Testing helium equilibrium between quartz and pore water as a method to determine pore water helium concentrations |
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| Institution: | 1. Psychiatry & Behavioral Sciences, Stanford University School of Medicine, 401 Quarry Rd, Stanford, CA 94305-5717, USA;2. Anesthesiology and Perioperative Medicine, Case Western Reserve University School of Medicine, 11100 Euclid Ave, Cleveland, OH 44106-6031, USA;3. Oral and Maxillofacial Surgery, Case Western Reserve University School of Dental Medicine, 2124 Cornell Rd, Cleveland, OH 44106-3804, USA;4. Honorary Curator and Laureate of the History of Anesthesia, Wood Library-Museum of Anesthesiology, American Society of Anesthesiologists, 1061 American Ln, Schaumburg, IL 60173-4973, USA;1. Dept. of Fisheries Management and Marine Research, Echebastar Fleet SLU, Muelle Erroxape s/n (Box 39), 48370 Bermeo, Spain;3. UMR CNRS 5805 EPOC, University of Bordeaux, 33615, Pessac, France;4. Otago Museum, 419 Great King Street, PO Box 6202, Dunedin 9059, New Zealand;5. IKERBASQUE, Basque Foundation of Science, 48013 Bilbao, Spain;6. DigitalGlobe, Inc., 2325 Dulles Corner Blvd, Suite 1000, Herndon, VA, 20171, USA;7. University of Las Palmas de Gran Canaria, Campus Universitario de Tafira, Edificio de Ciencias Básicas, Facultad de Ciencias del Mar, sn. 35017 Las Palmas de Gran Canaria, Spain |
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| Abstract: | The effectiveness of carbon capture and geologic storage depends on many factors, including and especially the permeability of the reservoir’s caprock. While caprock integrity is generally assumed if petroleum has been preserved, it is poorly constrained in reservoirs containing only saline waters, and CO2 leakage poses a potential risk to shallow aquifers. Naturally-occurring He accumulates in pore waters over time with the concentration being strongly dependent on the long term flux of fluid through the caprock. Furthermore, a small fraction of pore-water He diffuses into quartz and this may be used as a proxy for He concentrations in pore water, where dissolved gas samples are difficult to obtain, such as in deep sedimentary basins. In this paper He contained in quartz grains is measured and compared to previously measured pore water concentrations. Quartz was purified from core samples from the San Juan Basin, New Mexico and the Great Artesian Basin, South Australia. Quartz separates were heated at 290 °C to release He from the quartz. The quartz from the San Juan Basin and high purity quartz from the Spruce Pine Intrusion, North Carolina was repeatedly impregnated at varying pressures using pure He, heated and analyzed to build He sorption isotherms. The isotherms appear linear but vary between samples, possibly due to fluid inclusions within the quartz grains as high purity quartz samples partition only 1.5% of He that partitions into San Juan Basin samples. Concentrations of He in the pore water were calculated using the He-accessible volume of the quartz and the air–water He solubility. The mean San Juan Basin He pore water concentration was 2 × 10–5 cc STP He/g water, ∼400 times greater than atmospheric solubility. Great Artesian Basin samples contain a mean He concentration of 3 × 10–6 cc STP He/g water or 65 times greater than atmospheric solubility. However, pore water He concentrations in both the San Juan and Great Artesian Basins differ by up to an order of magnitude compared to samples collected with an alternate method. The reason for the offset is attributable to either partial saturation of the pore volume or a lack of He equilibrium between quartz and pore water. Coating of clay or other mineral phases on quartz grains, which tends to reduce the effective diffusion coefficient, may cause the latter. This technique of assessing permeability is promising due to the abundance of existing core samples from numerous basins where carbon sequestration may ultimately occur. |
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