ZTE Imaging for High-Resolution Characterization of the Shale Pore Structure and Fluid Distribution

Shale oil is mainly stored in the pores and cracks of mud shale reservoirs, which is the mainstream oil and gas resource development in the world today. Shale is a complex porous medium with low porosity and low permeability, which results in a short relaxation time of the nuclear magnetic resonance (NMR) signal. By characterizing the pore structure and fluid distribution of shale, one can effectively guide the exploration of shale oil. However, the NMR relaxation signal of shale decays so rapidly that it cannot be effectively characterized by conventional magnetic resonance imaging (MRI) methods. Zero echo time imaging (ZTE) is often used for imaging short T2 tissues under high field conditions, where the theoretical value of the echo time (TE) of the pulse sequence is zero. In this study, the ZTE technique is implemented under low-field NMR, and the ZTE sequence is combined with relaxation NMR to obtain local information for shale samples before and after fluid self-absorption. The results show that ZTE technology can be applied to obtain high-quality shale images, and the heterogeneity of these samples was characterized. The fluid signals inside the samples were monitored, and the pore structure and fluid distribution inside the shale were characterized on macroscopic and microscopic scales. This method provides a trustworthy experimental technique for shale characterization and will benefit the oil industry.