Fast spin echo MRI of reservoir core plugs with a variable field magnet

Abstract

Fast Spin Echo MRI is now widely employed in biomedicine for proton density and T₂ contrast imaging. Fast Spin Echo methods provide rapid data acquisition by employing multiple echoes to determine multiple k-space lines with single excitations. Due to the multi-exponential behavior of T₂ in typical porous media, and the strong dependence of T₂ on the details of the experiment, acquiring a proton density image with Fast Spin Echo methods requires favorable sample and acquisition parameters. In recent years, we have shown the value of pure phase encode Free Induction Decay based methods such as SPRITE. However, in a reservoir rock, a typical T₂* is hundreds of µs, whereas a typical T₂ is hundreds of ms. Hence, there is merit in considering spin echo-based MRI measurements such as the Fast Spin Echo for rock core plug studies.

A variable field superconducting magnet was employed in this study. This is a new class of magnet for MR/MRI. These magnets have the flexibility of operation in the field range of 0.01 Tesla to 3 Tesla. This is advantageous when working with rock core plugs, as it allows one to maximize sample magnetization, by increasing the static field while controlling magnetic susceptibility mismatch effects, and thereby T₂ and T₂*, through reducing the static field. The magnetic fields employed in the study were 0.79, 1.5, and 3 Tesla.

Measurements were undertaken on five brine-saturated reservoir rock core plugs (Bentheimer, Berea, Buff Berea, Nugget, and Wallace). The results show that Fast Spin Echo measurements are more sensitive than SPRITE methods in amenable samples and usually feature higher resolution. Quantification of saturation with Fast Spin Echo methods requires correction for T₂ attenuation. The results also show that 3 Tesla is too high a static field in general for rock core MRI studies with either method. While the current study is focused on five representative reservoir rock cores, the conclusions which result are general for MRI of fluids in porous media.