Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T2-y Method in Low-Field NMR

The low-field nuclear magnetic resonance (LF NMR) technique proves valuable in determining porosity, permeability, pore size, and wettability through T2 measurements in rocks. Recently, there has been growing interest in low-field nuclear magnetic resonance imaging (LF MRI) technology, which can provide sliced T2 distributions and position profiles for the oil industry. However, there is a lack of detailed observation and discussion on the relevant application limitations and influencing factors. This work addresses this gap by presenting a comprehensive experiment and numerical investigation aimed at exploring T2-y maps for high-porosity sandstone and low-porosity dolomite cores using a phase-encoded T2 imaging method. First, experiments were conducted to obtain sliced porosity and permeability for estimating rock heterogeneity along the core height. It was noted that T2 components shorter than 0.3 ms were overlooked, leading to underestimated NMR porosity when comparing MRI-projected T2 distributions with bulk T2 distributions. Then, typical micropore modeling and magnetization evolution were employed to simulate and discuss factors, affecting the accuracy of the MRI T2 spectra and image profiles. These factors include the off-resonance frequency caused by the external static magnetic field or internal field, the imaging-encoded time, the excitation pulse angle, the refocusing pulse angle, and gradient properties. The results showed that field inhomogeneity significantly influenced MRI-T2 relaxation, particularly at high off-resonance frequencies. It was found that minimizing the image-encoded time is ideal for measuring short relaxation components. Additionally, the excitation pulse angle greatly impacted the amplitude of the T2 distributions, whereas the refocusing pulse angle affected both the amplitude and the peak values of the T2 spectra, especially in higher magnetic field inhomogeneity. Increasing gradient strength and duration were beneficial for imaging profiles but detrimental to the T2 distribution and porosity. The investigation offers both practical guidance and theoretical insight for undertaking T2-y measurements in rocks, facilitating the optimization of the pulse sequence and the acquisition conditions, as well as the manipulation of the magnetization data.