US2618475A - Method of mining soluble salts - Google Patents

Description

Nov. 18, 1952 c, BUTLER, JR 2,618,475

METHOD OF MINING SOLUBLE SALTS Filed Feb. 24, 1949 2 SHEETS-SHEET 1 FIG. I FIG. Il'.

TEABRINE ---BRINE INVENTOR. CLARENCE A. BU TLER J'R.

C. A. BUTLER, JR

METHOD OF MINING SOLUBLE SALTS Nov. 18, 1952 2 SHEETS-SHEET 2 Filed Feb. 24, 1949 WAT E R FIG! -'- WATER FIGJIY WEAK BRINE couc. BRINE FIG. III

CONC. BRINE WATER FIG. '11.

INVENTOR. CLARENCE A. BUTLER JR.

MEZZAM Patented Nov. 18, 1952 UNITED STATES TENT OFFICE METHOD OF MINING SOLUBLE SALTS Application February 24, 1949, Serial No. 78,191

3 Claims.

This. invention relates to a method for mining soluble salts in subterranean deposits thereof.

It has heretofore been proposed to mine soluble salts in subterranean deposits by forcing a solvent into the deposit of salt, allowing the solvent to become substantially saturated with dissolved salt, and subsequently removing the solution from the cavity thus formed in the deposit.

Certain difficulties attend this method of mining salts, particularly where the salt is ordinary rock salt and the deposit thereof is several hundred feet beneath the earths surface. For example, the subterranean deposits of rock salt in the region of the Great Lakes are general- 1y located about LOGO-2,500 feet below the earths surface and are generally overlaid with layers of shale, and less commonly with layers of sandstone or limestone. Other deposits in the region of Texas and Louisiana occur in the shape of domes up to about three miles in diameter, found about 1.000 feet beneath the earths surface and estimated to be up to 40,000 feet in thickness.

The several different types of salt deposits require that different techniques be employed in the mining thereof in order that maximum efficiency may be obtained in the mining operation. However. whether the salt deposit is of the dome type or of the very thick layer deposit type as generally found in the Great Lakes region, or of the more shallow layer type, the dissolution of salt within the deposit, when continued over extended periods of time, removes the support for the overlying shale, sandstone or limestone. and thus when the horizontal cross-section of a mine cavity becomes sufiiciently large, a cave in of shale or stone may result, which cave in often severs the solvent inlet and brine take-off conduits, whereupon time-consuming and costly repairs must be made to put the well in operation again. Moreover, the mine cavity within the salt deposit must be of such form that insoluble materials associated with the salt in the deposit do not accumulate upon the sloping walls of the cavity and thereby overlay the soluble material with an insoluble mud and thus prevent the dissolution of the soluble salt in the solvent.

The method of the present invention for mining of a subterranean deposit of a soluble salt byintroducing a solvent into the salt deposit and withdrawing a solution of said salt in said solvent therefrom, includes the steps of providing a vertically elongated cylindrical cavity within said deposit, introducing solvent into said cavity below the top of said deposit, causing it to rise in said cavity to the top thereof in contact with partially saturated brine, but substantially out of contact with the soluble salt, causing it to descend to the bottom of said cavity in dissolving contact with said salt, and withdrawing a solution of said salt in said solvent from said cavity.

Thus, it is an object of the present invention to provide a method of recovering soluble subterranean deposits in the form of their solutions at maximum concentration of solution and minimum expense.

Another object is to recover such salts in a manner to leave in the deposit a relatively stable relation between the earth strata above the deposit and the remaining salts.

Still another object is to avoid the expense and loss of capacity which accompanies cave ins in such deposits.

A further object is tov withdraw solutions of such salts in a manner to leave a clean, substantially mud-free salt surface exposed to the unsaturated solvent at all times whereby the strength of the solution and hence, capacity of the well, is accordingly increased.

A more detailed consideration of the present invention is given in connection with the drawings attached hereto and forming a part hereof, in which:

Figs. I, II, and III are somewhat diagrammatic vertical sectional views of cavities in salt beds obtained by prior art methods and,

Figs. IV, V, VI, and VII are similar views of the various stages in the evolution of a cavity obtained by the mining method of the present invention. As shown in these figures, a casing may descend through the covering strata to the salt bed and be suitably cemented or otherwise secured to the non-salt bearing strata at the bottom thereof. This casing serves as a passage for introduction of brine and water conduits; as is Well understood, it has no specific mining func-.

tion of its own.

