MONOVALENT BRINES FOR USE AS WELLBORE FLUIDS

Abstract

The invention relates to a wellbore fluid, which is a monovalent brine comprising one or more alkali bromide salt(s) and one or more TCT-reducing additive(s) selected from the group consisting of alkali nitrates. A method of treating a subterranean formation, comprising placing the wellbore fluids of the invention in a wellbore in the subterranean formation is also provided.

Claims

1. A wellbore fluid, which is a monovalent brine comprising one or more alkali bromide salt(s) and one or more TCT-reducing additive(s) selected from the group consisting of alkali nitrates.

2. A wellbore fluid according to claim 1, wherein the monovalent brine comprises water and a binary salt mixture consisting of sodium bromide and alkali nitrate, said monovalent brine being of density in the range from 1.47 to 1.55 g/ml.

3. A wellbore fluid according to claim 2, wherein the monovalent brine is sodium bromide/sodium nitrate brine.

4. A wellbore fluid according to claim 3, which is sodium bromide/sodium nitrate brine of density from 1.49 gr/ml to 1.52 gr/ml and TCT below −5.0° C.

5. A wellbore fluid according to claim 4, comprising from 35 to 38% by weight sodium bromide and from 13 to 18% by weight sodium nitrate in aqueous solution.

6. A wellbore fluid according to claim 2, wherein the monovalent brine is sodium bromide/potassium nitrate brine.

7. A wellbore fluid according to claim 6, which is sodium bromide/potassium nitrate brine of density from 1.48 gr/ml to 1.51 gr/ml and TCT below −5.0° C.

8. A wellbore fluid according to claim 7, comprising from 35 to 42% by weight sodium bromide and from 7 to 10% by weight potassium nitrate in aqueous solution.

9. A wellbore fluid according to claim 1, which is a monovalent brine comprising water and a ternary salt mixture consisting of a first alkali bromide, a second alkali bromide and alkali nitrate.

10. A wellbore fluid according to claim 9, wherein the first alkali bromide is sodium bromide, the second alkali bromide is either lithium bromide, potassium bromide or cesium bromide and the alkali nitrate is sodium nitrate.

11. A wellbore fluid according to claim 10, wherein the monovalent brine is sodium bromide/lithium bromide/sodium nitrate brine of density higher than 1.47 gr/ml.

12. A wellbore fluid according to claim 11, with density from 1.47 gr/ml to 1.49 gr/ml and TCT below −5.0° C.

13. A wellbore fluid according to claim 12, comprising from 30 to 35% by weight sodium bromide; from 5 to 10% by weight lithium bromide; and from 6 to 10% by weight sodium nitrate in aqueous solution.

14. A wellbore fluid according to claim 10, wherein the monovalent brine is sodium bromide/potassium bromide/sodium nitrate brine of density higher than 1.51 gr/ml.

15. A wellbore fluid according to claim 14, with density from 1.51 gr/ml to 1.54 gr/ml and TCT below −5.0° C.

16. A wellbore fluid according to claim 15, comprising from 30 to 37% by weight sodium bromide; from 5 to 10% by weight potassium bromide; and from 6 to 10% by weight sodium nitrate in aqueous solution.

17. A wellbore fluid according to claim 10, wherein the monovalent brine is sodium bromide/cesium bromide/sodium nitrate brine of density higher than 1.53 gr/ml.

18. A wellbore fluid according to claim 17, with density from 1.53 gr/ml to 1.57 gr/ml and TCT below −5.0° C.

19. A wellbore fluid according to claim 18, comprising from 33 to 37% by weight sodium bromide; from 5 to 10% by weight cesium bromide; and from 5 to 10% by weight sodium nitrate in aqueous solution.

20. A wellbore fluid according to claim 1, having nearly neutral pH.

21. A method of preparing wellbore fluid with improved stability against crystallization, comprising formulating one or more alkali bromide(s) and at least one alkali nitrate in water to give a monovalent brine, where the nitrate is present in an amount sufficient to achieve TCT-lowering effect of not less than 5° C. compared to the corresponding nitrate-free brine of equal density.

