The present paper presents methods for injection of brine solution into the subsurface investigated under the Compressed Air Energy Storage Project of the Department of Energy in the United States of America. The principal objectives in this investigation were two fold:

  • Prevention of any possible pollution of ground water by the injected brine solution.

  • Avoidance of over pressurizing the solution-deposit zone by the injection operation which will result in:

    • induced undesirable fractures in both upper and lower confining formations, and

    • inefficient and uneconomical injection operation.

INTRODUCTION

The continental United States has many rock salt deposits, as well as salt lakes and natural brines. Salt is found in 4 major basins as well as several minor basins (Fig. 1). Taken together, this means that over one half of the states contain salt deposits (Lefond, 1969). The injection of brine into geologic formations can cause contamination of existing portable ground water. The contamination is due to either extensive lateral spreading or vertical upward of downward movement of the brine. The direction and magnitude of brine movement is a direct function of the total geologic environment surrounding the injection point. The total geologic environment embraces the rock types, fractures, rock strengths and permeabilities, in situ state of stress, ground water regime(s), and geochemistry. During brine injection containment is ideally obtained by ensuring that the suitable injection sequence, which is highly permeable, is over/underlain by impermeable rocks. These rocks restrict both the vertical upward and downward movement of the brine. The lateral extent is dependent on the uniformity, isotropic aspects, of the injection formation. The operating fluid pressures active within the migrating brine, must be known to ensure that they do not cause either, a) tensile fractures or b) extend pre-existing fractures within the over/underlying confining rocks. The promotion of fractures markedly increases the permeability of the low permeability confining rocks, and consequently contamination of previously isolated ground water by the brine.

CRITERIA FOR INJECTION FORMATION

The injection formation should be:

  1. an extensive sedimentary formation: unfractured sandstone, limestone, dolomite, and unconsolidated sands are the general lithologic types used for injection;

  2. at least several hundred feet thick;

  3. hundreds of square miles in extent in order to minimize injection pressures and to provide a buffer zone against migration of brine to discharge areas;

  4. an area of relatively simple geologic structure without complex folds and faults (synclinal sedimentary basins are favorable; high formation pressures are unfavorable);

  5. of uniform hydraulic properties with high enough permeability so that excessive injection pressures can be avoided (according to Warner in 1977, a porosity of at least 10% to 20% and permeability greater than 100 millidarcys, or 0.2 ft/day, is required for large-scale brine injection);

  6. of low or negligible lateral movement of formation fluids, to a discharge point (grouting may be necessary to lower the permeability so that lateral confinement is maintained);

  7. a zone that is below the level of fresh-water circulation with the surface

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