Study of Rock Physical Properties Characterization Methods in Porous Media Using Ultrashort Relaxation MRI

In the domain of petroleum industry, characterizing the internal pore structure and fluid distribution within rocks is crucial for guiding reservoir development. The integration of magnetic resonance imaging (MRI) with Laplace Nuclear Magnetic Resonance (NMR) technique in spatially resolved experiments is one of the effective methods for characterizing porous media in rocks. Traditional MRI techniques are limited by their lengthy echo times, which compromises their effectiveness in rocks possessing short relaxation times. Zero Echo Time (ZTE) imaging technology offers a robust solution to this challenge which typically deployed under ultra-high field conditions with a theoretical echo time of zero. We have successfully adapted ZTE technology for use at low field (0.47T). By integrating this with conventional time- domain nuclear magnetic resonance sequences, we have optimized the sequence design. This innovation not only facilitates the visualization but also enables the detailed mapping of internal fluids in short-relaxation-time rocks like shale, thereby establishing a customized characterization methodology tailored specifically to these media. In this study, we show the spatially resolved experimental results of a natural shale rock core sample utilizing ZTE imaging technology. It adeptly captures three 3D stereoscopic imaging, incorporating sample features and cross-sectional images from various directions. Then, after the regional segmentation of the coronal cross-section of the sample, a detailed spatial resolution experiment was conducted. This experiment enabled the acquisition of the shale sample's Laplace inversion spectra, thereby facilitating a more precise evaluation of its pore structure characteristics. Subsequent to inversion processing, the analysis successfully generated 1D T1 spectra and T2 decay curves of different internal regions. The T1 spectra peaks more effectively reveal the content and variations of solid organic matter within the sample than the T2 spectra. Quantitative analysis of the T2 decay curves enables precise calculation of the sample's internal porosity. Integrating this analysis with advanced mathematical statistical methods facilitates a thorough evaluation of the sample's internal pore structure and fluid distribution. Analyzing the T1 spectra alongside imaging facilitates a direct assessment of the composition and pore distribution across different regions within the sample, providing a qualitative evaluation of its heterogeneity. The integration of imaging with quantitatively derived data, including average porosity and standard deviation across various internal sections, results in a more refined assessment of the sample's heterogeneity.