CALCIUM-FREE CLEAR HIGH DENSITY FLUIDS

CROSS-REFERENCE

This application is a continuation-in-part of appli¬ cants' copending application, Serial No. 892,155, filed July 30, 1986.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to the preparation and use. of solids-free fluids for oil and gas drilling, completion and. workover operations. More particularly, the invention relates to new calcium-free fluids which may be used as com¬ pletion, packer and perforating media in oil and gas drilling and completion operations when formations have high carbonate and/or high sulfate ion concentrations.

Description of the Prior Art

Special fluids known as drilling fluids are used in the drilling, completion, and workover of oil and gas wells. These fluids ideally perform the following functions: transport drill cuttings or solids debris to the surface; suspend cuttings and solids in lost circulation zones; counteract formation pressure; maintain borehole stability; cool and lubricate downhole equip¬ ment; aid the suspension of tool string and casing; minimize corrosion; and minimize damage to formation permeability.

Use of these drilling fluids has greatly increased the efficiency of operations at the well. However, problems with certain applications of these fluids have been encountered. For

example, when used in completion operations, these fluids leave a deposit of acid-insoluble filter cake in the bore hole which blocks production and is difficult to remove. Further, use of these fluids may permit entry of fresh water mud filtrates which can promote the hydration of naturally occurring clay materials which swell in volume and restrict permeability. Finally, because of the high alkalinity of many of these fluids, precipi¬ tation of insoluble hydroxides occurs along the filtration path, impeding production. These problems have been partially overcome by underreaming or acidizing of the bore hole if the damage is not severe.

In recent years, however, specialized solids free completion and workover fluids have been developed to help prevent this type of damage to formation permeability. These solids-free fluids are placed across the production zone during completion and workover operations performing the same functions as- drilling fluids but minimizing formation damage. These solids-free completion fluids comprise concentrated salt-water solutions in the density range of about 10 to 21 pounds per gallon ("lb/gal" or "ppg") and may be used as perforation, gravel pack, packer, and workover media. Examples of these solutions include aqueous solutions of alkali and alkaline earth metal and zinc halides such as sodium chloride, sodium bromide, calcium chloride, calcium bromide, zinc bromide or mixtures thereof.

As disclosed in 1964 in U.S. Patent No. 3,126,950 ("'950"), concentrated solutions of zinc chloride and/or calcium chloride can be prepared and used as well completion fluids up to a density of about 17 lb/gal. As noted in the '950 patent, however, zinc chloride/calcium chloride solutions with densities greater than 14 lb/gal. have high ferrous metal corrosion rates and therefore cannot practically be used with most well and surface equipment. Further, solutions with densities in the

14 lb/gal. range are not highly effective for deep well drilling. As a result of these limitations, these completion fluids did not receive strong acceptance in the oil and gas industry.

Other solids-free completion fluids have been better received. These fluids comprise calcium bromide, calcium chlo¬ ride, and water and have densities up to 15.1 lb/gal. See Plonka, "New Bromide Packer Fluids Cut Corrosive Problems," World Oil, April 1972, and Paul and Plonka "Solids-Free Completion Fluids Maintain Formation Permeability," SPE 4655, Las Vegas, Septem¬ ber 30 - October 3, 1973. Unlike the fluids in the '950 patent, calcium bromide/calcium chloride fluids have very low corrosion rates, which can be further reduced with the addition of suitable corrosion inhibitors. Density limitations (15.1 lb/gal limit) and high crystallization point temperatures (68°F) of the calcium bromide/calcium chloride fluids, however, have made these fluids less than ideal for use in completion operations. Therefore demands for other new solids-free completion fluids have contin¬ ued.

Another new system of completion fluids in the density range of 15.0 to 19.2 lb/gal was disclosed in 1981 in U.S. Patent No. 4,292,183, ('"183"). The '183 patent teaches mixtures of zinc bromide, calcium bromide, calcium chloride, and water which contain corrosion inhibitors capable of reducing the corrosion rate of mild steel coupons to less than 10 mpy at 250°F.

Although the introduction of these various new comple¬ tion fluids have helped resolve many of the difficulties encoun¬ tered in completion and workover operations*, problems still remain. For example, use of completion fluids with significant zinc and calcium ion concentrations in subterranean wells con¬ taining carbonate or carbon dioxide result in precipitation of calcium and zinc carbonates. Further, it has been reported by

Shaughnessy, et al. in "Workover Fluids for Prudhoe Bay," February-July 1977 that the mixing of calcium chloride workover fluids with formation brines under certain conditions (i.e., at a pressure of 5000 pεi and a temperature of 220° F) can lead to the precipitation of calcium carbonate within reservoir rock and, therefore, to formation damage. These problems have been par¬ tially resolved by utilizing sodium bromide completion and workover fluids in place of calcium ion containing solutions. However, sodium bromide solutions can only be used in shallow wells where high formation pressures are not encountered. Further, more recently, carbon dioxide or carbonate containing wells have been discovered which require drilling and completion fluids with fluid densities of at least 14-20 lb/gal, density ranges which are well above those of sodium bromide.

