US 3792902 A
Plugging of solution-mining wells by mineral precipitation due to solubility changes associated with temperature and pressure changes of the solution-mining liquid is prevented by injecting a diluent into the well production tubing, or into an area surrounding the production tubing intake.
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Description (OCR text may contain errors)
[451 Feb. 19, 1974 METHOD OF PREVENTING PLUGGING OF 3,700,280 10/1972 Papadopoulos......................... 299/5 SOLUTION MINING WELLS  Inventors: Lawrence H. Towell, Woodland Primary ExammerErnest R. Purser Hills, Calif.; Jere R. Brew, Houston,
Assignee: Shell Oil Company, Houston, Tex.
Filed: Aug. 14, 1972 ABSTRACT 21 Appl. No.: 280,287
mining wells by mineral precipitation due to solubility changes associated with tempera- Plugging of solution- 52 us. Cl. 299/5.
ture and pressure changes of the solution-mining liquid is prevented by injecting a diluent into the well production tubing, or into an area surrounding the production tubing intake.
 References Cited I UNITED STATES PATENTS 3,523,582 8/1970 Fulford 166/310 X 5 Claims, 7 Drawing Figures PAIENTE FEBI 91914 sum vu or 4 MAX/MUM D/LUT/ON TEMPERATURE MAX/MUM PRODUCT/ON TEMPERATURE MIN/MUM PRODU C T/ON TEMPE RA TURE TEMPERATURE MIN/MUM D/LUTION I1 mmE mmmzmk FIG. 7
METHOD OF PREVENTING PLUGGING OF SOLUTION MINING WELLS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to recovering water-soluble minerals from subsurface deposits by solution mining and, more particularly, to the recovery of water-soluble carbonates and bicarbonates from subsurface-oil shale formations by solution mining.
2. Description of the Prior Art Recovery of water soluble minerals such as halite, trona, and nahcolite from subsurface deposits by solution mining is well known. Commonly, water, or other mineral solvent, is circulated through wells into mineral-containing formations to leach soluble minerals therefrom. Since the solubility of most commercially solution-mined water-soluble minerals and the rate of dissolution increases with increasing temperature, it is usually desired to use a heated solvent in order to maximize the amount of mineral dissolved per unit of solution mining fluid flowed into the well.
However, as hot mineral-containing mining fluid flows upwardly in the well, its temperature may drop due to heat losses in the wellbore. Additionally, its pressure drops due to a reduction in fluid head and the effect of friction associated with flow. These changes can affect the mineral carrying capacity of the mining fluid quite dramatically. In many cases, solubility is reduced to the extent that mineral precipitation occurs. This may result in eventual plugging of the production well or of portions of the mineral containing formation of relatively lower temperature than the temperature of injected solution-mining fluid.
SUMMARY OF THE INVENTION We have now discovered an improved method for producing water soluble minerals by solution mining in which precipitation of minerals in and around the production well due to temperature and pressure changes is prevented. The improvement comprises-injecting a diluent directly into the flow stream of mineral-laden solution-mining fluid at a point adjacent the bottom of the well under controlled conditions to prevent precipitation of minerals within the flow area of diluted solution-mining fluid. The diluent, advantageously, is an aqueous fluid and, preferably, is hot water of low alkalinity. The diluent may be injected continuously or intermittently directly into a production tubing string or into an area in communication with the production tubing string inlet.
In applying the invention in the solution mining of heat-sensitive bicarbonate minerals, such as trona or nahcolite, particular care must be taken to maintain the diluent temperature within specific controlled ranges to prevent deposition of carbonate minerals at one temperature extreme or bicarbonate minerals at the other temperature extreme.
BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-5 are sectional views illustrating various embodiments of well equipment suitable for the practice of this invention.
FIG. 6 is a graphical representation of sodium carbonate and sodium bicarbonate concentration expressed as equivalent sodium bicarbonate concentration, as a function of temperature and pressure in a sodium carbonate, sodium bicarbonate, water system.