All the figures are drawn from vertical sections of cavities obtained by operating scalemodel wells in accordance with both the prior art methods and the method of this invention and observing the course of erosion of the soluble salt within the deposit from time to time.

In Fig. I is shown one of the early methods employed in the mining of subterranean salt deposits comprising two concentric conduits ex tending into the salt deposit. The inner conduit penetrated. substantially the full depth of the salt. stratum, whereas the outer conduit exoperation of the wells.

tended only a short distance thereinto, and a solvent, such as water, was introduced into the salt deposit through the outer conduit and saturated brine flowed from the deposit through the inner conduit. As a result of this method of mining, after an extended period of operation, much of the soluble material at the top of the salt stratum dissolved out laterally, thereby destroying the support of the shale roof and leaving an inverted pinnacle or morning glory shaped cavity surrounding the inner brine takeoff conduit. As may be seen from Fig. I, a cave in of the roof cavity would block the free passage of brine to the brine take-01f conduit or sever the take-off conduit itself, necessitating time-consuming and costly repairs to refit the well for operation. Moreover, a relatively large area of the cavity is substantially horizontal or sloping only to a very slight degree. This condition naturally results in the insoluble material associated with the soluble salt in the deposit, blanketing especially the horizontal, but to a marked degree the slightly sloping areas with insoluble mud hereinabove referred to, thereby materially decreasing the well efficiency. Where the salt stratum is relatively shallow, for example, of the order of 50-75 feet, cave in occurs, as a result of this method of mining, relatively frequently, thereby decreasing the overall efficiency of well operation.

In Fig. II is shown another method which has been proposed for the purpose, comprising extending the outer of two concentric conduits into the salt stratum to a level somewhat above that of the brine take-off conduits and, in addition to the water forced into the salt deposit to dissolve salt therefrom, air or other non-solvent is also forced into the mine cavity. The air or other non-solvent rises to the roof of the cavity, thereby protecting the soluble salt in this region from the action of the solvent. In this manner dissolution of the salt is effect-ed largely radially from the water and non-solvent inlet conduits, whereby the mine cavity takes on the shape of a very large disc. Fig. II shows a series of such cavities resulting from working the salt deposit from the base thereof upwardly towards the roof. By this method of operation, a salt stratum may be worked extensively over a wide radius before there which has been proposed for preventing "cave in, as well as the undesirable effects of insoluble mud deposits overlaying the salt during the In accordance with this proposal, the inner of two concentric conduits extends substantially to the full depth of the salt stratum while the outer of the two conduits extends only a slight distance thereinto. Solvent is introduced through the inner of the two conduits and substantially saturated brine is removed from the mine cavity near the top of the salt stratum through the outer of the two conduits. As may be seen from Fig. III, the mine cavity formed by such procedure is an elongated cylinder, and while a relatively small area of the overlying shale roof is left unsupported and exposed to the cavity, thereby permitting comparatively safe operation of the well for relatively extended periods of time, the capacity of the well, i. e. the volume output of substantially saturated brine, is extremely low even in salt strata of substantial depth. As shown in Fig. 111 of the drawings, the rising stream of solvent in the central portion of the cavity surrounding the solvent inlet conduit can only increase in concentration of solute by diffusion from the more saturated brine in the area of the cavity nearer the walls thereof and by mixing with the brine of said area, the mixing being caused by the mechanical contact of the brine and solvent. However, unless the flow through the well is extremely slow and thus the well operates at low capacity, the obtaining of desired saturated brine is substantially impossible.

In contrast to these prior art methods of mining soluble salts in subterranean deposits, the method of the present invention introduces solvent into the salt stratum and, as illustrated in Figs. IV-VII, removes brine therefrom in such a manner that the mine cavity acquires the shape of a paraboloid of revolution. Certain definite advantages of operation are thereby obtained. For example, the steep slope of the boundary walls of the mine cavity in the region where the brine solution travels downwardly thereagainst prevents the deposition of the insoluble mud in this region, thus insuring continued high capacity of the mining operation, since this slope of the boundary walls of the mine cavity is maintained for the most part at an angle greater than the angle of repose of the mud deposit during substantially the entire period in which the salt is mined. In addition, the roof of the mine cavity is well-supported during most of the period of the operation and a relatively large cross-sectional area is provided at the base of the mine cavity, whereby the danger of cave in and the damage ordinarily attending this event is substantially lessened.

In preparing for the mining operation in accordance with the present invention, the overlying layers of clay, shale, rock, etc., may be penetrated in any suitable manner, such as drilling, until the subterranean salt deposit is reached. Thereupon, a suitable casing may be inserted into the drilled hole to a maximum depth as shown to the top of the salt stratum though, as is well understood, where a hard stratum exists directly above the salt bed, the depth of the casing is a matter of choice. Thereafter, the salt deposit is preferably drilled to a suitable depth to allow for the placing of solvent inlet a brine take-off conduits therein. Ordinarily, in the Great Lakes region, the salt strata are in general between about and 650 feet in depth and are accordingly generally drilled to substantially their full depth, as illustrated in Fig. IV of the drawings. The initial cavity provided by such drilling operations within the salt deposit proper is enlarged usually by "washing, which consists in introducing a pair of concentric pipes, the inner of which reaches to substantially the bottom of the cavity and the outer of which is at or near the top. Solvent, such as water, is introduced through the inner pipe and withdrawn through the outer pipe until the desired dimensions of the cavity are obtained, which dimensions may be evidenced by the concentration of salt in the brine issuing from the well. The pipes may be suitably situated adjacent each other in the casing but the concentric arrangement is usually considered most convenient. In the early part of the washing operation (see Fig. theatres, of the salt deposit in contact with the solventis relatively small and the concentration of salt in the brine. issuing from the well is relatively low, whereas during the final period of the "washing operation (see Fig; V), the area of the salt deposit contacted with solvent is usually larger and the brine issuing from the well during this period approaches saturation. The washing step may require several weeksor'months, in full scale operation, depending noon the depth of the salt stratum and the rate at which the solvent is introduced into the initial cavity; It is' preferable in the practice of the. method of the. present invention during the washing step that the solvent be introduced. into. the mine cavity very near the. base thereof and. that the. brine. be withdrawn. from the. cavity in the region of the top of the salt stratum, as illustrated in Figs. IV and V of the drawings.

At the conclusion of the washing step, in accordance with the method of the present in-- vention, particularly where the solvent and brine conduits are arranged concentrically, solvent is introduced into the mine cavity through the outer of. the two concentric conduits and very near the base of the mine cavity, as illustrated in Fig. VI of the drawings. The solvent, being considerably lighter than brine, then rises in the region of the solvent conduit to the roof of the salt stratum, becoming progressively more satu-- rated by diffusion of solute from the brine already in the mine cavity, and by mechanical mixing therewith, and then passes radially toward the vertical walls of the cavity, as indicated in Fig. VI. The partially saturated brine thus formed passes downwardly along the vertical cavity wall in contact with the solute, becoming increasingly concentrated with dissolved salts, whereby the brine thus formed becomes more saturated and falls more rapidly toward the base of the mine cavity and, except for the portions thereof, which become associated with the incoming solvent as noted above, is withdrawn through the inner of the two concentric conduits, which opens into the mine cavity at a level somewhat lower than the conduit through which the solvent was introduced. Preferably, the distance between the openings of the concentric conduits, after the washing period, is so controlled that it is greater than five times the width of the annulus between the conduits and less than half the depth of the mine cavity, preferably less than thereof. Specifically, where the depth of the stratum is of the order of 100 feet or more, the solvent inlet conduit preferably terminates from one to ten feet above the terminus of the brine take-off conduit in the cavity. Gradually, the mine cavity takes on the form of a paraboloid of revolution of large diameter and having a very steep slope in the boundary wall, as illustrated in Fig. VII, whereby the insoluble material associated with the soluble salts in the deposit falls away from the dissolving area toward the base of the cavity, in which region it is ineffective in decreasing the efficiency of the mining operation.

A particular advantage of the method of the present invention arises from the fact that salt at the top of the stratum is always contacted with relatively saturated brine rather than with solvent (see Fig. VII). The rate of solution of the salt at the top of the cavity is accordingly low and hence, the cavity only attains cave in size after extended use of the well. This may be 6 contrasted. with the prior art practice (viz. Fig. I) and considered with the fact that the well of the present method has high capacity whilepossessing this advantage.