22. A method according to claim 21, wherein the alkali bromide comprises sodium bromide and the alkali nitrate is sodium nitrate.

23. A method according to claim 21, wherein the alkali bromide comprises sodium bromide and the alkali nitrate is potassium nitrate.

24. A method of treating a subterranean formation, comprising placing the wellbore fluids of claim 1 in a wellbore in the subterranean formation.

25. (canceled)

26. (canceled)

27. (canceled)

Description

IN THE DRAWINGS

[0039] FIG. 1 is a TCT versus concentration plot of sodium bromide brine.

[0040] FIG. 2 shows TCT versus density plots of sodium bromide/sodium nitrate brines.

[0041] FIG. 3 shows TCT versus density plots of sodium bromide/potassium bromide/sodium nitrate brines.

[0042] FIG. 4 shows TCT versus density plots of sodium bromide/lithium bromide/sodium nitrate brines.

[0043] FIG. 5 shows TCT versus density plots of sodium bromide/cesium bromide/sodium nitrate brines.

[0044] FIG. 6 shows TCT versus density plots of sodium bromide/sodium nitrate, sodium bromide/lithium nitrate and sodium bromide/potassium nitrate brines.

[0045] FIG. 7 shows cyclic polarization curves to illustrate the noncorrosive behavior of sodium bromide/sodium nitrate solution.

EXAMPLES

[0046] Methods

[0047] Density measurements: an empty, clean and dry volumetric flask of 25.00 ml (+0.04 ml) was weighed on an analytical balance (+0.0001 gr). The flask was filled up to 25.00 ml with the desired solution. The outer wall of the flask was cleaned and dried. The flask was observed to ascertain that there were no air bubbles in the solution. The full flask was weighed on the analytical balance and its weight was recorded. The temperature of the solution was also measured, and the density was calculated (temperature of measurements reported herein is 26±1° C.).

[0048] TCT measurements: 25-30 ml of the test sample, pinch of a nucleation agent and magnetic stirrer were placed in a 50 ml beaker. Silicon oil (˜50 ml) was introduced into a 250 ml jacketed glass cup. The 50 ml beaker with the sample was inserted into the 250 ml jacketed glass cup (with the silicon oil inside) connected to a Huber silicon oil circulation thermostat in such a way that the sample was cooled by the silicon oil in the cup; the thermocouple was immersed into the solution and the cup was covered with the cap. The 250 ml jacked glass cup with the beaker inside was placed on the stirring plate and stirring of the solution was started. The initial temperature in the thermostat was set at 10° C. and was decreased gradually during the measurements down to −20° C. if needed. When crystals began to form, the corresponding temperature was written down (FCTA—first crystal to appear), afterwards the temperature slightly increased, indicating the TCT.

[0049] Bromide concentration in the solution: measurements were made by direct potentiometric titration using a silver electrode and 0.1M AgNO.sub.3 titrant solution after adding 2N HNO.sub.3.

[0050] Nitrate concentration in the solution: analysis by Ion Chromatography (IC). The analysis was done against an external standard, Dionex Seven Anion standard P.N. 057590. The instrument used was a Dionex ICS-2100 with an AS-9HC column.

Example 1

Solubility of NaNO.SUB.3 .in NaBr Solution and Properties of the NaBr/NaNO.SUB.3 .Salt Solution

[0051] Sodium bromide clear brine of 1.496 gr/cc (12.48 ppg) density contains 46.4% by weight sodium bromide in solution. The TCT of the brine is 6.5° C. and its pH is 7.3.

[0052] To this clear sodium bromide brine (100 g) was added sodium nitrate (11 gr). The added sodium nitrate dissolved swiftly in the sodium bromide brine at room temperature under stirring, to form NaBr/NaNO.sub.3˜10% solution of 1.55 gr/cc (12.94 ppg) density.

[0053] Water (7.8 gr) was added, to afford a NaBr/NaNO.sub.3 clear solution of 1.496 gr/cc (12.48 ppg) density with TCT of −8.7° C., i.e., reduction of about 16 degrees compared to the single salt sodium bromide solution of 1.496 gr/cc (12.48 ppg) density.