FIG. 7 is a graphical representation illustrating maximum and minimum dilution fluid temperature and maximum and minimum total diluted solution-mining fluid temperature as a function of dilution ratio for a particular nahcolite-water system at a pressure of 600 psi and originally at a temperature of 4l0F.
DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIGS. l5, in which like numerals represent like elements, we see a subsurface earth formation 10 containing at least one zone or layer 11 rich in at least one water-soluble mineral such as nahcolite. Nahcolite zone 11 is penetrated by at least one well 12 which has been completed in a conventional manner with cemented casing 13. The casing is provided with one or more preforations 14 for opening the interior of the casing into communication with the nahcolite zone 11. The well is provided with a production tubing string 15 for flowing mineral-containing fluid from the well. Additionally, a fluid flow path is provided for injecting dilution fluid from the surface into the well to a point adjacent the bottom of the production tubing 15. In the embodiments of FIGS. 1, 2 and 3, this flow path is formed by a separate dilution fluid conduit l6, l7 and 18, respectively. In the embodiments of FIGS. 4 and 5, this flow path is provided by the annular space 19 between the casing 13 and the production tubing 15.
This annular space 19 is closed by pack-off means 23 above the perforations 14 through which fluid is produced from formation 10. Suitable flow regulating means, such as pressure actuated valve 21 (FIG. 4) or flow control orifice 22 (FIG. 5), is provided for regulating the flow of dilution fluid into the production tubing string at a point adjacent the lower end thereof. Flow through such flow regulating means can be controlled by adjusting the pressure in the annular space surrounding the production tubing 15. For example, a pressurized gas may be maintained in this annulus, the pressure being adjusted to regulate the rate of flow through the pressure actuated value 21, which may be a spring loaded check valve, or through orifice 22.
To remove nahcolite from the zone 11, a hot solution mining fluid, such as hot water or steam, is flowed into contact with the zone 11 to leach nahcolite therefrom. This hot, now mineral-laden, fluid then flows into the casing 13 of the well through preforations l4 and is lifted to the surface through production tubing 15.
The solution-mining fluid may be injected through the well 12 in alternating sequence with the production of fluids according to the method of this invention, or it may be injected simultaneously with fluid production. For example, in well equipped as shown in FIGS. 1-3, the solution mining fluid may be injected through the space 19 surrounding the dilution fluid injection string 16-18 and the production tubing string 15. This fluid may flow into contact with the nahcolite zone 11 through a set of perforations 20 in the casing 13 near the top of formation 10. In another embodiment, hot solution mining fluid may be injected through a separate injection well (not shown), flowed through the formation l0, and the produced up the well 12. FIGS. 4 and 5 illustrate equipment configurations well suited to this production scheme.
As mineral-containing, solution-mining fluid rises to the surface through production tubing 15, its temperature falls due to heat losses to material in the well 12 and to surrounding formations. Simultaneously, its pressure falls due to reduction of fluid head. These temperature and pressure changes can reduce the capacity of the solution mining fluid to carry dissolved minerals. This may result in mineral precipitation in the tubing 15.
For example, FIG. 6 shows dissolved sodium bicarbonate and sodium carbonate concentration expressed in' equivalent pounds of Nal-ICO per pound of water as a function of hot water temperature and pressure for a nahcolite-water system (i.e. any Na- CO content is expressed in terms of the number of pounds of Nal-ICO containing the same amount of Na). If solution-mining water contacting zone 11 is at a temperature of 325F and pressure of 1,500 psi, the water can carry 0.42 pounds/pounds of dissolved sodium bicarbonate (NaH- CO If at the earth surface pressure has been reduced to 500 psi and temperature to 290F, the liquid can then carry only 0.35 pounds/pound. It follows that 0.07 pounds per pound of sodium mineral will be precipitated within the production tubing as the solution mining fluid rises to the surface if no steps are taken to prevent this precipitation. Therefore, according to the method of this invention, a diluent such as fresh water is introduced into the flow stream of substantially saturated solution-mining fluid downhole near the production tubing intake 24. For example, the diluent can be co-mingled with the solution-mining fluid at the points shown in any of FIGS. l-S to dilute the produced fluid before it is cooled or subjected to pressure reductions which would cause the precipitation of dissolved minerals.
