Evan reported in 1966 that during 1961, 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 appear 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.
Evans concluded that the Rocky Mountain Arsenal Pressure Injection Disposal well was drilled for the purpose of disposing of contaminated waste water,
which is a by-product of chemical warfare products manufactured at the Arsenal. During the month following the initiation of injection of waste water into the almost vertically fractured Precambrian rocks there were two earthquakes with epicenters in the Arsenal area.
A summary of the evidence relating the Arsenal injection program with the earthquakes:
1. The first earthquakes observed during the present century with epicenters in the Denver area were recorded during the month following the initiation of the Arsenal injection program.
2. Since the initiation of the injection program in march 1962, 150 million gallons of waste have been injected into the Arsenal well, and there have been 710 earthquakes (to 1 October 1965).
3. The majority of the earthquake epicenters are located within five miles of the Arsenal well. All epicenters determined from four-or-more station data
are within seven miles of the well.
4. There is evidence that the earthquake activity is taking place along a plane that dips eastward and passes beneath the Arsenal well at a depth of 6.5 miles (Wang, 1965).
5. When the Arsenal injection program is considered on the basis of high, medium, low, or no injection, there is a correlation between the fluid injected and earthquake frequency.
6. The best correlation of earthquake frequency with fluid injected occurred during July, August and September 1965, when relatively large amounts of fluid were injected at higher pressures and for longer periods of time than previously.
7. A statistical analysis (Bardwell, 1966) is cited that suggests a mathematical relationship between Arsenal earthquakes and volumes of waste injected into the Arsenal well.
The volume of fluid injected appears to be affecting the Precambrian reservoir only for a limited distance from the well bore, and rough estimates of the energy released by a single earthquake suggest that relatively minor rock movement is involved. The Precambrian reservoir receiving the Arsenal waste is highly fractured granite gneiss of very low permeability. The fractures are nearly vertical. The fracture porosity of the reservoir is filled with saltwater. Reservoir pressure is 900 pounds sub-hydrostatic. It appears that movement is taking place in this fractured reservoir as a result of the injection of water at pressures from 900 to 1,950 pounds greater than reservoir pressure.
Hubbert and Rubey (1959) point out that rock masses in fluid-filled reservoirs are supported by solid stress and the pressure of interstitial fluids. As fluid pressure approaches lithostatic pressure, the shear stress required to move rock masses down very gently slopes approaches zero. It appears that these principles offer an explanation of the rock movement in the Arsenal reservoir. The highly fractured rocks of the reservoir are at rest on steep slopes under a condition of sub-hydrostatic fluid pressure. As the fluid pressure is raised within the reservoir, frictional resistance along fracture planes is reduced and, eventually, movement takes place.
The elastic wave energy released is recorded as an earthquake. The earthquake hypocenters indicate the point at which movement is taking place and injected fluid is triggering this movement, then there is evidence that open fractures exist at depths of 12 miles under conditions of lower fluid pressure-overburden weight ratios than has formerly been considered possible (Secor, 1965). Evans concluded that the high angle of this fracture system is an important factor. Because the fractures are almost vertical, only a small part of the lithostatic pressure is acting to force the fractures closed, and they can remain open under conditions of lower fluid pressure, and at greater depth than if they were horizontal or inclined at a lower angle.
The time lag between fluid injection and earthquake frequency was assumed to be due to the unsteady state of percolation of fluid through the fractures in the reservoir due to the opening and closing of these passages resulting from the applied pressure of the injected waste. Evans also assumed that as fluid continues to be injected into this reservoir, fluid pressure will be increased at greater distances from the well bore, and rock movement will be occurring at
ever increasing distances. In the present case, Evan concluded that a stable situation in this Precambrian reservoir is being made unstable by the application of fluid pressure.
However, it is interesting to speculate that the principle of increasing fluid pressure to release elastic wave energy might have an application in the subject of earthquake modification. That is, it might someday be possible to relieve the stresses along some fault zones in urban areas by increasing the fluid pressures along the zone using a series of injection wells. The accumulated stress might thus be released at will in a series of non-damaging earthquakes instead of eventually resulting in one large event that might cause a major disaster.
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