Disposal of waste fluids into the subsurface may affect the mechanical properties and strength of the rock mass in two important ways. When fluid is chemically inert, strength and ductility are reduced by increasing pore pressure. When fluid is chemically active, the strength of the rock mass is further reduced through modification of the cohesive strength of its constituent grains in contact with the fluid.
Several experiments reveal that the strength of rocks and propagation of minute surface cracks are highly dependent on the moisture content of the rock. Dilute solutions of aluminum and ferric iron salts, in addition to water, react with the surface structure of quartz and silicates and weaken the surface silicon-oxygen bonds by hydrolysis. The result is a reduction in surface energy, surface cohesion, and breaking strength. The coefficient of internal friction, however, remains unaltered.
The frictional characteristics of already-broken rocks may be significantly altered by the introduction of chemically active fluids. Because of such manmade earthquakes as those near Denver and Rangely, Colorado, it is obvious that more sophisticated tests of rock properties should be made, particularly regarding the influence of pore-fluid pressure and chemistry, if problems caused by unexpected rock failure are to be eliminated.
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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.