Monitoring Salt Precipitation in Porous Media Using 23Na MRI: Implications for CO2 Storage

Salt precipitation, also known as salt-out poses a challenge for CO2 storage in saline aquifers, dueto potential injectivity loss. The primary tool to study this phenomenon in porous media has been X-raymicro-Computed Tomography (X-ray µCT). However, this imaging method uses non-representative fluidsand salts to create attenuation contrast. Furthermore, it is limited in field of view and sample size. MagneticResonance (MR) offers a non-invasive method for probing salt precipitation using industry-standard coreplug samples. 23Na MR/MRI provides quantitative sodium content, which can be combined with 1H MR togive sodium concentration in the brine phase. Experiments were conducted on Bentheimer sandstonessaturated with 8 wt% NaCl brine, with dry N2 as a model gas for CO2. Salt precipitation was monitoredusing a variable field magnet allowing to switch between 1H and 23Na MR measurements. The 23Na MRsignal was resolved using both T2* and T1 relaxation times, enabling quantitative differentiation of sodiumcontent in solution and solid phases. Phase-encode imaging methods were employed to selectively imagesolution and crystalline sodium. Initially, brine was displaced due to viscous flow of gas. Fluid displacementwas well separated from the subsequent evaporation and salt precipitation stages. A complete conversion ofthe residual solution sodium into crystalline sodium was observed. 1D and 2D images were acquiredshowing the spatial distribution of sodium during and after drying. Crystallization was localized at the dryingfront near the sample inlet. This salt accumulation was attributed to capillary backflow to transport sodiumto the drying front, as evidenced by the contrasting spatial profiles of 23Na and 1H content in solution. Themethods presented using 23Na MRI have a great potential for salt precipitation studies, paving the way forapplications in CO2 storage and other petroleum-related processes.