CO₂ Hydrate in Porous Media: A Quantitative NMR Method to Detect Formation, Dissociation, and Localization

Our objective is to characterize the potential hydrate formation during CO2 injection in a depleted gas reservoir for which both the pressure and temperature can be low and within the hydrate stability zone; such hydrate formation may impact the gas injectivity. For this purpose, we developed a new NMR setup in which the temperature is particularly well controlled using a Peltier-based system inserted into the NMR probe. This setup allows us to impose slow temperature ramps (down to 0.1 °C/h), useful for observing quasi-static hydrate dissociation at a given pressure and thus measuring thermodynamic hydrate equilibrium points (Teq and Peq). The NMR measurements enable the continuous determination of the liquid water fraction within the porous media studied as well as the liquid content vertical profile. The samples are small cylinders of diameter 5 mm and maximum length 20 mm. The hydrate formation is detected by a decrease in the liquid water fraction. In a quartz sand pack at a constant pressure of 31 bar, the hydrate formation was studied by varying the temperature between 7 and −10 °C. The residual liquid volume fraction after hydrate formation was 0.05%. We estimated that a temperature increase rate no larger than 1 °C/h is necessary to properly determine the hydrate equilibrium temperature (Teq). In the large pores of the Bentheimer sandstone also studied, the largest temperature Teq of the hydrate stability zone at 21 bar is not significantly different from the one measured in the bulk phase from the literature.