Example 2 (Comparative)

Solubility of NaNO.SUB.3 .in CaBr.SUB.2 .52% Solution

[0054] Calcium bromide clear brine of 1.71 gr/cc (14.2 ppg) density contains 52% by weight calcium bromide in solution. The TCT of the brine is −17° C. and its pH is 6.5.

[0055] To this clear calcium bromide brine (100 g) was added sodium nitrate (11 gr). Only partial dissolution of the added salt was observed. The experiment was repeated, this time with a lesser amount of sodium nitrate (˜5 gr). Most of the added amount dissolved at room temperature. However, to achieve full dissolution of the −5 gr sodium nitrate in calcium bromide 52% solution, it was necessary to heat the solution.

Example 3 (Comparative)

Solubility of NaNO.SUB.3 .in MnBr.SUB.2 .58% Solution

[0056] Manganese bromide clear brine of 1.90 gr/cc (15.86 ppg) density contains ˜58% by weight manganese bromide in solution. The TCT of the brine is 4.7° C. and its pH is 3.

[0057] To this clear manganese bromide brine (100 g) was added sodium nitrate (11 gr). Dissolution of sodium nitrate was observed, followed by color change, from pink-reddish to brown, and formation of a brown precipitate a few hours later. The observations indicated oxidation of Mn.sup.2+ to Mn.sup.3+ and generation of a brown precipitate of Mn.sub.2O.sub.3.

Example 4

Preparing and Testing NaBr/NaNO.SUB.3 .Solutions with Varying NaNO.SUB.3 .Concentration

[0058] A saturated sodium bromide solution was prepared by charging a vessel with solid sodium bromide (530 gr) and water (470 gr). The mixture was stirred to achieve dissolution of the salt. To obtain full dissolution, an additional amount of water was gradually added, affording a clear fluid which contained ˜48% by weight sodium bromide in solution. The total weight was >1000 g.

[0059] Three portions were taken from the saturated sodium bromide solution, each consisting of 320 gr. Sodium nitrate was then added to each of these sodium bromide solutions in the following amounts: 16 gr, 32 gr and 48 gr, to create three stock sodium bromide solutions that contained ˜5%, ˜9% and ˜13% by weight sodium nitrate, respectively.

[0060] Each of the three stock solutions was divided into 50 gr portions, which were diluted by the addition of small amounts of water (1-1.5 gr, 2-2.5 gr, 3-3.5 gr, 4-4.5 gr, 5-5.5 gr).

[0061] Compositions and properties (density and TCT) of the stock solutions and diluted solutions are tabulated in Table 1 (because test solutions were prepared by dilution of stock solutions of ˜5%, ˜9% and ˜13% NaNO.sub.3 concentration as described above, NaNO.sub.3 concentrations are actually slightly lower; for simplicity, the ˜5%, ˜9% and ˜13% values are indicated in Table 1 and the accompanying graph).

TABLE-US-00001 TABLE 1 NaBr single NaBr/~5% NaNO.sub.3 NaBr/~9% NaNO.sub.3 NaBr/~13% NaNO.sub.3 NaBr d TCT NaBr d TCT NaBr d TCT NaBr d TCT % g/cc ° C. % g/cc ° C. % g/cc ° C. % g/cc ° C. 47.0 1.510 11.5 43.9 1.526 9.5 ND 1.555 14.7 39.7 1.556 8.5 46.6 1.503 8.2 43.1 1.512 3.9 42.1 1.522 2.5 38.5 1.537 0.1 46.4 1.497 5.2 41.8 1.497 −2.9 40.4 1.509 −1.4 38 1.526 −3.4 45.5 1.488 0.9 41.0 1.481 −10.9 40.2 1.496 −10.4 37.2 1.512 −10.2 45.2 1.483 −0.7 40.5 1.469 −16 38.9 1.486 −13 36.6 1.499 −15.6 44.6 1.476 −3.3 38.6 1.478 −16.2 36.5 1.494 −16.7

[0062] Results are presented in graphical form in FIG. 2. TCT versus density plots show a linear relationship across the density range under consideration for all four systems, with roughly comparable slopes, as indicated by the four parallel straight lines. However, the TCT of sodium bromide/sodium nitrate brines are lower than that of the corresponding (i.e., of comparable density) sodium bromide brine. Furthermore, lower TCTs can be achieved with an increasing concentration of sodium nitrate in the multi-salt solution: the presence of ˜5%, ˜9% and ˜13% by weight of sodium nitrate in sodium bromide solution leads to a TCT reduction of ˜8.5° C., ˜13.5° C. and ˜21.5° C., respectively, compared to the single salt solution.

