Monitoring of saturation changes and salt precipitation during CO2 injection using pulsed neutron-gamma logging at the Ketzin pilot site

During injection of CO2, monitoring of the subsurface saturation changes is required. For well logging in cased boreholes only a limited number of techniques such as radiometric pulsed neutron-gamma (PNG) logging are applicable. The conventional PNG saturation model mainly considers a displacement process. But during CO2 injection additional processes such as evaporation and salt precipitation are expected to occur as a result of the mutual solubility between brine and CO2. For this purpose an extended PNG saturation model for NaCl-brines is developed and applied to a time-lapse PNG monitoring data set from the Ketzin site. The results show that for the observation well further away from the injection well, the conventional displacement saturation model is valid, with average CO2 saturations below 60%. In contrast, the data from the injection well shows that both evaporation and salt precipitation have occurred. Here, the largest CO2 saturations with values up to 100% are determined locally. The results of the extended saturation model indicate that dry-out regions, where only CO2 and halite with saturations up to 1.4% exist, and maximum halite saturations up to 14.1% occur in the vicinity of the brine levels. The halite saturation distribution in the injection well seems to be controlled by changes in the injection regime associated with changing brine levels, lithological heterogeneities, and capillary effects. PNG monitoring in combination with the extended saturation model is suited to determine displacement and evaporation/precipitation processes for CO2 storage operations.