NMR Relaxation Mechanisms of Fluids in Rock Pores

Three distinct relaxation mechanisms—free relaxation, surface relaxation, and diffusion relaxation—occur in the NMR relaxation of fluids within rock pores, and all three typically coexist simultaneously.

1. Free Relaxation

Free relaxation refers to the intrinsic bulk relaxation of a fluid, with relaxation times determined by the fluid’s physical properties (such as viscosity and chemical composition) and the surrounding environment (including temperature and pressure).

In petroleum NMR studies, the solid nature of rock surfaces usually results in surface relaxation being more dominant than bulk relaxation for fluids in the pores. However, in hydrophilic rock pores where oil and gas are non-wetting, in the presence of fractures that reduce fluid-solid contact, or for highly viscous fluids such as heavy oil, free relaxation of the fluid cannot be ignored. In such cases, both free and surface relaxation effects must be considered.

2. Surface Relaxation

Surface relaxation refers to the relaxation mechanism at the rock pore surfaces, specifically arising from interactions between the pore fluid and the solid rock surfaces.

3. Diffusion Relaxation

During Brownian motion, molecules undergo self-diffusion. Diffusion relaxation describes the relaxation behaviour of protons caused by molecular diffusion in the presence of a magnetic field gradient.

The role of these three relaxation mechanisms depends on the type of pore fluid, pore size, pore structure development, the physical properties of the pore surface, and the wettability of the rock grains.

Typically, in hydrophilic rocks, the T₂ relaxation time of water is primarily governed by surface relaxation. For heavy oil, T₂ relaxation is mainly controlled by free relaxation. In the case of light oil, T₂ relaxation is influenced by both free and diffusion relaxation, and correlates with oil viscosity. For natural gas, due to the diffusive nature of gas molecules, T₂ relaxation is predominantly dictated by diffusion relaxation.