Monitoring of fluid–rock interaction and CO2 storage through produced fluid sampling at the Weyburn CO2-injection enhanced oil recovery site,Saskatchewan, Canada |
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| Institution: | 1. Dept. Earth Sciences, Downing Street, Cambridge CB2 3EQ, UK;2. Shell Global Solutions International, Kessler Park 1, 2288 GS Rijswijk, The Netherlands;3. Department of Earth Sciences, South Parks Road, Oxford OX1 3AN, UK;4. Sorbonne Universités, UPMC Univ. Paris 06, CNRS, Institut des Sciences de la Terre de Paris (ISTeP), 4 place Jussieu, 75005 Paris, France;5. CRPG, Centre de Recherches Pétrologiques et Géochimiques, 15 rue Notre Dame des Pauvres, 54501 Vandoeuvre lès Nancy, France;6. International Affairs and Cooperative Education, Prince of Songkla University, Surat Thani Campus, Thailand;7. Department of Earth Sciences, Earth Sciences Centre, University of Toronto, Toronto, Ontario M5S 3B1, Canada;8. Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK |
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| Abstract: | The Weyburn Oil Field is a carbonate reservoir in south central Saskatchewan, Canada and is the site of a large CO2 injection project for purposes of enhanced oil recovery. The Weyburn Field, in the Mississippian Midale Formation, was discovered in 1954 and was under primary production until secondary recovery by water flood began in 1964. The reservoir comprises two units, the Vuggy and the Marly, and primary and secondary recovery are thought to only have significantly depleted the Vuggy zone, leaving the Marly with higher oil saturations. In 2000, PanCanadian Resources (now EnCana), the operator of the field, began tertiary recovery by injection of CO2 and water, primarily into the Marly. The advent of this project was an opportunity to study the potential for geological storage of CO2.Using 43 Baseline samples collected in August 2000, before CO2 injection at Weyburn, and 44 monitoring samples collected in March 2001, changes in the fluid chemistry and isotope composition have been tracked. The initial fluid distribution showed water from discovery through water flood in the Midale Formation with Cl ranging from 25,000 to 60,000 mg/L, from the NW to the SE across the Phase 1A area. By the time of Baseline sampling the produced water had been diluted to Cl of 25,000–50,000 mg/L as a result of the addition of make up water from the low TDS Blairmore Formation, but the pattern of distribution was still present. The Cl distribution is mimicked by the distribution of other dissolved ions and variables, with Ca (1250–1500 mg/L) and NH3 (aq) increasing from NW to SE, and alkalinity (700–300 mg/L), resistivity, and H2S (300–100 mg/L) decreasing. Based on chemical and isotopic data, the H2S is interpreted to result from bacterial SO4 reduction. After 6 months of injection of CO2, the general patterns are changed very little, except that the pH has decreased by 0.5 units and alkalinity has increased, with values over 1400 mg/L in the NW, decreasing to 500 mg/L in the SE. Calcium has increased to range from 1250 to 1750 mg/L, but the pattern of NW–SE distribution is altered. Chemical and isotopic data suggest this change in distribution is caused by the dissolution of calcite due to water–rock reactions driven by CO2. The Baseline samples varied from ?22 to ?12‰ δ13C (V-PDB) for CO2 gas. The injected CO2 has an isotope ratio of ?20‰. The Monitor-1 samples of produced CO2 ranged from ?18 to ?13‰, requiring a heavy source of C, most easily attributed to dissolution of carbonate minerals. Field measured pH had increased and alkalinity had decreased by the second monitoring trip (July 2001) to near Baseline values, suggesting continued reaction with reservoir minerals.Addition of CO2 to water–rock mixtures comprising carbonate minerals causes dissolution of carbonates and production of alkalinity. Geochemical modeling suggests dissolution is taking place, however more detail on water–oil–gas ratios needs to be gathered to obtain more accurate estimates of pH at the formation level. Geological storage of CO2 relies on the potential that, over the longer term, silicate minerals will buffer the pH, causing any added CO2 to be precipitated as calcite. Some initial modeling of water–rock reactions suggests that silica sources are available to the water resident in the Midale Formation, and that clay minerals may well be capable of acting as pH buffers, allowing injected CO2 to be stored as carbonate minerals. Further work is underway to document the mineralogy of the Midale Formation and associated units so as to define more accurately the potential for geological storage. |
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