Earthquakes have been clearly linked to subsurface fluid injection in wells at two places—near Denver at the Rocky Mountain Arsenal waste-disposal well and at the Rangely oil field in northwestern Colorado. The theory linking fluid pressure to earthquakes is based on the effective stress concept—i.e., that increases of pore pressure reduce the effective normal stress across fracture surfaces. At both the Rocky Mountain Arsenal and Rangely, evidence exists for sub-stantial tectonic shearing stresses in the reservoir rock, although stress was below the critical value necessary to cause failure. At Denver, fluid injection relieved a fraction of the frictional resistance to shear fracture and earthquakes resulted. There is presently no way to determine before drilling whether injection at a given site will produce earthquakes. At Denver and Rangely, the earthquakes appear to have originated entirely along preexisting faults. In placement of injection wells, existing faults should be avoided. Seismic surveillance during injection can provide early warning of inadvertently triggered earthquakes, and palliative measures then can be taken. At Rangely, the earthquakes have been drastically reduced in frequency by reducing pore pressures in the hypocentral region. On the basis of this experience, it appears that seismic activity due to waterflooding in oil fields can be controlled without seriously disrupting production of oil.
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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.