The effect of brine supply on CO₂ injectivity impairment due to salt precipitation

Salt clogging has been recognized as an important factor for injectivity impairment in the CO₂ injection wells. While only a small amount of field experience with salt clogging has been reported to date, a large number of experimental and numerical studies have been published on the topics of injectivity impairment, drying mechanisms in porous media and salt precipitation due to CO₂ or gas injection. The motivation for our work was to provide a better insight into which combinations of petrophysical properties and operational parameters would enable long term CO₂ injection with minimal injectivity impairment caused by salt precipitation. Two supercritical CO₂ injection experiments into brine-saturated Berea sandstone cores were performed in a core flooding setup at reservoir conditions. Salt concentration was moderate, at around 10 wt%. X-ray computed tomography scanning of the cores was performed prior to and after the flooding experiments. We focused on recreating conditions in the near-well region where CO₂ injection rate is high, and brine back-flow rate is low, and thus evaporative regime is expected. The novelty in our experimental approach was that we introduced a continuous brine supply through a thin borehole in the outlet end of the core. This mimics a situation where the dry-out zone close to the injection well is (partially) replenished by brine from the surrounding reservoir. Complete salt clogging occurred in both experiments. The main difference between the two experiments was that CO₂ injection rate was higher in the second experiment. In the first experiment, CT-imaging shows that salt precipitated in sheets with a helical structure mostly in the lower half of the core. In the second experiment, the salt accumulation was displaced further towards the core outlet and concentrated around the brine injection borehole. The average permeability across the core samples decreased substantially in both cases: by 99 % in the first experiment and by 96 % in the second. Our results confirm that salt clogging is a complex process where different parameters, such as salinity, CO₂ and brine injection rates, permeability, pressure and temperature, can have significant effect on whether injectivity impairment occurs.