The seismic activity triggered by injection into the RMA well at Derby, and the earthquakes that continued after injection was stopped should serve as a warning to the waste disposal engineer that no underground disposal project is routine. project is routine. Introduction

From 1962 until 1966, waste fluids were injected into the pre-Cambrian formations northwest of Denver, Colo. Since injection started into the 12,000-ft well, earthquakes have been observed in the area. This seismic activity, although different in character, continued when injection was stopped. Consideration has been given to removal of the waste fluids from the well to reduce earthquake magnitude and frequency. This article is a case history of the well, its operation, the observed earthquakes, and a discussion of possible mechanisms. Considerable information is available in various reports. See the references for more detailed information than can be given here.

Drilling and Completion

The Rocky Mountain Arsenal (RMA) disposed of waste waters by various means until 1961, when the deep waste disposal well was completed. Before the well was completed eight geologic formations were considered as possible injection reservoirs: the Hygiene sandstone, the Codell sandstone, the "J" sandstone, the Dakota sandstone, the Morrison formation, the Lyons formation and the Fountain formation. The hole was to be drilled to a total depth to penetrate the pre-Cambrian rocks. pre-Cambrian rocks. During drilling it was found that the potential reservoirs were tightly cemented with silica and that Cambrian was then considered as a disposal prospect. Waste disposal through wells is practiced in many states. The RMA disposal well is unique in that it is by far the deepest (12,000 ft) and in that the reservoir consists of fractures in crystalline rock as opposed to sedimentary rocks in other projects. Operating conditions of the well fall within the range of all wells. The maximum wellhead injection pressure for the RMA well was 1,100 psi during an injection rate of 300 gal/min. The greatest depth of other waste disposal wells is 7,650 ft; the maximum wellhead pressure is 400 psi, and the maximum injection rate is 700 gal/ min. During the drilling of the well, lost circulation was observed in the Lyons formation (9,582 to 9,772 ft). Eight instances of mud losses were noted. These ranged from a 10- to a 320-bbl loss. Cores indicated the formation to be fractured. The Fountain formation (9,772 to 11,880 ft) appeared to be highly fractured to a depth of 10,400 ft, with some fracturing at greater depths. Lost circulation was frequently experienced while drilling through the Fountain, mostly between 9,960 and 10,250 ft. The lowest mud loss took place at 11,080 and was the most serious one in the well, with some 1,350 bbl lost. The well was cased to 11,171 ft and subsequent drilling did not encounter lost circulation until after a core was cut in the pre-Cambrian (11,950 ft total depth).

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