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With a yearly discharge of liquid industrial waste of more than 14 trillion gallons before treatment, underground injection is a valuable tool to aid industry with the problems of storing and treating its generated waste.
Disregarding mechanical factors, reservoir characteristics exert the greatest influence on operating safety and economics of injection systems. Ideally, a host reservoir should be a uniform salaquifer of large areal extent, substantial thickness, high porosity and permeability, and low pressure with adequate overlying and underlying aquicludes containing fluids compatible with injected wastes. There should be a minimum of faulting and abandoned wells near the injection site.
Compressibility of water, rock, and gas in solution, in addition to any previously removed fluids, provide the space necessary for injection into the host reservoirs. The pressure effect at various distances from the wellbore, for given times and volumes of injected fluids, is important in predicting long-range reservoir performance, effect on inadequately plugged wells in the vicinity, effect of injection near potentially valuable mineral deposits, and unintentional formation fracturing. Potentiometric levels and gradients should be determined for host reservoirs to help analyze and anticipate fluid movement and monitoring methods needed.
With over 300 waste-injection wells in operation today, the vast majority are injecting into reservoirs at depths of less than 6,000 ft. Approximately 70% are injecting into sandstone and unconsolidated sand and about 25% are injecting into limestone and dolomite, with the remaining percentage injecting into crystalline or evaporite sequences. Porosities in these wells range from under 10% to over 30%; permeabilities range from under 1 md to over 5 Darcys, giving a wide range of potential injection volumes and pressures and associated operating conditions.
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