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3,779,602 3 4 believed that this increase in cavity growth rate with and an effective reduction in the equivalent nahcolite tcmperature is at least in part due to more rapid [her- saturation of the system. Thus at 400°F, leaching oper- mat fracturing at higher temperatures of oil shalt su r- ations can be maximized if pressure in the cavity 21 is rounding discreet nahcolite nodules. Such fracturing maintained at about 1,000 psi. To maintain this pres- allows injected aqueous fluid to reach the nahcolite 5 sure, it is necessary to artifically lift fluid from the cav- nodule and leach it from the formation leaving an ex- icy 21 i! the fluid head of solution-mining Iluid in pro- posed oil shale face which is in turn thermally fractured duction tubing I7 is greater than 1,800 psi. Therefore, opening up communication to yet another nodule. the well ]3 is provided with a gas lift systtm az hereto~ As can be seen in FIG. 1, for temperatures below fore described. 250°F, growth rate of cavity radius is quite low, less 10 FIG. 4 shows a well 22 extending into the formation than 0.08 feet per day; whereas at 300°F, growth rate 10 that is completed in a manner particularly advanta• is almost doubled to 0.15 feel per day, Thus, for.maxi• genus for the practice of this invention. The well 22 is , mum mineral removal, cavity temperature should be completed with casing 23 which extends into the , maintained above 250°F and preferably above 300°F. nahcolite-containing formation 10. The casing 23 is ce- We have also found that in solution-mining nahcolite IS mcntcd in plate with cement 24 and perforated adja• from an oil shale formation with aqueous Iluid, the rate cent formation ]0 with perforations 25 to open the in- o(mincral recovery can be maximized by selecting an terior o(the casing into communication with the forma- operating temperature for maximum desired cavity [ion 10. ° grow[:[ rate az by reference to FIG. 1, and then during A liquid production tubing string 26 and a gas pro• operation adjusting cavity pressure to a pressure at 20 duction tubing string 27 extend into the well from the which the sodium carrying capacity of the aqueous surface. The liquid production tubing string 26 prefera- leaching fluid is a maximum for the selected cavity tam- bly terminates at the point adjacent the bottom of the pera[ure. This pressure is less than that required to hy- interval of the formation 10 to be treated whereas the draulically fracture the formation and is greater than gas production tubing string 27 terminates at a point the pressure at which nahcolite decomposition to so- 2S above the Tower end of the liquid production tubing 26 dium carbonize, carbon dioxide and water is maxi- but below the perforations 25. The interior of the cas- mized. ing is preferably sealed to fluid flow by packvff means Operating in this manner can significantly reduce the such as packer 28 at a point above the terminal ends of energy requirement for carrying out the process since the two tubing strings 26 and 27 and below the perfora- heat can be carried to the formation by relatively low 30 [ions 25. pressure steam. Additionally, water requirements are The liquid production tubing string is provided with reduced because the total amount of sodium mineral means for lilting liquid from the formation 30 to the removed from the cavity 21 by a given volume of leach- surface. This may be a down-hole pump or gas lift ing luid is maximized. means (az illustrated in FIG. 4) in which a gaz injection The particular selected {caching temperature will 3 S string extends into the well 22 and is connected in com- vary from operation to operation depending upon eco- munication with production tubing 26 at a point near nomic conditions and the desired cavity growth rate (or the lower end of [hat tubing. The particular point of in- each particular case. Operating pressure for a particu- tersection will be determined by the fluid head desired tar selected temperature is determined from pressure, to be maintained in liquid production string 26. temperature, saturation relationships such as those 4 0 In operation, hot aqueous fluid having a temperature given in FIG. 2. That figure shows total sodium concert- greater than 250° and preferably greater than 300°F is [ration in pounds of nahcolite per pound of water for injected into casing 23 through conduit 30 and then a sodium carbonate saturated, sodium carbonate/- down the razing until it passes through perforations 25 sodium bicarbonate-water system. The graph rel7ecLS into the formation 10. This fluid leaches nahcolite Crom the amount of nahcolite removed from a nabcolite for- < s the formation creating a cavity 3] which may be filled motion which is present in the solution even though the with fragmented particles of oil shale and nahcolite 32. actual composition of the solution includes both so- The aqueous fluid advantageously contains high pro- dium bicarbonate and sodium carbonate generated by portion of steam which upon contacting the formation ' nahcolite decomposition. Best results are obtained by 10 condenses to form a liquid phase capable of carrying operating at the pressure for which the isobar in[erseces s0 dissolved mineral in solution. Simultaneously with the - the upper dashed curve a[ the selected operating rem- injection of steam down Ne casing 22, liquid is pro- ~ pcratu re. Good results are obtained at pressures vary- duced from the lower pan of the cavern 31 through ing az much as IO percent above or below this pressure. production tubing string 26 and gas is produced from Looking at FIG. 2 for a tcmperature o! 400°, it can b h d ss the cavern 31 through gas production tubing string 27. e seen c at at that temperature an about 200 psi only The production rate of these Iluids is preferably ad- sodium bicarbonate is present in the solution (as given justed to maintain the pressure in the cavern 31 at a by the lower dotted line of the Figure) and [hat the particular preferred value for the selected temperature total amount of equivalent nahcolite dissolved is operation. The removal of gas through the tubing 26 around 0.55 pounds per pound of water. However as 60 draws both steam and COr from the tavern 31. This re• , pressure is incre azed, the amount of sodium bicarbon- suits in a reduction o(the partial pressure of CO, in the act in the systtm increases until at about 1,000 psi, the cavern and further promotes the decomposition of nah- f total sodium content is equivalent to about 1.25 pounds _ entice (NaHCOs) to sodium carbonate, COr, and water per pound of water even though sodium carbonate sat- (e.g., 2 NnHCO~ -° Na,COs + CO, + Hr0 ). uration remains the safie. Further pressure increase to 65 Both FIGS. J and 4 illustrates single well systems !or a pressure for which the exiention of an isobar would the practice of this invention. However, it should be un- interxct the a00°F isotherm above the upper dotted derstood that Iwo or more wells may at any one time line results in the precipitation of sodium bicarbonate be in communication with any particular cavern 21 or