Hydrodynamics of Fluid Injection1
A more complete theory than has heretofore been available for the flow of slightly compressible fluids in multilayered porous systems predicts the effects of injecting into a permeable layer (aquifer) and also the consequent effects in adjacent confining beds of relatively low permeability (aquitards). Pressure buildup in a multilayered system depends on the degree of communication that develops within the system between aquifer and aquitard. This communication can be characterized in terms of dimensionless parameters β and r/B, which are functions of the permeabilities, storage coefficients, and bed thicknesses. Analytical equations for a three-layer system have been evaluated for typical values of β and r/ß, and the results have been verified by independent numerical methods.
This new theory provides a basis for understanding the hydrodynamics of fluid injection in multilayered systems. It also leads to the ratio method of determining hydraulic properties of aquitards; applications of the method to two examples of field data provide confirming data. By considering multilayered systems under quasi—steady-state conditions, a conservative method of evaluating leakage through an aquitard can be made. One can therefore determine how effectively fluids injected in any given aquifer are retained within that layer. Such knowledge is of vital importance if environmental problems resulting from an unexpected migration of toxic or otherwise undesirable fluids are to be avoided.
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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.