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<br />
<br />PROCESS FOR SOLUTION hflN[NC NAHCOLfTE
<br />3,779,602
<br />2
<br />BRIEF DESCRIPTION OF THE DRA WINCS
<br />BACKGROUND OF THE INVENTION
<br />Field of the Invention
<br />This invention relates to the field of producing mint
<br />als from subsurface formations; and more particular
<br />to a process for solution mining nahcolite from subs
<br />face oil shale formations
<br />Description of the Prior Art
<br />The recos•ery of water-soluble minerals from subs[
<br />(ace deposits 6y solution mining with aqueous fluids
<br />well known. In such a process, aqueous fluid is Ilowi
<br />down a well into contact with a subsurface deposit. TI
<br />solution dissolves some of the soluble mineral. TI
<br />mineral-containing solvent is then flowed to the surfai
<br />where it is [reated to remove the dissolved miner.
<br />e.g., by evaporation.
<br />The solubility of most commercially interestir
<br />water-soluble minerals increases with increasing to
<br />perature. Therefore, aqueous solution-mining fluid
<br />often heated to increase its mineral carrying capaci
<br />before it is injec«d into a subsurface mineral deposi
<br />For example, U.S. Pat. No. 1,649,385 issued Nov. I
<br />! 927, to H. Blumenberg, Jr. [caches a method of sol
<br />[ion-mining crystallized boron compounds by using
<br />mixture of hot air and steam.
<br />In the western United States, there are large subsu
<br />Cace oil shale formations which contain substanti
<br />amounts of water-soluble, heat-xnsitive bicarbonat
<br />minerals such az trona and nahcolite. These mines
<br />are present both in inter-bedded substantially pure so
<br />able mineral layers and as dispersed nodules in certai
<br />layers which predominently contain oil shale.
<br />It is known that these heat-sensitive, vs•ater-solubl
<br />minerals can be solution-mined with hot aqueous solo
<br />[ions. See, for example, U.S. Pat. 3,050,290, issue
<br />Aug. 21, 1962, to N. A. Caldwell et al. A eo-pendin
<br />commonly assigned application of T. N. Beard, Ser
<br />No. 75,009, filed Sept. 24, 1970, teaches a method o
<br />producing oil from such mineral-containing oif-shal
<br />forma[ions which includes permeabilization of the for
<br />motion by dissolution oC mineral with hot aqueous solo
<br />[ion.
<br />SUMMARY OF THE INVENTION
<br />We have now found that the process of «moving
<br />heat-sensitive, water-soluble bicarbonate minerals
<br />from subsu rtace oil shale deposits by solution-mining
<br />with hot aqueous solutions is improved by injecting
<br />steam into the formation at a xlec[ed temperature
<br />greater than 250°F, and advantageously, greater than
<br />300°F, to leach water-soluble mineral from the forma-
<br />tion; maintaining the temperature of fluid in the
<br />leached zonc greater than 250'F; and adjusting pres-
<br />sure in the leached zonc Io a particular optimum pres-
<br />sure for the selected tcmperature.
<br />The optimum pressure is that pressure at which the
<br />sodium mineral-carrying capacity of the aqueous leach-
<br />ing fluid is at a maximum. At pressures below the opti-
<br />mum, excessive conversion of bicarbonate material to
<br />carbonate with attendant precipitation of carbonate
<br />leads to a reduced rpineral-carrying capacity. At higher
<br />pressures than the optimum, conversion of bicarbonate
<br />material to carbonate is inhibited and the mineral-
<br />earrying capacity o! the leaching fluid is thereby re-
<br />duced.
<br />FIG. I is a graphical representation of cavity growth
<br />rate versus cavity temperature for a nahcolite leaching
<br />5 operation conducted in a nahcolite-containing oil shale
<br />formation.
<br />FIG. 2 is a graph of sodium content expressed as
<br />equivalent pounds of nahcolite per pound of water for
<br />a sodium carbonate saturated, sodium bicarbonate-
<br />10 water system az a func[ion of tcmperature.
<br />FIG. 3 is a schematic view, partly in cross xenon, of
<br />a solution-mining well equipped for the practice of this
<br />invention.
<br />FIG. 4 is a schematic view, partly in cross-xenon, of
<br />t 5 another well system for use in the practice of this inven-
<br />tion.
<br />DESCRIPTION OF A PREFERRED EMBODIMENT
<br />m- Referring to FIG. 3, we see a subsurface oil shale for-
<br />is 20 motion ]0 containing strata 11 of substan[ially pure
<br />ty nahcolite (NaHCOs) and strata 12 which are predomi-
<br />t nantly oil shale but which contain a substantial amount
<br />5 0( nahcolite, e.g. 20 to 40 percent nahcolite dispersed
<br />in discreet nodules.
<br />a 25 Asolution-mining well 13 extends into the oil shale
<br />formation ]0 from the earth surface. The well IJ has
<br />r been completed in a conventional manner with cazing
<br />al 14 xaled in place with cement 15. Asolution-mining
<br />e fluid injection tubing string ]6 and a solution•mining
<br />Is J0 fluid production tubing string 17 are extended into the
<br />I well 13. The lower end of the injection tubing 16 is
<br />n preferabl}• positioned adjacent the top of a zone 9 of
<br />the oil shale formation ]0 to be solution-mined. The
<br />c lower end of the production tubing string is preferably
<br />IS positioned near the bottom of the zone 9.
<br />d Pack-oft means such az packer 18 may be positioned
<br />in [he casing 14 above the lower end of the tubing
<br />g string 16. Production tubing string 17 is provided with
<br />suitable means for lifting solution-mining fluid to the
<br />f 40 surface. For example, pumping apparatus may be posi-
<br />e tinned adjacent the bottom of production string 17 or
<br />the production string 17 may be equipped for gas lift az
<br />shown in FIG. 3. In the embodiment illustrated, a pres-
<br />sure actuated gas lift valve 19 is operatively connected
<br />45 to production tubing 17 at a point above packer 18, A
<br />conduit 20 for injection gaz is connected to the cazing
<br />14 at the surface. To lilt fluid in the tubing 17, gas is
<br />injected through conduit 20 into casing 14. When the
<br />50 pressure of this gas exceeds a certain threshold value,
<br />valve 19 opens and admits gas into the interior of tub•
<br />ing 17, This gas lightens the column of Iluid in tubing
<br />17 thereby reducing the pressure necessary to cause
<br />fluid to flow from the bottom of tubing 17 to the earth
<br />55 surface.
<br />To solution mine nahcolite from to rmation 10, hot
<br />aqueous solution-mining Iluid, preferably low quality
<br />steam, is injected down tubing 16. This fluid contacts
<br />water-soluble minerals in the formation 10 and dis•
<br />60 solves them thereby forming a leached zone and, evcn-
<br />lually, a cavity 21. The cavity 2l may be at leazt par-
<br />tially filled with fragmented panicles of oil shale and
<br />nahcolite 22.
<br />We have found that in leaching formations similar to
<br />65 that shown in FIG. 1 with steam, the cavity growth rate
<br />varies logarithmically with the cavity temperature as
<br />shown in FIG. 1 and that cavity growth rate is only
<br />slightly dependent upon the rate of fluid injection. It is
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