Low-Field NMR Investigation of the Dynamic Adsorption–Desorption Process of Shale Gas

Adsorbed and free methane not only occur in the shale gas enrichment and accumulation process but also in the decompression–desorption–diffusion–seepage process during shale gas production. In this work, we elaborate the optimization and combination of low-field nuclear magnetic resonance (NMR) and isothermal adsorption experiments on shale samples to quantitatively identify multiphase methane in the adsorption–desorption process. NMR signatures from individual matrix components, which comprise shale, are investigated to gather data to better estimate its petrophysical parameters. The T₂ signal is the highest for montmorillonite, followed by Illite, and the T₂ signals of bitumen and chlorite are relatively similar. Kaolinite and kerogen have the lowest NMR signals because they do not contain crystal water. The ¹H signal of kerogen mainly stems from its methyl, methylene, carboxyl, and hydroxyl groups. The types of clay minerals will profoundly affect the wettability of pores, thereby further influencing the relaxation mechanism of shale. By analyzing the T₂ spectrum of the injected and produced CH₄ from the shale sample, we revisit the possible mechanisms that could arise in shale and examine the adsorption–desorption process based on the tested sample. The results reveal that methane is present in three different states in shale, among which T₂ (0.1–1 ms) indicates the adsorbed methane and T₂ (3–20 ms) indicates the free methane in pores, along with T₂ (80–110 ms), indicating the free methane in fractures. The practical exploitation process of shale gas exhibits notable pressure and desorption hysteresis characteristics. The T₂ spectrum of methane in the different states in the adsorption–desorption process can be better simulated with a mathematical model. The ultimate goal of this study is to develop a holistic multidisciplinary methodology to assess multiphase methane in the adsorption–desorption process of shale gas.