Changing the resonant nucleus by altering the static field, compensation of γ and B0 effects in T2 and T2* measurements of porous media

Abstract

Multinuclear ¹H, ¹³C, and ²³Na magnetic resonance (MR) has many advantages for studying porous media systems containing hydrocarbons and brine. In recent work, we have explored changing the nucleus measured, keeping the Larmor frequency constant, by changing the static magnetic field B₀. Increasing the static B₀ field distorts the field in the pore space due to susceptibility mismatch between the matrix and pore fluid. Distortion of the magnetic field in the pore space scales with the applied static field. The gradients that result from the spatial variation of the distorted field will also scale with B₀. The equations that describe the inhomogeneous broadening in T₂* show that the MR result depends on $\gamma$B₀. The diffusion through internal field gradients effect on T₂ depends on the product of $\gamma$ and G, with G depending on B₀.

Increasing the static field to bring a nucleus with lower $\gamma$ into resonance at the same frequency will result in the products $\gamma$B₀ and $\gamma$G being constant, and therefore, inhomogeneous broadening and diffusion attenuation effects in porous media are predicted to be constant. We explore the T₂* hypothesis with ²³Na and ¹H measurements of brine in porous reservoir core plugs. We explore the diffusion through internal field gradients effect hypothesis with ¹H and ¹³C measurements of decane saturated glass beads.

The nuclei chosen for study: ¹H, ¹³C, and ²³Na are the three most important nuclei for studies of fluids (brine and hydrocarbons) in reservoir core plugs. These three nuclei have a common resonance frequency of 33.7 MHz at static fields of 0.79 T, 3.19 T, and 2.99 T, respectively. All three fields are readily achieved with our variable field superconducting magnet.