87Sr/86Sr in waters from the Lincolnshire Limestone aquifer,England, and the potential of natural strontium isotopes as a tracer for a secondary recovery seawater injection process in oilfields |
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| Institution: | 1. Département du Génie des Systèmes Horticoles et du Milieu Naturel, Université de Sousse, Institut Supérieur Agronomique de Chott Meriem, Sousse, Tunisia;2. Université de Strasbourg, CNRS, LHyGeS UMR 7517, Strasbourg, France;1. School of Earth Sciences, Wills Memorial Building, University of Bristol, Bristol BS8 1RJ, UK;2. Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA;3. Departmento Geociencias Marinas y Ordenación del Territorio, Facultad de Ciencias del Mar, Universidad de Vigo, 36310 Vigo, Spain;4. Massachusetts Institute of Technology, 45 Carleton Street, Building E25, Room 625, Cambridge, MA 02142, USA;1. Department of Geoscience, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, T2N 1N4, Canada;2. Geological Survey of Canada, Natural Resources Canada, 3303-33 Street N.W., Calgary, Alberta, T2L 2A7, Canada |
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| Abstract: | The Sr isotope composition of formation waters is a sensitive indicator of diagenetic processes in the host sediments, mixing processes between different bodies of water, and the connectivity of hydrological systems. The87Sr/86Sr ratio of present seawater is constant worldwife, while formation waters in hydrocarbon reservoirs have various values, depending on the aforementioned effects, in most cases different from modern seawater. This forms the basis of anatural tracer technique for seawater injection projects, involving characterization of the87Sr/86Sr ratios and Sr contents of formation waters in the reservoir before injection commences, followed by monitoring of these parameters in the produced water as injection proceeds. This method is best suited to reservoirs in which the formation waters have low Sr concentrations and87Sr/86Sr ratios much higher or lower than seawater. Available data for reservoir formation waters suggest that breakthrough recognition could be expected at <10% seawater in many sandstone reservoirs, while the method would be less sensitive in carbonate reservoir or situations where the formation waters had interacted with evaporites, as the associated waters tend to have high Sr contents. In heterogeneous but well-mapped reservoirs, it may be possible to obtain information about flow paths/mechanismsbefore breakthrough. Combination with other chemical and isotopic tracers creates a very powerful tool, the Sr method acting as a safeguard should the batch of water containing the conventional tracers be overtaken by subsequently injected seawater. The Sr method could also be used for injection projects that were begun without the addition of tracers. A natural analogue of a water injection process is found in the Jurassic Lincolnshire Limestone aquifer in England, where rapidly moving fresh meteoric water mixes progressively with an older saline formation water. The87Sr/86Sr data enable quantitative modelling of this mixing process. The infiltrating fresh water becomes progressively modified by dissolution of detrital carbonate and calcite cement in the limestone, with depth becoming increasingly dominated by Sr derived from the more soluble detrital components. The saline formation water contains water molecules of meteoric origin and an87Sr/86Sr ratio much higher than Jurassic seawater or marine carbonate; the solute content has been influence by interaction of the water with non-carbonate phases. |
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