It is thus a primary object of the present invention to develop high density completion fluids that may be successfully used in sulfate and/or carbonate-containing wells, in the density range of 11.5 to 20.5 lb/gal.

It is a further object of the invention to develop high ^" density completion fluids having pH values in the range of 1.0 to 7.5 for use in sulfate and/or carbonate containing wells.

An additional object of the invention is to develop high density calcium-free completion fluids for use in carbonate and/or sulfate containing wells which are economical.

Another object of the present invention is to develop high/density calcium-free completion fluids which may also contain corrosion inhibitors and viscosifying agents for downhole applications.

Further objects and uses of the present invention will also be obvious from the following disclosure.

SUMMARY OF THE INVENTION

The foregoing objects, advantages and features of this invention may be achieved with high-density calcium-free fluids adapted for use as completion, packing, and perforation media in well formations having high carbonate and/or sulfate concen¬ trations comprising aqueous solutions of zinc bromide and one or more alkali metal bromide having densities in the range of about 11.5 to about 20.5 lb/gal and pH values in the range of about 1.0 to 7.5. Suitable alkali metal bromides include bromides of lithium, sodium and potassium as well as mixtures thereof. These solutions may also contain corrosion inhibitors to provide a non-corrosive environment for downhole applications, and viscosi- fiers for more effective use.

In its method aspect, the present inyention involves injecting a high density calcium-free fluid into wells having a high carbonate and/or sulfate ion concentration.

The novelty of the fluids of this invention is that, contrary to the expectations of those skilled in the art, solu¬ tions obtained by substituting one or more alkali metal bromides for calcium bromide in zinc bromide/calcium bromide fluids may be used without precipitation of zinc salts when applied to carbon¬ ate and/or sulfate containing formation brines.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Calcium-free solutions for use as completions fluids in oil and gas formations containing high carbonate and/or sulfate

ion concentrations have been prepared. These solutions comprise mixtures of zinc bromide and one or more alkali metal bromide and water and have densities in the range of 11.5 to 20.5 lb/gal, preferably about 11.5-19.2 lb/gal, and pH values of about 1.0 to 7.5, preferably about 2.5 to 5.5.

Suitable alkali metal bromides which may be used in accordance with this invention include sodium bromide, potassium bromide, and lithium bromide. Mixtures of alkali metal bromides, especially mixtures of sodium and potassium bromides, may also be employed.

The relative concentrations or amounts of the different salt constituents used in these completion fluids are not criti¬ cal and may be determined by convenience so long as the density and pH limitations are maintained. Desirably the compositions of zinc bromide and alkali metal bromides (LiBr, KBr, or NaBr) in these calcium-free fluids are about 1.0 - 77.0 wt.% and 1.0 - 54.0 wt.%, respectively. Preferably these compositions are 2-56% zinc bromide and 14-54% alkali metal bromide(s) by weight of the overall compositions.

In the case of the zinc bromide/sodium bromide fluids, the densities lie in the range of about 12.5 to 19.2 lb/gal. Density of zinc bromide/potassium bromide fluids lie in the range of about 11.5 to 19.2 lb/gal. Zinc bromide/lithium bromide fluids have densities lying in the range of about 13.5 to 18.0 lb/gal., and fluids composed of zinc bromide/sodium bromide/potassium bromide have densities of about 13.0 to 18.0 lb/gal.

The calcium free solutions of the present invention may be prepared by mixing a zinc bromide/alkali metal bromide base fluid with one or more monovalent alkali metal bromide solutions.

The zinc bromide/alkali metal bromide base fluids may be pre¬ pared by combining solid zinc bromide and water with a solid alkali metal bromide or an aqueous solution thereof. The zinc bromide/alkali metal bromide base fluids may also be prepared by dissolving dry alkali metal bromides(s) in aqueous zinc bromide solutions. For example, a 17.5 lb/gal. ZnBr _^ /NaBr base fluid is prepared by adding 14.8g water to 60.8g 77 wt% ZnBr, solution, and then dissolving 24.4g 97% NaBr in the resulting solution to prepare lOOg fluid. