Cavernleaching Techniques

The fundamental technique of cavern development involves drilling and cementing concentric casings into the salt dome, then drilling an uncased hole to expose the salt for dissolution. Two concentric leaching strings of pipe provide circulation of water through the well to dissolve the salt (Figure 7). Water is injected through either the inner leach string or the annulus, depending upon the leach phase. As water is injected and cavern development proceeds, a flow of brine is circulated back to the surface. The result is the development of a cavern by dissolution. The brine is filtered for solids removal, then pumped through a pipeline to brine disposal wells located off the dome. Alternatively, this high purity brine may be used in chemical production activities.

The rate at which the cavern enlarges depends on two parameters, the flow rate and the brine concentration achieved within the cavern. These parameters are essentially a function of the water injection rate, existing cavern volume, contact surface between salt and water, and method of leaching.

The two basic leaching methods are direct circulation and reverse circulation. The direct circulation leaching method is used in the first phase of the leaching process to enlarge the initial bore hole and form the cavern chimney and sump. Direct circulation involves the injection of raw water through the inner leach tubing suspended near the bottom of the cavern and the withdrawal of brine through the annulus positioned above the raw water injection point. With this method, maximum diameters occur near the bottom of the cavern, and the diameters decrease toward the cavern top.

The reverse circulation method is used in the final stage for development of the upper area of the disposal cavern. In reverse circulation, water is injected down the annulus between the inner and outer suspended strings, causing brine to circulate into the inner string below the raw water injection point. Reverse circulation causes cavern enlargement in the top half of the cavern. Cavern shapes resulting from reverse circulation depend on the positions of the suspended strings within the cavern. By raising or lowering the strings, salt dissolution can be controlled to achieve favorable shapes. In this manner, direct and reverse leaching result in a nearly cylindrical cavern.

Both direct and reverse leaching methods will dissolve salt on all exposed salt surfaces. To prevent salt dissolution above the planned cavern top, the salt in this region is protected from leaching by injection of a material that is immiscible and lighter than water. This material blankets the exposed salt at the cavern top, prevents leaching of the salt from around the cemented casing, and protects the casing from internal corrosion. The blanket is injected in the annulus

WELLHEAD

RAW WATER

4C0C W

BRINE

GROUND LEVEL

GROUND LEVEL

BLANKET LEVEL

NOT TO SCALE

Figure 7 Direct injection process.

Figure 7 Direct injection process.

BLANKET LEVEL

NOT TO SCALE

between the last cemented casing and the leaching string. Careful control of blanket withdrawal near the end of the leaching program permits the development of an arched roof in the caverns to enhance roof stability.

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