Experimental study of CO2/CH4 distribution in shale rock samples during adsorption/desorption reaction by low-field NMR

Shale is attracting more attention than ever because it can act as a cap rock for CO2 storage as well as a source rock for hydrocarbon resources known as shale gas. In particular, it has been known that the enhanced gas recovery (EGR) technology that enhances the recovery of CH4 by injecting CO2 can be applied to shale gas production. Gas in shale is known to exist in phases of free gas and adsorbed gas, and the adsorption tendency of CO2 is higher than that of CH4. Because of these unique characteristics, CO2 injected into shale induces desorption of CH4 (natural gas production) and remains in adsorbed phase (CO2 storage) at the same time. In other words, shale can also serve as a CO2 storage site. Since shale has a complicated pore structure and a very small pore size, research on the fluid flow or CO2-CH4 adsorption-desorption mechanism within shale has not been well investigated yet.

In this experimental study, Low-field NMR was used to analyze the characteristics of NMR signals of gases present in shale and how they change according to various gas pressures. In addition, the CO2-CH4 adsorption-desorption mechanism was analyzed by observing how the signal characteristics due to adsorption and desorption change as CO2 was injected into a shale sample saturated with CH4 gas. Through this study, it was confirmed that the NMR signal obtained from shale sufficiently reflects the phase and amount of gas, and that the progress of the adsorption-desorption reaction can be quantitatively analyzed. The results of this experiment can be used as important analytical data to understand the behavior of gas in shale, which is essential for shale gas recovery enhancement and CO2 storage.