Circulation Patterns of Saline Groundwater Affected by Geothermal Heating—as Related to Waste Disposal1
Published:January 01, 1972
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H. R. Henry, F. A. Kohout, 1972. "Circulation Patterns of Saline Groundwater Affected by Geothermal Heating—as Related to Waste Disposal", Underground Waste Management and Environmental Implications, T. D. Cook
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In recent years, considerable interest has been focused on deep saline aquifers as reservoirs for the disposal of liquid waste. In general, the water in such aquifers is already in motion, being controlled by three sets of gradients: the hydraulic-pressure gradient, the geothermal gradient, and the salt-concentration gradient. In thick aquifers, interaction of these gradients may induce gravity convection currents which are not generally present in shallow constant-density fluid systems. The fate of the waste liquids entrained in such aquifer systems will depend, among other things, upon the state of motion in the aquifer before injection and the modification of this state by the injection process.
The Floridan aquifer underlying peninsular Florida is more than 2,000 ft (610 m) thick and provides a field situation for comparison with mathematical and hydraulic models. The aquifer is exposed to cold seawater where truncated by the deep trenches of the Gulf of Mexico and the Florida Strait. Natural upwellings of warm saline water and observations of temperature and salinity in wells suggest that the seawater flows inland at depth then upward into shallow parts of the aquifer, and, after mixing with fresh water, it flows seaward again to form a large, geothermaHy heated, convective flow cycle.
To develop predictive techniques for injection of waste into the deep part of this massive aquifer, a hydraulic sand model was built to simulate a saline aquifer, a geothermal source, freshwater recharge, and waste-injection wells. The studies show stream lines, velocities, and temperature/salinity distributions. The governing equations—namely, the hydraulic-flow equation, the diffusion equation for salt and injected contaminants, and the heat-diffusion equation—are solved simultaneously on a high-speed digital computer. Obtaining theoretical solutions comparable to the model data requires choosing correct empirical values of coefficients of salt and heat diffusion.
At the time of this writing, theoretical solutions have been obtained for the hydraulic model which qualitatively bear a remarkable correspondence to observed distributions of temperature and salinity in the Floridan aquifer. Work is progressing to obtain quantitative correspondence to the field observations and to extend the predictive techniques to solutions of practical problems of waste injection.
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Underground Waste Management and Environmental Implications
This publication consists of papers based on oral presentations at a symposium of the same name co-sponsored by the United States Geological Survey and the American Association of Petroleum Geologists. A wide range of technical issues are covered, as well as regulatory and liability concerns. Documentation of two areas in Colorado where earthquakes had resulted from subsurface fluid injection set the stage for modern debates regarding possible similar results elsewhere. A wide range of fluid compositions are subject to subsurface waste disposal. The largest volumes are brines separated during the production of oil and gas wells, but acid-water and industrial wastes of all types can be disposed in significant quantities in local areas. Large hydraulic fracture treatments never recover all of the injected fluids, and the chemical additives in the fluid that remains underground can be a concern. The subsurface injection of radioactive waste is a topic for three of the papers. The possible need for sequestration of carbon dioxide was not a significant concern at the time and was not covered, but many of the papers provide insight into the issues related to modern proposals. When fluids are injected under pressure into subsurface aquifers, they interact in numerous ways. The fluids can potentially migrate for long distances and potentially interfere with other uses for the native aquifer fluids. If the aquifer cannot transport all of the fluids away, the buildup in pressure can cause fracturing of the rock. Differences in composition between the injected and native fluids can cause chemical reactions to occur; in some cases these can be desirable in that they can immobilize certain solutes in mineral form. The long-term environmental consequences are a common theme in many of the papers because of the recognition that the disposed fluids would become a permanent fixture in subsurface aquifers and could have long-term consequences for their future utilization.