USING NOBLE GASES TO DEFINE CO2 BEHAVIOUR IN GEOLOGICAL CO2 STORAGE ANALOGUES

The chemically inert noble gases provide a unique insight into the physical and chemical processes operating on multi-phase basinal fluid systems. Noble gas abundance and isotopic composition in groundwater that has equilibrated with the atmosphere is well understood, while noble gases derived from radioactive decay in the crust, such as ⁴He and ⁴⁰Ar, are also produced and often released from source minerals at known relative elemental compositions. The development of phase interaction models that describe how the different noble gases are distributed between water and other fluid phases (‘partitioning’) allows us to identify the physical processes operating in the system of interest. These concepts have recently been applied with great success to understanding, for example, the control of the groundwater on the behaviour of CO₂ in naturally occurring CO₂ gas fields used as analogues for carbon capture and storage (CCS) targets [1,2,3].

The accuracy of the phase interaction models is very much dependant on the accuracy of the noble gas partition coefficients. We present new experimental data for noble gas partitioning in a CO₂-water binary system at pressures and temperatures relevant to carbon capture and storage (CCS) targets (323-377 K and 89-134 bar). Data show systematic deviations from partitioning in the reference pure noble gas-water system. The deviation correlates with carbon dioxide density (170-656 kg/m³) and is up to -54%, 76%, 106% and 290% for He, Ar, Kr and Xe respectively for a CO₂ density of 656 kg/m³. This is interpreted as the CO₂ phase acting as a polar solvent inducing polarisation in the noble gases. A Gibbs-Ensemble Monte Carlo molecular simulation is also used to investigate partitioning in binary CO₂-water system. The model accurately replicates both the CO₂-H₂O system and low pressure noble gas Henry constants. The focus is now on using the new observations to calibrate model CO₂-Noble gas interactions at high CO₂ density with a view to using the model approach to accurately predict noble gas partitioning in a wider range of natural analogue and engineered CO₂-water systems.

[1] Gilfillan et al. (2009) Nature 458 614-618 [2] Gilfillan et al. (2008) GCA 72 1174-1198 [3] Zhou et al., (2012) GCA 86 257-275