Quantitative MR/MRI in porous media with a variable field magnet

Abstract

Quantitative characterization of fluids and fluid transport within reservoir core plugs is integral for Special Core Analysis (SCAL) and petrophysical interpretation for reservoir evaluation in the petroleum industry. Magnetic Resonance (MR) and Magnetic Resonance Imaging (MRI) are non-invasive, non-destructive analytical techniques uniquely well suited to quantifying fluid behavior in porous media.

This thesis develops quantitative MRI methods for core plugs, compares MRI derived porosity with conventional porosity measurements and introduces a multinuclear MR approach for SCAL applications. Quantitative 23Na MRI tracer measurements were employed to evaluate transport in porous media, while 23Na MR relaxation lifetime measurements on brine saturated core plugs were used to evaluate information beyond a tracer measurement. This thesis utilizes a purpose-built variable field superconducting magnet, that offers advanced measurement capabilities that will be valuable to industry professionals and academic researchers.

Investigation of magnetic susceptibility mismatch and B0 effects on MR relaxation lifetimes and image quantification in core plugs indicate that quantitative imaging of fluid content is limited by multi-exponential T2 relaxation lifetimes in porous media. 3D Centric Scan SPRITE MRI measurements were more straightforward compared to 3D FSE MRI measurements and provided a more reliable quantitative imaging.

Multinuclear measurements were employed to measure 1H, 13C, and 23Na, maintaining a constant Larmor frequency, while varying B0. The results demonstrated that when the product 𝛾B0 is constant, both T2* and T2 effects due to diffusion through internal field gradients are constant. The finding challenges the conventional notion that increasing B0 to accommodate lower 𝛾 nuclei complicates MR measurements.

23Na MR/MRI studies offer new insight for petrophysical studies. 23Na MRI was used as a quantitative tracer technique to study solute transport in brine saturated core plugs. Spatially resolved profiles reveal heterogeneities and dispersion behaviour that are not captured by conventional breakthrough curves. A systematic investigation of 23Na relaxation lifetimes in brine saturated sandstones under controlled wettability and saturation conditions was undertaken. 23Na MR relaxation lifetimes showed no dependence on wettability but varied with saturation state and lithology.