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

Abstract

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 T 2 signal is the highest for montmorillonite, followed by Illite, and the T 2 signals of bitumen and chlorite are relatively similar. Kaolinite and kerogen have the lowest NMR signals because they do not contain crystal water. The 1 H 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 T 2 spectrum of the injected and produced CH 4 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 T 2 (0.1−1 ms) indicates the adsorbed methane and T 2 (3−20 ms) indicates the free methane in pores, along with T 2 (80−110 ms), indicating the free methane in fractures. The practical exploitation process of shale gas exhibits notable pressure and desorption hysteresis characteristics. The T 2 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.

Adsorption Models for Shale Gas: A Mini-Review

“…Copyright 2017, Elsevier. Panels (e) and (f) were reproduced with permission from ref . Copyright 2021, American Chemical Society, Washington, DC.…”

“…Yao et al 107 provided a method based on NMR to directly measure methane adsorption capacity and adsorption isotherm of shale, without the application of the EOS for methane or volumetric corrections of any type. Zhou et al 93 used low-field NMR and isothermal adsorption experiments on shale samples to quantitatively identify multiphase methane in the adsorption−desorption process. Jiang et al 108 used NMR to quantitatively classify the pore system of shale gas reservoir by combining high-speed centrifugation with cores drying gradually, as shown in Figure 7.…”

Recent Advances in Multiscale Digital Rock Reconstruction, Flow Simulation, and Experiments during Shale Gas Production

“…The T 2 spectrum was divided to represent adsorbed methane and free methane in shale, and the absolute and relative amounts of free and adsorbed methane gases could be measured. 228,245 246 The dynamic fluid interactions/exchange process between adsorbed methane and injected CO 2 was investigated. The combination of NMR and core experiments can also characterize the occurrence state of water phase.…”

Shale Formation Damage during Fracturing Fluid Imbibition and Flowback Process Considering Adsorbed Methane

“…The production process of shale gas includes desorption, diffusion and seepage [27]. If one of the three processes is changed, the production process of shale gas will definitely be affected.…”