NIUMAG
NMR relaxation mechanism of rock pore fluid
Three different relaxation mechanisms, free relaxation, surface relaxation and diffusion relaxation, exist in the NMR relaxation of rock pore fluids. Generally, the three relaxation behaviors exist simultaneously.
1. Free relaxation
Free relaxation, that is, the body relaxation phenomenon unique to fluids, the relaxation time is determined by the physical properties of the fluid (viscosity, chemical composition, etc.) and the environment (temperature, pressure, etc.) where the fluid is located.
In the process of nuclear magnetic research in the petroleum industry because the rock surface is solid, the fluid surface relaxation in the rock pores is usually stronger than the body relaxation. However, when the oil and gas in the pores of the hydrophilic rock belong to the non-wetting phase, the existence of cracks in the rock causes less contact between the fluid and the solid surface, and the viscosity of the fluid such as heavy oil is high, the free relaxation between the fluid and the rock pores is impossible. Ignore, the influence of free relaxation and surface relaxation needs to be considered at the same time.
2. Surface relaxation
The relaxation mechanism of the rock pore surface is the surface relaxation, which is specifically manifested as the relaxation phenomenon between the pore fluid and the rock-solid surface.
3. Diffusion relaxation
Molecules undergo self-diffusion during the process of Brownian motion. Diffusion relaxation is the relaxation characteristic of protons in a gradient magnetic field due to molecular diffusion.
Conditions such as the type of pore fluid in the rock, pore size, pore development structure, petrophysical properties of the pore surface, and the wettability of the rock particle surface determine whether the three relaxation mechanisms work for the fluid in the pores.
Generally, for hydrophilic rocks, the T2 relaxation time of water in pores is mainly determined by surface relaxation; for heavy oil, the T2 relaxation time is mainly determined by free relaxation; while the T2 relaxation time of light oil is It is jointly determined by free relaxation and diffusion relaxation and is related to the viscosity of oil; natural gas due to the diffusion characteristics of gas molecules, its T2 relaxation time is mainly controlled by diffusion relaxation.