Preferably, the fresh water diluent has a very low level of hardness (e.g., contains substantially no calcium or magnesium ions) in order to prevent downhole carbonate scale deposition. In solution mining of sodium minerals, such as nahcolite, the dilution water should have a relatively low level of alkalinity as well, so that fluid handling problems are not compounded by the fact that large volumes of water are necessary to reduce alkalinity.
The diluent is preferably added in sufficient amount to substantially prevent mineral precipitation in production tubing 15. The amount of diluent required will vary with changes in diluent temperature. Where the solubility of the mineral being mined increases with increased temperature, less diluent will be required as diluent temperature is increased.
In many cases it is desirable to heat dilution fluid prior to injection down the dilution tubing string 14. Heated dilution fluid will reduce the temperature reduction in the production tubing string 15 and thereby reduce the tendency of precipitate to form within this string. This, in turn, reduces the amount of diluent required to prevent precipitation.
Advantageously, heated dilution fluid is flowed into the well 12 in counter-current heat exchange relationship with produced solution-mining fluids. The dilution fluid heats the produced fluid in tubing 15 by conduction as it passes down the dilution fluid tubing 14. This heating further supplements precipitation preventing effects from diluent injection.
In this regard the embodiment of FIG. 1 wherein dilution water is flowed down a tubing string 16 concentric with the production tubing string 15 is particularly preferred. This configuration provides an ideal tube-in shell heat exchanger. For most practical operating parameters, the dilution string temperature becomes the same as the temperature of the fluid entering the production tubing intake. In such a situation sufficient diluent should be added to dilute the produced fluid stream adjacent the bottom of production tubing 15 to a mineral content lower than saturation content at the earth surface. For instance, in the above mentioned example, if the temperature of the diluent added is such that no temperature changes in the system are created by addition of this diluent, then no deposition will occur in the production tubing 15 as long as sufficient diluent is added to reduce the combined NaHCO and Na CO content of the produced fluid in the production tubing 15 adjacent the bottom of the tubing 15 to 0.35 or less pounds of sodium bicarbonate per pound of water.
Where the dilution fluid added is at a temperature lower than that of the solution-mining fluid with which it is inter-mixed, care must be taken to ensure that the temperature decreasing effect of the dilution fluid does not diminish the capacity of the total mixture to carry water-soluble minerals more than the increased carrying capacity attributable to the presence of dilution fluid. FIG. 7 illustrates this for a nahcolite system at 600 psi with an initial solution mining fluid sodium concentration of 1.3 pounds/pound and an initial temperature of 410F. The lower-most curve gives minimum temperature of the dilution fluid as a function of dilution ratio. The adjacent curve gives minimum temperature of the mixed fluid stream which may be attained without precipitation. It can be seen that a 2:1 dilution ratio precipitation will occur if the dilution fluid temperature is below 220F or if the temperature of the mixed stream falls below 280F. It can also be seen that at larger dilution ratios precipitation can be avoided by adding a diluent of substantially lower temperature.
In dealing with heat-sensitive water-soluble minerals, care must be taken not only to ensure that minimum temperatures are maintained for a given dilution ratio but also to stay below certain maximum temperatures at that ratio. FIG. 6 reflects the fact that in a nahcolite (Nal-ICO water system with increases in temperature or decreases in pressure, nahcolite decomposes to sodium carbonate (Na cfi and carbon dioxide (CO For example, considering a system having a combined sodium bicarbonate, sodium carbonate concentration of 0.8 pounds/pound and a pressure of 500 psi, FIG. 6 indicates that at 340F the solution is saturated at 0.72 pounds/pound. Therefore, sodium bicarbonate will precipitate. If temperature is raised to 350F the system becomes capable of dissolving as much as 0.85 pounds/pound; therefore, there will be no precipitation. However, if temperature is increased to 410F, Na CO concentration due to decomposition of NaHCO has increased to the extent that the system is once again saturated. At temperatures above 4l0F the system is super-saturated with NaCO and precipitation of this mineral will occur.
