NMR Measurements for Gas Adsorption Characterization on Shale: State of the Art and Perspectives

Abstract

Gas adsorption characterization in shale is essential for shale gas reserve estimation and exploitation. While the adsorption capacity for containing all the gas in shale nanopores is needed, traditional volumetric and gravimetric methods can only provide the excess adsorption, which is smaller and thus needs to be adjusted. NMR relaxometry offers a new possibility to directly measure the adsorption capacity based on a relaxation time contrast between the gas inside and the gas outside the nanoscale pores in the shale. The accuracy of this method has been validated by comparison with another method combining the volumetric/gravimetric method and molecular simulation based correction. The current understanding of the NMR relaxation mechanism of shale gas and the development of the experimental procedure of this method are introduced here. The results of related research using these methods are reviewed, and several future concerns are proposed in the end.

Quantifying Gas Storage and Transport in an Intact Core Sample Using Low-Field NMR and Pressure Measurements

“…The T 2 results (Figure A) were reported as an equivalent water volume and show the peak of the bulk gas signal moving later with higher pressures. The relaxation mechanisms of the bulk gas methane in the sample chamber are dominated by spin-rotation. , Spin-rotation relaxation causes the relaxation times to increase with increasing gas density, which is related to pressure, which was observed for ethane in Figure A. Other NMR relaxation mechanisms caused by the interaction of the gas and the rock core sample are discussed below.…”

NMR Relaxation of Gas Adsorbed in Microporous Material

“…Here, we consider the nuclear magnetic resonance (NMR) relaxometry for this task due to its sensitivity to both the chemical environment and the molecular motion. , The NMR relaxation time can be acquired by the high-field spectrometer alongside the spectra and also the low-field equipment which is economical and more adaptable to extreme testing conditions . Owing to these merits, this technique has already been widely used in research and industry to study the fluid confined in porous media such as reservoir rocks, construction materials, and foods. ,, However, the traditional NMR relaxation theories for confined fluids may not apply to gas adsorption in a microporous material. It has been observed that the NMR relaxation times T 1 and T 2 of gas confined in micropores can be higher than the bulk phase, , which contradicts the traditional theories predicting that confined fluids always relax faster. , There is still a lack of a quantitative explicit model for NMR relaxation of gas adsorbed in microporous material, so although NMR relaxometry has been used to study the adsorption capacity, ,, molecular movement, − and pore structure of multiple gas adsorption systems, the information that can be extracted from the results is limited.…”

Virtual Special Issue of Recent Advances in Shale Gas Exploration and Exploitation

“…For the fluids in nanopores, hydrocarbons are adsorbed near the boundary, while H 2 molecules are freely distributed, which makes H 2 molecules more likely to be released to the bulk region. Zeng et al reviewed the practical applicability of NMR relaxometry for gas adsorption capacity based on a relaxation time contrast between the gas inside and the gas outside the nanoscale pores in the shale. The current understanding of the NMR relaxation mechanism of shale gas and the development of the experimental procedure of this method are discussed.…”