Pore-Scale Imaging to Quantify the Evolution and Reduction in Trapped CO2 due to Ostwald Ripening.

Geological carbon storage is a key strategy for mitigating climate change, but the long-term stability of trapped CO2 remains uncertain. Transport of dissolved CO2 in the aqueous phase can cause the rearrangement of capillary-trapped CO2 in the pore space, which is called Ostwald ripening. Using high-resolution three-dimensional X-ray imaging, we visualized the in situ evolution of CO2 ganglia in reservoir sandstone during storage and quantified its impact on trapped CO2 saturation. Pore-scale imaging showed the concurrent shrinkage and growth of CO2 ganglia, reduced morphological complexity, and enhanced connectivity, resulting from Ostwald ripening. Ganglia exhibited a size-dependent response: small ganglia dissolved and disappeared, intermediate ones shrank or grew, and large ganglia stabilized with occasional fragmentation. After waiting for 58 h with no flow, originally residual CO2 reconnected, and subsequent brine injection led to a decrease in saturation from 22.8% to 15.6%, consistent with previous estimates based on pore-scale modeling. This work suggests that measurements that ignore the effect of Ostwald ripening overestimate the residual saturation by a factor of approximately a third.