Thus, as illustrated by the upper two curves of FIG. 7 for that sodium bicarbonate system, the dilution fluid temperature must be maintained below a certain maximum given by the uppermost curve to prevent Na CO deposition upon the addition of diluent to a solution of nahcolite in water. The adjacent lower curve gives the maximum temperature of the mixed fluid stream as a function of dilution ratio. For the 2:1 example discussed above, the dilution fluid temperature must be above about 220F but below about 530F and the temperature of the diluted production fluid must be in the range of about 300480F.
The particular temperature range for a given heatsensitive mineral mining process varies with changes in system pressure and dilution ratio. Given solubility in formation such as that of FIG. 6, one skilled in the art should have no problem determining particular temperature limits for a given heat-sensitive mineral-water system. It should be pointed out that in high tempera ture nahcolite systems, solubility is quite sensitive to pressure, thus, at 450F a given volume of water can dissolve 0.47 pounds of sodium bicarbonate per pound of water at 500 psi and 1.22 pounds of sodium bicarbonate per pound of water at 1,800 psi. Thus, one must be careful in designing a system in which fairly large pressure decreases are incurred as high temperature fluids are lifted in a well bore to ensure that from point to point along the path of conditions through which the solution passes, no precipitation occurs.
The method of solution-mining water soluble minerals heretofore described is particularly applicable to insitu oil-shale processing projects in which nahcolite or other water soluble mineral is leached from a nahcolite-rich oil shale formation in conjunction with a hydrocarbon recovery process. There, leaching fluid temperature may be quite high (250550F or higher) because the solution-mining fluid, in addition to leaching nahcolite, may be simultaneously used as an oilshale thermal-fracturing agent or as a kerogenpyrolyzing fluid. Heat losses can be quite severe as this high temperature fluid is withdrawn from a subsurface formation through a well. Dilution of this fluid with fresh water prevents well equipment plugging by mineral precipitation.
We claim as our invention:
1. In a method of mining from a subsurface earth formation a heat sensitive bicarbonate which decomposes to solution with increasing temperature, wherein heated water is flowed into contact with said bicarbonate to remove said bicarbonate from the formation by dissolution and wherein the resulting bicarbonatecontaining aqueous solution is lifted to the earth surface through a well, the improvement which comprises, prior to lifting said aqueous solution to the earth surface,
mixing water with said aqueous solution at a point adjacent the bottom of the well at temperature high enough to prevent bicarbonate deposition in the well but lower than a maximum temperature at which carbonate precipitation occurs. 2. The method of claim 1 wherein the well is cased with a tubular casing and said bicarbonatecontaining solutio is lifted through a production tubing string within the casingv and including the steps of:
positioning pack-off means in the space between the casing and the production tubing at a point above the inlet of said bicarbonate-containing solution into the production tubing;
providing pressure-responsive flow control means in the production tubing string above the pack-off means for admitting water into the production tubing from the space surrounding the production tubing;
injecting water into the well through the space between the production tubing and the casing;
and adjusting the rate at which water flows through the pressure-responsive flow control means by adjusting pressure in the space between the production tubing and casing.
3. The method of claim 1 which includes, prior to mixing, the step of heating the water to a temperature at least as high as the temperature of said bicarbonatecontaining aqueous solution with which the water is to be mixed.
4. In a method for recovering nahcolite and hydrocarbons from a subsurface, nahcolite-containing oilshale formation of the type wherein the oil-shale is permeabilized by injecting an aqueous nahcolite solvent into contact with the formation at a temperature sufficient to thermally fracture the oil shale and wherein hot nahcolite-containing nahcolite solvent is withdrawn from the formation through a well production tubing string, the improvement comprising:
injecting water into the lower portion of the production tubing string at a temperature high enough to prevent bicarbonate deposition in the well but lower than a maximum temperature at which carbonate precipitation occurs.
5. The method of claim 4 wherein the water contains substantially no calcium or magnesium ions.