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

Abstract

The low-field nuclear magnetic resonance (LF NMR) technique proves valuable in determining porosity, permeability, pore size, and wettability through T 2 measurements in rocks. Recently, there has been growing interest in low-field nuclear magnetic resonance imaging (LF MRI) technology, which can provide sliced T 2 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 T 2 -y maps for high-porosity sandstone and low-porosity dolomite cores using a phase-encoded T 2 imaging method. First, experiments were conducted to obtain sliced porosity and permeability for estimating rock heterogeneity along the core height. It was noted that T 2 components shorter than 0.3 ms were overlooked, leading to underestimated NMR porosity when comparing MRI-projected T 2 distributions with bulk T 2 distributions. Then, typical micropore modeling and magnetization evolution were employed to simulate and discuss factors, affecting the accuracy of the MRI T 2 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-T 2 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 T 2 distributions, whereas the refocusing pulse angle affected both the amplitude and the peak values of the T 2 spectra, especially in higher magnetic field inhomogeneity. Increasing gradient strength and duration were beneficial for imaging profiles but detrimental to the T 2 distribution and porosity. The investigation offers both practical guidance and theoretical insight for undertaking T 2 -y measurements in rocks, facilitating the optimization of the pulse sequence and the acquisition conditions, as well as the manipulation of the magnetization data.