Impact of efflorescence on internal salt precipitation dynamics during injection of gases in porous rocks.

Porous geological formations play an important role as storage media for CO and H. Brine in these formations can evaporate during gas injection, leading to salt precipitation. Estimating how and where salt precipitates is important to understand its influence on injectivity. So far, studies on gas injection scenarios primarily focused on formation dry-out inside the pore space. However, salt precipitation on external rock surfaces exposed to dry-out (e.g., the wellbore interface) can strongly influence drying dynamics as well. The occurrence of salt precipitation on these surfaces and its relation to the internal precipitation dynamics remain unclear. Therefore, we conducted experiments injecting dry gas into Bentheimer sandstone at two flow rates in the advective regime and for two different brine salinities. We tracked precipitation on both the gas injection surface and in the pores via μCT imaging. Salt precipitation on the surface occurred for all cases, and precipitation within the pores exhibited heterogeneous distribution, with higher salt accumulation at higher salinities and lower flow rates. Porosity decreased by less than 3 % for all cases, nevertheless accompanied by up to 20 % reduction in permeability. Our experiments suggest a strong correlation in time between precipitation dynamics within the pores and on the injection surface. We discovered a sudden decrease in brine concentration inside the pores during dry-out, with a simultaneous increase in the rate of precipitated salt volume on the surface, highlighting a previously unrecognized interaction. This work provides insight into complex drying and precipitation dynamics during gas injection, which carries important implications for well injectivity impairment in field operations.