum and establish a nuclear magnetic porosity correction model for shale reservoirs. In this paper, the shale of the second member of the Funing Formation in the Subei Basin is used as the research object. The porosity-permeability relationship and main mineral composition of the reservoir are clarified through petrophysical property experiments and X-ray diffraction analysis. The positions of bound water and hydroxyl groups in the clay minerals are identified using drying, water-saturated, and centrifuged 2 MHz two-dimensional (2D) nuclear magnetic resonance (NMR) experiments. The relationships between gas-measured porosity, the difference between water-saturated porosity and NMR porosity, and different mineral contents, especially the significant effects of iron-bearing minerals (including iron dolomite, chlorite, and illite) on porosity correction, were discussed, and nuclear magnetic resonance experiments of 2 MHz chlorite, illite, and kaolinite with sands mixtures were carried out to verify the effect of iron mineral content on NMR T2 spectra and porosity. Combined with the principle of NMR and the magnetic susceptibility of different minerals, the T2 relaxation time is corrected, and the nuclear magnetic T2 spectrum and porosity correction model are established.