Pore space of tight sandstone samples exhibits fractal characteristics. Nuclear magnetic resonance is an effective method for pore size characterization. This paper focuses on fractal characteristics of pore size from nuclear magnetic resonance (NMR) of tight sandstone samples. The relationship between the fractal dimension from NMR with pore structure and water saturation is parameterized by analyzing experimental data. Based on it, a pore structure characterization and classification method for water-saturated tight sandstone and a water saturation prediction method in a gas-bearing sandstone reservoir have been proposed. To verify the models, the fractal dimension from NMR of 19 tight sandstone samples selected from the gas-bearing tight sandstone reservoir of Shahejie Formation in Nanpu Sag and that of 16 of them under different water saturation states are analyzed. The application result of new methods in the gas-bearing tight sandstone reservoir of Shahejie Formation in Nanpu Sag shows consistency with experimental data. This paper has facilitated the development of the NMR application by providing a non-electrical logging idea in reservoir quality evaluation and water saturation prediction. It provides a valuable scientific resource for reservoir engineering and petrophysics of unconventional reservoir types, such as tight sandstone, low porosity, and low permeability sandstone, shale, and carbonate rock reservoirs.

Relative permeability is a crucial parameter for reservoir fluid-producing evaluation. In clay-free rock, water flow and electric current are dependent on the same controlling factors. The impact of pore structure on rock electrical properties and fluid flow is significant when calculating relative permeability from electrical properties in complex pore structure sandstone. In this paper, the pore geometry of complex pore structure sandstone is equivalent to tubular-and micromembrane-like shapes. A conduction model and a fluid flow model for two shapes of pores are studied. The study result shows that the flow path of water is the same as the conductive path in tubular-like-shaped pores; however, in micromembrane-like-shaped pores, water does not flow but electric current flows. Based on the above, the relative permeability calculation method from electrical properties in complex pore structure sandstone is proposed. A total of 16 rock samples selected from a complex pore structure reservoir of Dongying Formation in Nanpu Sag are used to verify the validity of the relative permeability calculation method. Rock samples are classified into five pore size distribution types according to NMR experimental data; for each type, relative permeability is calculated using the new method. The result shows that the new method-calculated relative permeability is in accordance with the experimental values and parameter calculation accuracy is improved. The relative permeability calculation method from electrical properties in complex pore structure sandstone has good application prospects and can provide important guidance for high-precision evaluation of complex pore structure reservoirs and research of rock mechanical characteristics.

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Pore structure is the primary control factor of pore fluid distribution and flow capacity. It is an important content in reservoir evaluation. The capillary pressure curve is a major method to obtaining pore structure characteristics. The NMR T2 spectrum reflects pore size distribution. Both of them have a connection. Investigating methods of converting the NMR T2 spectrum into a capillary pressure curve is the key step to use nuclear magnetic resonance (NMR) logging data to quantitatively characterize the pore structure. The pore throat size distribution obtained by the NMR T2 spectrum and capillary pressure curve has fractal features. However, studies about the relationship between the capillary pressure curve and NMR T2 spectrum by the fractal characteristic have not been reported. This paper studies the relationship between the capillary pressure curve and NMR T2 spectrum by the fractal characteristic and proposes a method of converting the NMR T2 spectrum to the capillary pressure curve. To fully consider the influence of the pore structure, the dimension coefficient is set to be a variable that is related to the pore structure. The proposed model is verified by 20 rock samples in Nanpu sag, China by comparing the new method of calculate capillary pressure curves and experimental capillary pressure curves. Results show that the new method improves the accuracy of the conversion method between the NMR T2 spectrum and capillary pressure curve in the complex pore structure reservoirs. This research provides a new way to enrich pore structure characteristic parameters for high-precision reservoir evaluation and has certain guidance and reference significance for the research work of petrophysics, reservoir evaluation, and NMR logging data application.