Defining Noble Gas Partitioning for Carbon Capture and Storage Environments

For viable CCS implementation variables such as CO₂ dissolution rates, reactions with the host rock and the extent of groundwater interaction must be accurately constrained. Noble gases play an important role in these systems [e.g. 1,2]. Their application, however, requires accurate Henry’s constants within dense CO₂-H₂O systems. Current interpretations use pure noble gas-H₂O partitioning data [3,4] and assume CO₂-noble gas interactions are negligible, even at high (>700 kg/m³) CO₂ densities [2].

To test this assumption we experimentally determined noble gas CO₂-H₂O partitioning for the 170-656 kg/m³ CO₂ density range; representative of most CCS environments. Contrary to assumption, CO₂ density significantly affected noble gas partition coefficients. For helium, increasing CO₂ density resulted in a negative deviation trend from CO₂-free values whilst for argon, krypton and xenon strong, positive deviations were observed. At 656 kg/m³ these deviations were -35%, 74%, 114% and 321% respectively. This is interpreted as the CO₂ phase acting as a polar solvent inducing polarisation in the noble gases. Deviation trends are well defined using a 2^nd order polynomial. The effect of a dense CO₂ phase can now be incorporated into existing noble gas models.

We also present results from a Gibbs-Ensemble Monte Carlo molecular simulation to model partitioning for this binary system. This fundamental technique makes predictions based on the pair-potentials of interaction between the molecules. Here it gives the phase compositions and Henry coefficients for noble gases. With a proven ability in accurately replicating both the CO₂-H₂O system and low pressure noble gas Henry constants the focus is now on fully optimising the model to match high pressure observations.

[1] Gilfillan et al. (2009) Nature 458 614-618 [2] Gilfillan et al. (2008) GCA 72 1174-1198 [3] Crovetto et al. (1982) J.Chem.Phys. 76 1077-1086 [4] Ballentine et al. in Porcelli et al. (eds.) (2002) Rev.Min.Geo. 47 539-614.