[0063] Owing to the effect of sodium nitrate on TCT, sodium bromide and sodium nitrate can be formulated, over a useful density range (say, 1.47 g/cc up to 1.54 g/cc), to give solutions stable against crystallization down to approximately −20° C.

Example 5

Preparing and Testing NaBr/KBr/NaNO.SUB.3 .Solutions

[0064] To a saturated sodium bromide 48% solution (300 gr) prepared as described above was added potassium bromide (30 gr), followed by the addition of water to obtain a clear fluid. A few samples (50 gr each) were taken and diluted with water to produce sodium bromide/potassium bromide solutions spanning a density range from 1.536 to 1.470 g/cc.

[0065] Another set of samples was prepared by the addition of sodium nitrate (35 gr) to a stock sodium bromide/potassium bromide solution, followed by addition of water and removal of non-dissolved solids by filtration, to obtain a clear brine of 1.593 gr/cc density, which contained 36.7% by weight sodium bromide, 8% by weight potassium bromide and 8.6% sodium nitrate in solution. This clear brine served as a stock solution to create a series of diluted solutions by the addition of small quantities of water. Compositions and properties of the sodium bromide/potassium bromide and sodium bromide/potassium bromide/sodium nitrate solutions are set out in Table 2.

TABLE-US-00002 TABLE 2 NaBr/KBr NaBr/KBr/NaNO.sub.3 NaBr KBr d TCT NaBr KBr NaNO.sub.3 d TCT % % g/cc ° C. % % % g/cc ° C. 40.0 8.7 1.536 14.3 36.7 8 8.6 1.593 14.7 ND ND 1.520 8.8 ND ND ND 1.568 5.4 ND ND 1.503 2.3 35.2 7.8 8.2 1.560 1.6 38.3 8.3 1.489 −2.2 ND ND ND 1.546 −3 ND ND 1.479 −5.8 ND ND ND 1.530 −7.7 36.6 8.7 1.470 −8.9 33.5 7.4 7.4 1.517 −13.5

[0066] The graph of FIG. 3 indicates that a ternary blend composed of sodium bromide/potassium bromide/sodium nitrate can be formulated in water to give a clear brine of >1.5 g/cc density, exhibiting surprisingly low TCT, i.e., from −15° C. to −5° C. By contrast, in the absence of sodium nitrate, solids are crystallized out of sodium bromide/potassium bromide binary solutions of >1.5 g/cc density at a temperature in the range from +5° C. to +15° C. That is, across a useful density range [>1.5 g/cc], TCT can be reduced by ˜20° C. with the aid of sodium nitrate.

[0067] The graph of FIG. 3 further depicts a pair of straight lines representing TCT versus density plots for sodium bromide alone and sodium bromide/sodium nitrate, demonstrating a decrease of 13.5° C. in TCT owing to the addition of sodium nitrate, as previously discussed for Example 4.

[0068] The results shown in FIG. 3 underscore the strong effect of sodium nitrate addition on the TCT of a sodium bromide/potassium bromide brine.

Example 6

Preparing and Testing NaBr/LiBr/NaNO.SUB.3 .Solutions

[0069] To a saturated sodium bromide 48% solution (320 gr) prepared as described above was added lithium bromide (32 gr), followed by addition of water to obtain a clear fluid. A few samples (50 gr each) were taken and diluted with water to produce sodium bromide/lithium bromide solutions spanning a density range from 1.513 to 1.456 g/cc.

