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Use of organic and inorganic solutes in tracing oil-field salt-water sources; delineation of recent versus historic sources



Use of organic and inorganic solutes in tracing oil-field salt-water sources; delineation of recent versus historic sources



Proceedings of the Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection and Remediation Conference 2000(Pages 331-347



Salt water from both natural and anthropogenic sources impacts ground and surface water in west Texas. Potential oil-field sources include former evaporation pits, historical shallow completion of surface casings, and casing or plugging failure within boreholes that allows connection between fresh and saline zones. Waterflooding to enhance oil recovery may result in discharge of salt water into potable ground-water zones if well completion does not ensure fluids are confined to the injection zone. Our hypothesis is that differences in water chemistry, including the presence of soluble organic constituents such as benzene, and ratios of major ions and metals can be used to distinguish between old sources, such as pits, and more recent sources. BTEX constituents should be low in waters originating from old pit sources because of biodegradation, sorption, and volatilization over time. At our study area near the southeastern limit of the Ogallala aquifer, ongoing seepage of saline ground water from springs impacts vegetation and water quality in shallow wells. The source of the salinity is unknown, however, the oil-field site includes active and abandoned oil wells, old evaporation pits, and active and plugged salt-water injection wells. The oil field currently uses waterflooding techniques to enhance oil recovery. Natural background salinities in the Ogallala are relatively low, with dissolved chloride of 100 to 400 mg/L. Ground water, surface spring waters, and produced water samples were analyzed for dissolved solids, major cations, major anions, and BTEX using standard analytical methods. Hydrogeology of the site was defined with results from 15 monitoring wells. Sampling location data were entered into a geographic information system (GIS) database, along with the locations of current and former oil and injection wells, and local springs and surface water bodies. Locations of former evaporation pits were extracted from georeferenced air photos from 1957 and 1963 and added to the GIS. Two ground-water zones of differing geochemistry were found outside of the background area. The first zone, located immediately upgradient of the seep area, contained the highest salinity water. Chloride concentrations varied from 5,000 to over 25,000 mg/L. The highest benzene concentrations were also found in this zone, varying from 1000 to 7000 mu g/l. This is less than the benzene concentrations found in the produced water (avg. 12,000 mu g/L). These highest benzene values along with the presence of toluene correlate closely with the highest chloride zone within the aquifer. Chloride:bromide ratios within the high-chloride zone were similar to produced water ratios. Relative concentrations of calcium, magnesium, sulfate, and iron also tended to resemble patterns noted in the produced water samples. No clear trends were noted in metals such as barium and vanadium, between background, the saline zone, and the produced water. Chromium and nickel were undetected above method detection limits in all samples. A second zone exhibited low benzene concentrations in ground water (<5 to 34.2 mg/l). These low benzene concentrations correlated with moderately elevated chloride concentrations (778 to 5779 mg/l)., and chloride:bromide ratios approximately one-half of the high-chloride zone. Relative concentrations of other ions tended to be different from produced water, but also were different from the background zone. For example, sulfate and calcium were typically twice the concentrations found in background waters. Nitrate was also detected at low levels in this area. This second zone, while upgradient and side-gradient of the first, is also downgradient of several former pits, the inferred sources. Water originating from the pits would date from before 1960 (the time of pit closure) allowing time for evolutionary changes in water chemistry. Low benzene concentrations and a lack of toluene, ethylbenzene and xylenes may reflect biodegradation in this area. The intermediate levels of salinity are much higher than background but much lower than the high salinity zone. The elevated BTEX zone and the elevated chloride zone were superposed on the map of well locations using GIS, enabling several oil wells and one former injection well to be targeted for further study for source identification.

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