INTRODUCTION AND OVERVIEW
The natural accumulation of liquid and gaseous hydrocarbons in confined anticlinal or domal porous strata makes possible the economic recovery of these resources. The reverse process of storing natural gas within these stratigraphic traps has found widespread acceptance and is well understood. Storage of other gases such as air, hydrogen, and helium is also under consideration because of the regional availability of these low cost natural containments (Allen and Gutknecht, 1980). The storage of compressed air in porous media reservoirs is being investigated as a technique to convert lower cost off-peak base load electricity into mechanical energy for subsequent reconversion during high demand periods. In addition, dissolved salt and mined hard rock caverns are being examined for this potential application. In the porous medium concept shown in Figure 1 an air bubble is generated within an aquifer by delivery of air at a pressure higher than the hydrostatic discovery head so that a volume of water equal to the volume of compressed air would be displaced from the near-wellbore region. Subsequent withdrawal and expansion of the air mass through a conventional combustion turbine-generator would provide electrical energy above base load output to meet peak demand.
FIGURE 1. Schematic of an Aquifer System (Available in full paper)
In present CAES plant designs, air is injected at a temperature close to that of the natural reservoir. This requires cooling to remove the heat of compression (reducing the injected air temperature from about 350]]x0B0;C to below 100°C). On the generation side of the cycle, a fossil-fueled combustion turbine is required to reheat the compressed air. However, possible storage at elevated temperatures is of great interest because the efficiency increase from utilizing the heat of compression could eliminate petroleum firing during power generation. This would enhance CAES, which is presently restrained by rising prices of hydrocarbon fuel, decreasing availability of fuel, governmental restrictions on use of oil and natural gas for electric generation. The necessity also exists to dehydrate the near-wellbore air storage zone fairly rapidly (Stottlemyre et al, 1979).
Most of the uncertainties associated with this storage concept concern the subsurface reservoir and the injection-delivery well system. Although most experience derived from natural gas storage can be applied to aquifer compressed air storage, several differences exist between the two systems (Weinstein et al, 1978). The storage of air for CAES involves daily or weekly, rather than seasonal, cycling. Air has at least twice the viscosity of natural gas; and air storage at elevated temperatures may be desirable. Frequent pressure, temperature, and humidity cycles may have detrimental impacts on the aquifer rock matrix. In addition, the aerobic subsurface environment produced by air storage may cause formation problems by oxidation of inorganic and organic substances.
Aquifers are suitable sites for compressed air storage because of low construction costs and widespread availability (Weinstein et al, 1978). The storage volume of interest consists of interconnecting pores, microcracks, channelways, permeable bedding planes, and joints which characterize the porous medium. An impermeable caprock and some form of structural trap is required to contain the air.