Noble gases as tracers of injected CO2 in the Cranfield enhanced oil recovery field

[摘要] Identifying how injected CO2 is retained underground is a fundamental challenge for carbon capture and storage. Developing tracers that are cheap and widely applicable will increase confidence that stored CO2 remains in place. This PhD examines the applicability of the isotopic composition of noble gases (He, Ne, Ar, Kr and Xe) that are present as minor natural constituents in CO2, as tracers of the fate of injected CO2. The Cranfield oil field (MS, USA), into which natural CO2 is injected for enhanced oil recovery (EOR), was developed as a site for a parallel study of carbon capture and storage, and is the focus of this research. Samples of gas from the transported CO2, and the injection and production wells were taken 18 and 45 months after the commencement of injection in July 2008. Neon isotope data are consistent with simple binary mixing between the injected and in situ natural gas. This relationship allows the Ne isotope composition of the pre-injection gas in Cranfield to be determined. Coherent correlations between Ne, He and Ar isotopes allow the natural gas end-member composition to be calculated as well. The noble gas isotopic ratios (3He/4He = 0.05 RA, where RA is the atmospheric value of 1.39 x 10-6, 20Ne/22Ne = 9.62, 21Ne/22Ne = 0.0384, 40Ar/36Ar = 836 and 40Ar*/4He = 0.09, where 40Ar* is the sum of the radiogenic and mantle derived 40Ar) of the natural gas in Cranfield are typical of natural gases derived from the continental crust.Helium isotope ratios and the 40Ar*/4He ratio notably correlate with CO2 concentrations, indicating that the noble gas fingerprints of the injected gas are preserved, and may offer utility as a tracer of the CO2. The He and Ar isotope systematics of the four sampled wells that have the lowest CO2 concentrations identify the loss of a significant amount of CO2 from the free gas phase. The amount of loss in each of the four wells can be quantified from the measured 3He/4He and 40Ar*/4He ratios and changes in the CO2/3He values. Losses vary between 22% and 96%, with good agreement between the different methods. It is notable however, that these four wells do not have significant gas production, and do not contribute significantly to the total amount of produced and re-injected gas. So, even though there is a significant loss from these wells, the total amount of CO2 lost is estimated to be only ~0.1% of the total injected gas, equivalent to 10kt gas. Notwithstanding this, the new data indicate that, across the entire field, CO2 is retained as a free phase and stratigraphic trapping is the most important storage mechanism. The fractionation of 40Ar*/4He, CO2/3He and δ13CCO2 in the CO2-poor samples is consistent with dissolution in water.The non-radiogenic noble gases (20Ne, 36Ar, 84Kr, 132Xe) originate from the atmosphere and are present in the gas, water and oil phases in the reservoir to differing degrees. It has been revealed that groundwater degassing, induced by CO2 injection plays an important role in fractionating 20Ne/36Ar, 84Kr/36Ar and 132Xe/36Ar at the early stage of injection, but a large heterogeneity in the degree of degassing has been observed throughout the reservoir. Some wells have shown 100% water degassing, while others are close to 0%. Oil degassing, and therefore the active CO2 – oil contact, became important during the later phase of injection, which is consistent with the fact that more CO2 injection was required to degas the oil than water. Temporal variations in the non-radiogenic noble gas ratios and 3He/4He are indicative of the evolution of the oil displacement efficiency. This fully agrees with the injection – production well data recorded in the field during sampling. This suggests that noble gases can also be used as a reservoir engineering tool to better understand the interaction of CO2, water and oil in the subsurface not only during CO2 storage but also to track EOR operations.

[发布机构] University:University of Glasgow;Department:Scottish Universities Environmental Research Centre

[关键词] Carbon capture and storage, geological storage, geochemical tracing, carbon isotope, noble gas isotope, mass spectrometry