Tracing injected CO2 in the Cranfield enhanced oil recovery field (MS, USA) using He, Ne and Ar isotopes

The He, Ne and Ar isotopic composition of gases collected in 2009 and 2012 from 13 production wells, injection wells and the CO₂ supply pipeline at the Cranfield CO₂-enhanced oil recovery field (MS, USA) have been measured in order to determine the extent to which they trace the fate of injected CO₂ in the reservoir. In the absence of samples of CO₂ pre-injection reservoir gas we use the Ne isotope composition of the production and injection well gases to determine the isotopic composition of the natural gas. The noble gas isotopes display binary mixing trends between the injected CO₂ and a CH₄-rich natural gas that is characterised by radiogenic He, Ne and Ar isotope ratios.³He/⁴He and ⁴⁰Ar^*/⁴He ratios (where ⁴⁰Ar^* represents the non-atmospheric ⁴⁰Ar) display coherent relationships with CO₂ concentrations that can be used to trace and quantify the injected CO₂ in an engineered site over a sustained period of injection. The presence of a small amount of air-derived Ar, from a non-atmospheric source, in many gas samples rules out using ⁴⁰Ar/³⁶Ar to track the injected CO₂. The noble gases identify the loss of a significant proportion of the CO₂ from the gas phase sampled by five production wells in 2009. Using ³He/⁴He and ⁴⁰Ar^*/⁴He ratios to reconstruct the major gas composition, it appears that between 22% and 96% of the CO₂ has been lost in individual wells. This study demonstrates that the naturally occurring noble gases have the potential to trace the fate and quantify the sequestration of CO₂ at injection sites.