Abstract The pressure injection disposal well completed for the U. S. Corps of Engineers at the Rocky Mountain Arsenal, 10 miles northeast of Denver, exemplifies a revolutionary concept in industrial waste disposal. This method is one which has been used by the oil industry for many years. The well has two major distinctions: it is the deepest well drilled in the Denver Basin, and it may provide a precedent for solving waste disposal problems which are anticipated in the future industrial growth of the Denver area. The well was drilled on the east flank and near the axis of the Denver Basin and penetrated Tertiary through pre-Pennsylvanian sediments. Drilling was completed at a depth of 12,045 feet in Precambrian gneiss. The sediments from Tertiary through Pennsylvanian are in normal sequence. They show only minor variations in thickness and lithology from sections in adjacent areas. The occurrence in this part of the Denver Basin of pre-Pennsylvanian sediments, possibly Ordovician in age, was unexpected and may shed valuable light on the paleography of the Denver area. Lost circulation was a major drilling problem in the Paleozoic sediments and in the Precambrian gneiss. Slow penetration rates resulting from the induration of the Paleozoic sediments necessitated the use of special hard-formation drilling bits. Core information shows that the Permian and Pennsylvanian sediments are fractured and have a rock matrix of low porosity and permeability. Drill-stem tests indicate that the Paleozoic sediments contain low-pressure reservoirs.

Abstract During 196l, a deep well was drilled at the Rocky-Mountain Arsenal northeast of Denver, Colorado, to dispose of contaminated waste water. The well is bottomed in 75 feet of highly fractured Precambrian gneiss. Pressure injection of waste water into the fractured Precambrian rock was begun in March 1962. Since the start of fluid injection, 710 Denver-area earthquakes have been recorded. The majority of these earthquakes had epicenters within a five-mile radius of the Arsenal well. The volume of fluid and pressure of fluid injection appears to be directly related to the frequency of earthquakes. Evidence also suggests that rock movement is due to the increase of fluid pressure within the fractured reservoir and that open fractures may exist at depths greater than previously considered possible.

Abstract Aspects of the statistical problem of relating frequency of earthquakes in the Denver area to injection of waste water by the Rocky Mountain Arsenal are discussed. Behavior of cumulative time series for earthquake frequency and volume of water are analyzed. A regression-correlation analysis leads to an exponential model approximating the relationship between these two variables. Also considered are statistical tests of the differences in mean number of earthquakes per month for three periods of waste injection.