The effect of heterogeneity identifying the leakage of carbon dioxide in a shallow aquifer: an experimental study

Abstract:

Carbon dioxide (CO₂) leakage into the shallow aquifer is one of the main concerns at a CO₂ sequestration site. Various hydrogeochemical parameters have been suggested to determine the leakage (i.e., pH, EC, Alkalinity, Ca and δ13C). For the practical point of view, direct and continuous measurement of the dissolved CO₂ concentration at the proper location can be the most useful strategy for the CO₂ leakage detection in a shallow aquifer. In order to enhance possibility of identifying leaked CO₂, monitoring location should be determined with regard to the shallow aquifer heterogeneity. In this study, a series of experiments were conducted to investigate the effects of heterogeneity on the dissolved CO₂ concentrations. A 2-D sand tank of homogeneous medium sands including a single heterogeneity layer was designed. Two NDIR CO₂ sensors, modified for continuous measuring in aquatic system, were installed above and below the single heterogeneous layer (clay, fine and medium sand lenses). Also, temperature and water contents were measured continuously at a same position. Bromocresol purple which is one of the acid-base indicator was used to visualize CO₂ migration. During the gas phase CO₂ injection at the bottom of the sand tank, dissolved CO₂ in the water is continuously measured. In the results, significant differences of concentrations were observed due to the presence of heterogeneity layer, even the locations were close. These results suggested that monitoring location should be determined considering vertical heterogeneity of shallow aquifer at a CO₂ leakage site.