[0070] Another set of samples was prepared by the addition of sodium nitrate (32 gr) to a sodium bromide/lithium bromide stock solution, followed by addition of water, following which non-dissolved solids were removed by filtration and a clear brine of 1.528 gr/cc density was collected, which contained 34.9% by weight sodium bromide, 6.5% by weight lithium bromide and 8.14% sodium nitrate in solution. This clear brine served as a stock solution to create a series of diluted solutions by the addition of small quantities of water.

[0071] Compositions and properties of the sodium bromide/lithium bromide and sodium bromide/lithium bromide/sodium nitrate solutions are set out in Table 3.

TABLE-US-00003 TABLE 3 NaBr/LiBr NaBr/LiBr/NaNO.sub.3 NaBr LiBr d TCT NaBr LiBr NaNO.sub.3 d TCT % % g/cc ° C. % % % g/cc ° C. 37.9 9.9 1.513 21.1 34.9 6.5 8.14 1.528 9.5 ND ND 1.495 15.8 ND ND ND 1.512 2.7 36.6 9.7 1.483 10.8 34 6.5 8.14 1.497 −2.9 ND ND 1.470 5.3 ND ND ND 1.483 −7.2 35.4 9.1 1.456 −1.7 33.2 6.1 7.77 1.476 −11.8 1.468 −14

[0072] Addition of lithium bromide to sodium bromide solution increases significantly the TCT of the mixture compared to the single salt sodium bromide brine. The effect of the addition of sodium nitrate on the TCT of sodium bromide/lithium bromide brine is shown graphically in FIG. 4. Also in this type of alkali bromide blend, sodium nitrate produces a useful effect: sodium bromide/lithium bromide/sodium nitrate solution is stable against crystallization down to temperatures of from −15° C. to −5° C., i.e., about 18° C. lower than TCTs of equivalent (i.e., of equal density) sodium bromide/lithium bromide binary solutions. However, it is seen that the straight line corresponding to the sodium bromide/sodium nitrate clear fluid is positioned below that of the sodium bromide/lithium bromide/sodium nitrate system, indicating that over the density range under consideration, lower TCTs were measured for the sodium bromide/sodium nitrate binary blends than for lithium-containing ternary blends.

Example 7

Preparing and Testing NaBr/CsBr/NaNO.SUB.3 .Solutions

[0073] To a saturated sodium bromide 48% solution (300 gr) prepared as described above was added cesium bromide (30 gr), followed by addition of water to obtain a clear fluid. A few samples (50 gr each) were taken and diluted with water to produce sodium bromide/cesium bromide solutions spanning a density range from 1.611 to 1.531 g/cc.

[0074] Another set of samples was prepared by the addition of sodium nitrate (33.5 gr) to a sodium bromide/cesium bromide stock solution, following which non-dissolved solids were removed by filtration and a clear brine of 1.615 gr/cc density was collected, which contained 36.7% by weight sodium bromide, 8.7% by weight cesium bromide and 8.9% sodium nitrate in solution. This clear brine served as a stock solution to create a series of diluted solutions by the addition of small quantities of water.

[0075] Compositions and properties of the sodium bromide/cesium bromide and sodium bromide/cesium bromide/sodium nitrate solutions are set out in Table 4.

TABLE-US-00004 TABLE 4 NaBr/CsBr NaBr/CsBr/NaNO.sub.3 NaBr CsBr d TCT NaBr CsBr NaNO.sub.3 d TCT % % g/cc ° C. % % % g/cc ° C. 43.1 9.8 1.611 16.8 36.7 8.7 8.9 1.615 9.4 ND ND 1.585 8.3 Nd ND ND 1.589 0.6 ND ND 1.568 1.8 35.3 8.2 8.6 1.571 −7.1 40.0 9.0 1.551 −5.8 ND ND ND 1.551 −11.8 39.5 8.8 1.531 −11.8 34.5 8.3 8.3 1.546 −13.4

[0076] The information gleaned from the graph of FIG. 5 is that the sodium bromide/cesium bromide/sodium nitrate brine displays TCTs that are about 8° C. lower than sodium bromide/cesium bromide brines of equal density. This magnitude of reduction of TCT is smaller than previously observed for the other blends of alkali bromides. Nevertheless, incorporation of cesium bromide shifts the density towards higher ranges, say, >1.55 g/cc relative to cesium-free bromide mixtures, such that sodium bromide/cesium bromide/sodium nitrate blends can be formulated in water to give a clear brine of >1.55 g/cc density that does not crystallize out solids down to temperatures in the range from −15° C. to −5° C.