The above-described sequences of events, both in prior art and wells of the present invention, are verified by setting up scale-models of salts wells, using large blocks of solid rock salt, drilling the blocks to a suitable depth near one face of a block, placing a window of glass or other suitable material over that face of the block nearest the drill hole, and sealing the salt block and window against loss of brine solution. Thereafter, in the case of demonstration of the present invention, the drill hole is washed in the manner abovedescribed finally the Well is operated, employa dye in the solvent water used in the. mining operation so that the cavity contour, as well as the path described by the diffusing solventin its course through the well cavity, is accuratelyobserved when that portion of the salt block nearest the window has dissolved away, exposing the mine cavity.

While there have been described in detail certain forms of the invention and embodiments of its practice, the invention is not to be understood as being limited to the detailed disclosure as it is realized that changes within the scope of the invention are possible, and it is further intended that each step in the following claims shall refer to all equivalent steps for accomplishing the same result in substantially the same or equivalent manner, it being intended to cover this invention broadly in whatever form its principle may be utilized,

What is claimed is:

l. The method of mining a soluble salt in a subterranean deposit, which includes the steps of initially providing a vertically elongated cylindrical cavity extending to a substantial depth within said deposit, providing conduits in said cavity through which a solvent for said salt is introduced into said cavity and a solution of said salt is withdrawn therefrom, introducing a solvent for said salt into said cavity at the base thereof and withdrawing a solution of said salt from said cavity at the top thereof, thereby enlarging said cavity throughout its length to a substantially greater diameter than the initial cavity, thereafter introducing said solvent into said cavity at a point of relatively high salt concentration in said solution near the bottom of said cavity, and withdrawing said solvent with said salt dissolved therein from said cavity at a point adjacent the bottom of said cavity lower than said point of introducing said solvent, whereby said solvent rises to the top of said cavity in a substantially vertical path and mixes with said salt solution, and whereby solvent and solution move from the top of said cavity downwardly along the boundary walls thereof in dissolving contact with said deposit to the bottom of said cavity.

2. The method of mining a soluble salt in a subterranean deposit, which includes the steps of initially providing a vertically elongated cylincavity within said deposit, providing concentric conduits through which solvent is introdnced into said cavity and a solution of said salt in said solvent is withdrawn therefrom, extending the inner of said conduits substantially to the full depth of said cavity and the outer of said conduits to the upper level of said cavity, initially introducing solvent through said inner conduit and removing a solution of said salt in said solvent from said deposit through said outer conduit,

thereby to enlarge said cavity throughout its length to a substantially greater diameter than the initial cavity, thereafter extending said outer of said conduits into said cavity to a level near the base thereof but above the level of the terminus of said inner conduit, the distance between termini of the conduits being so controlled as to be greater than 5 times the width of the annulus between the conduits and less than half the depth of the mine cavity, thereafter introducing further quantities of solvent into said cavity through said outer conduit whereby said solvent rises to the top of said cavity in a substantially vertical path and mixes with solution as it rises, and whereby solvent and solution move from the topof said cavity along the boundary walls thereof in dissolving contact with said deposit to the bottom of said cavity, while a solution of said salt in said solvent is withdrawn from said cavity through said inner conduit, and continuing thusly to mine said salt in said deposit.

3. The method of mining a soluble salt in a subterranean deposit, which includes the steps of providing an inner conduit extending for a substantial depth in the said deposit, providing an outer conduit concentric with said inner conduit and terminating at substantially the upper limit of said deposit, introducing a solvent for said salt through the inner of said conduits and withdrawing solvent through the outer of said conduits to form a cylindrical shaped cavity in said deposit by action of said solvent moving from the termination of said inner conduit to the termination of said outer conduit, resetting said outer conduit in position substantially at the bottom of said cavity but above the termination of said inner conduit and thereafter introducing solvent through said outer conduit and withdrawing a solution of said salt in said solvent through said inner conduit, whereby said solvent rises to the top of said pre-formed cavity in a substantially vertical path and mixes with solution of said salt as it rises and whereby solvent and solution move from the top of said cavity downwardly along the boundary walls thereof in dissolving contact in said deposit to the bottom of said cavity.

CLARENCE A. BUTLER, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 273,623 Smith Mar. 6, 1883 287,909 Cook Nov. 6, 1883 1,121,225 Bradley Dec. 15, 1914 2,009,534 Trump July 30, 1935

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