www.acsprf.org

The time scale of fluid migration in the crust is a crucial factor in addressing the development of oil and hydrocarbon reservoirs as well as assessing the risks of artificially injected CO₂ leakage from subsurface reservoirs. This project examines the feasibility of using the noble gas radionuclides as chronometers to determine fluid residence time in the crust. The isotopic abundances of noble gas radionuclides in CO₂ (and hydrocarbon, if possible) reservoirs will be determined for the first time, and will be interpreted in the context of fluid migration in the crust together with stable noble gas chemical and isotopic compositions. The progress over the first grant period is two-folds:

(i) Improvement in field gas separation technique

One of technical difficulties in measuring noble gas radionuclides is the large quantity of noble gases required, and an efficient removal of carbonic gases in the field is essential for successful sampling. We evaluated the efficiency of our field gas-purification technique not only through our field experiences but also through laboratory experiments. The laboratory test on the efficiency of the CO₂ absorbent (calcium hydroxide) revealed that the rate of CO₂ absorption significantly drops after absorption proceeds to half-quantity of the stoichiometric expectation. This result implies that the rate of absorbent replacement becomes the time-limiting factor at sampling sites with high gas flux such as production wells. Water vapor also needs to be crudely removed from the gas samples prior to the gas separation system mentioned above, and the re-generation of the water condenser had similarly been a sampling rate-limiting process. For these reasons, we made significant technical improvements in our field gas sampling systems.

The water condenser re-generation process was automated using an optical liquid sensor. We also constructed a proto type of a gas purification system that does not require adsorbent replacement. The system applies an established gas-separation technology, pressure-swing adsorption. It is controlled by a programmable logic controller, and has operational flexibility so that it can also be used for separation of hydrocarbon in near future. Using the proto type apparatus, we are currently investigating the most optimal adsorbent, temperature, pressure and timing for the separation of CO₂ and CH₄ from noble gases. In order to adapt the gas processing system that is being developed, we also modified the field gas handling line and the control unit. We plan to start field sampling as soon as we complete the optimization of the sampling apparatus.  

Two undergraduate students participated in this part of the project as their first laboratory work experience over the summer, and made important contributions.

(2) Geochemical modeling of noble gas radionuclide in crustal fluids  

Argon-39, which we plan to study in this project, is a radioactive isotope of Ar with half-life of 269 years. This isotope has a source in the upper atmosphere via cosmic-ray induced spallation and neutron activation of stable Ar (atmospheric ³⁹Ar/Ar ~ 8.1× 10^-16). Due to its atmophile nature, the produced isotope resides predominantly in the atmosphere where it become isotopically well-mixed within a few years. It serves as a chronometer when atmospheric Ar is entrapped (or partitioned) and subsequently isolated in a subsurface fluid reservoir and decays. It has been recognized as an ideal chronometer of hydrological reservoirs such as glacial ice, groundwater and seawater. Unlike other noble gas radionuclides of interest, it is also produced in the crust by the nuclear reaction ³⁹K(n,p)³⁹Ar. Due to its short half-life, the activity of ³⁹Ar approaches zero after ~1,000 years, which defines the time scale of this chronometer in usual application. In some studies of ³⁹Ar in groundwater or spring water, however, there is an isotopic abundance of ³⁹Ar in excess of the atmosphere, suggesting that abundant ³⁹Ar produced in the crustal reservoir rock is transferred into the fluid phase. This excess ³⁹Ar is usually considered an obstacle of groundwater ventilation age dating. However, an addition of the crustal ³⁹Ar, if quantified, will make this relatively short-lived radionuclide more interesting and potentially useful for tracing longer time scales than expected. We investigated the ³⁹Ar isotopic abundance in crustal fluid at various rate of water-rock interaction theoretically, and examined its applicability by applying the model to data in previous literatures.  

The key results of the geochemical model investigations are: (i) Silicate rocks in the Earth’s crust are isotopically highly enriched in ³⁹Ar compared to the atmosphere; thus, crustal fluids are very sensitive to the input of ³⁹Ar from the reservoir rocks; (ii) The evolution of ³⁹Ar/⁴⁰Ar* (where “*” denotes radiogenic) ratios in the lithospheric reservoir is relatively simple whereas ratios the to atmospheric component (³⁶Ar or total ⁴⁰Ar) are much more complex; and (iii) The combination of three Ar isotopes, ³⁷Ar, ³⁹Ar and ⁴⁰Ar, sets constraints on the rate of Ar input from the reservoir rocks to the fluid, the rock porosity and the time of transport, which is a considerable improvement compared to the dating method using ⁴⁰Ar alone.

A surprising and important conclusion of this study is that ³⁹Ar is potentially useful for tracing a much wider time range (up to million years) than what is expected (1000 yrs) from its half-life. It enables us to determine the time it takes for fluids to migrate from one site to another, and thus, the migration velocity. Furthermore, the spatial distribution of these parameters would provide detailed changes in the migration velocity, possibly reflecting an existence of structural traps, low porosity zone, faults and fractures.

In summary, we have made significant progress in our sampling technique as well as in understanding the behavior of ³⁹Ar in the crust and the strategy of data interpretation. The field sampling will take place upon completion of optimizing the sampling apparatus.