Example 8

Preparing and Testing NaBr/MNO.SUB.3 .Solutions (M=Li, Na and K)

[0077] A set of solutions was prepared by the methods described above, to investigate the effect of the alkali nitrate (LiNO.sub.3, NaNO.sub.3 and KNOB) on the TCT of sodium nitrate/alkali nitrate systems. The concentration of the added alkali nitrate was ˜9%. Compositions (sodium bromide/alkali nitrate concentrations) and properties of the solutions (density and TCT) are set out in Table 5.

TABLE-US-00005 TABLE 5 NaBr alone NaBr/~9% LiNO.sub.3 NaBr/~9% NaNO.sub.3 NaBr /~9% KNO.sub.3 NaBr d TCT NaBr LiNO.sub.3 d TCT NaBr NaNO.sub.3 d TCT NaBr kNO.sub.3 d TCT % g/cc ° C. % % g/cc ° C. % % g/cc ° C. % % g/cc ° C. 47.0 1.510 11.5 42.5 10.2 1.544 15.8 ND ND 1.555 14.7 42.2 9.4 1.544 6.2 46.6 1.503 8.2 ND ND 1.530 8.6 42.1 10.4 1.522 2.5 ND ND 1.528 −0.3 46.4 1.497 5.2 ND ND 1.514 3.7 40.4 8.5 1.509 −1.4 40.0 9.1 1.511 −5.3 45.5 1.488 0.9 40.3 8.2 1.500 −2.2 40.2 8.2 1.496 −10.4 ND ND 1.500 −11.5 45.2 1.483 −0.7 ND ND 1.489 −7.0 38.9 8.0 1.486 −13 38.6 8.6 1.484 −16.3 44.6 1.476 −3.3 38.9 9.1 1.476 −12.3 38.6 8.1 1.478 −16.2 1.469 −6.7 ND ND 1.463 <−18

[0078] Graphical representation is provided in FIG. 6. The results indicate that a TCT reduction effect is achieved by the addition of different alkali nitrates to sodium bromide.

Example 9

Corrosion Experiment

[0079] The effect of the presence of sodium nitrate in sodium bromide brine on the corrosivity of the solution was evaluated using three-electrode cell arrangement, where the working electrode, (consisting of the tested specimen) was made of carbon steel ST-37. Platinum and Ag/AgCl (in 3.5 M KCl) served as counter and reference electrodes, respectively. The instrument used for the measurements was VersaSTAT 3 equipped with V3-Studio software package. Nitrate-free sodium bromide solution of 1.491 gr/cc density (pH-7.6) and 39.5% sodium bromide/7.7% sodium nitrate solution of 1.496 gr/cc density were tested.

[0080] Cyclic polarization curves for ST-37 in sodium bromide and sodium bromide/sodium nitrate solution are shown in FIG. 7 (red and blue curves, respectively). Tafel analysis is used to determine corrosion potential (E.sub.corr) and corrosion current (I.sub.corr), obtained by the intersection of the linear sections corresponding to anodic and cathodic currents. Results are tabulated in Table 6.

TABLE-US-00006 TABLE 6 Solution E.sub.corr (mV) I.sub.corr (μA) NaBr 690.57 146.64 NaBr/NaNO.sub.3 738.24 103.2

[0081] Generally, the behavior of carbon steel ST-37 in contact with the two brines is similar, as is demonstrated by the comparable E.sub.corr and I.sub.corr values and the general shape of the curves. A difference in favor of the NaBr/NaNO.sub.3 brine is observed across the anodic branch, with the mixed NaBr/NaNO.sub.3 brine showing increased passivation range. The formation of negative hysteresis for both solutions indicates that pitting—significant corrosion occurring in a small area—has not been developed.