Reinfection of formation waters creates few chemical problems if no large change in temperature has occurred, no gas or vapor has separated, and access of air has been prevented. The fluids already have had an opportunity to react with the minerals to the point of compatibility. Injection of incompatible fluids, however, may cause chemical problems; thus, prediction of fluid-mineral reactions should be attempted.
Real geologic systems chosen where reactions are known to occur, and where the reactant minerals and fluids and product minerals and fluids can be identified and analyzed, rarely have stable equilibrium (minimum Gibbs free energy) conditions. In some situations, metastable equilibrium conditions have been found. The assumption of a stable equilibrium state is an unwise assumption for a model, especially at temperatures of 100°C and below.
The general lack of attainment of equilibrium in no way impairs prediction of reactions, however. Admittedly, there is no a priori thermodynamic method of predicting what phase will react, nor can the necessary departures from equilibrium (ΔGR) be predicted before a reaction will occur at a significant rate, because thermodynamic arguments are time independent. However, in geologic systems now accessible, enough reactions are occurring that, by observation, an empirical knowledge will provide a base for predicting reactions.
Reactions should be studied in both reaction directions. Most reactions are asymmetric in that the ΔGR required to dissolve most phases is of different values than the ΔGR necessary to form the mineral from solution. The reactions should all be treated as congruent. All the species in a solution generated by complete solution of the solid must be considered. Using in-congruent reactions (reactions producing a new solid directly from the reactant solid) introduces the unwarranted assumption of equilibrium.
In general, hydroxide species such as amorphous silica, limonite, and brucite, as well as simple carbonates, have been found to dissolve or precipitate in natural systems where ΔGR values are 1 kcal or less. More complex silicates including serpentine, kenyaite, and magadiite may not precipitate from solutions where ΔGR values favor supersaturation by as much as 7 kcal. Sulfides such as covellite, chalcocite, and pyrite may not precipitate even where supersaturation exceeds 30 kcal, although some sulfide minerals will dissolve readily where oxidation of sulfur can take place.
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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.