Compatibility of Injection Fluids with Reservoir Components1
Deep-well disposal of wastes has proved to be feasible in certain geologic areas. The success of such an operation depends on a thorough investigation of all pertinent parameters before starting fluid injection. Possible interactions of waste fluids with reservoir fluids and rocks are among factors that should be understood.
Possible reactions between formation water and injection fluid may require that buffer solutions be used to help prevent the development of precipitates. Such precipitates can cause plugging of pore spaces to reduce rock permeability.
Some formations contain clays that may swell or migrate when contacted by foreign solutions. Subsequent bridging of the migrating clays at flow restrictions may cause severe plugging of natural flow channels and shorten the useful life of the well. Proper pH control of injected fluid can minimize clay migration. It is also desirable to maintain a constant chemical composition of the injected fluid at all times.
In some instances, injection fluid will react chemically with the reservoir rock, and such reactions could eventually result in a plugging action. The effect of precipitates on injectivity will depend on flow geometry of the reservoir, and matrix permeability will be more seriously affected than fracture permeability. Thus, an operator must be selective in the type of formation into which any waste is injected. Because there are many reservoirs with marked differences in composition and properties, and many types of wastes to dispose of, it is mandatory to investigate thoroughly each specific combination before injection is started.
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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.