NIUMAG
NMR Relaxation Mechanisms of Rock Pore Fluids
Three different relaxation mechanisms, free relaxation, surface relaxation and diffusion relaxation, exist in the NMR relaxation of rock pore fluids, and generally the three relaxation behaviors coexist.
1. Free relaxation
Free relaxation is the unique bulk relaxation phenomenon of fluid, and its relaxation time is determined by the physical properties of the fluid (viscosity, chemical composition, etc.) and the environment in which the fluid is located (temperature, pressure, etc.).
In the process of NMR research in the petroleum industry, because the rock surface is solid, the surface relaxation of the fluid in the rock pores is usually stronger than the bulk relaxation. However, when the oil and gas in the pores of the hydrophilic rock belong to the non-wetting phase, the fractures in the rock lead to less contact between the fluid and the solid surface, and the viscosity of the fluid such as heavy oil is relatively high, the free relaxation phenomenon between the fluid and the rock pores cannot be Neglect, at this time, the effects of free relaxation and surface relaxation need 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 expressed as the relaxation phenomenon between the pore fluid and the rock-solid surface.
3. Diffusion relaxation
Molecules will perform self-diffusion motion in the process of Brownian motion, and diffusion relaxation is the relaxation characteristic of protons in a gradient magnetic field due to molecular diffusion.
The type of pore fluid in the rock, pore size, pore development structure, pore surface petrophysical properties, and rock particle surface wettability determine whether the three relaxation mechanisms play a role in the pore fluid.
Generally, for hydrophilic rocks, the T2 relaxation time of water in pores is mainly determined by surface relaxation; for heavy oil, its T2 relaxation time is mainly determined by free relaxation; while for light oil, the T2 relaxation time is It is jointly determined by free relaxation and diffusion relaxation and is related to the viscosity of oil. Due to the diffusion characteristics of gas molecules, the T2 relaxation time of natural gas is mainly controlled by diffusion relaxation.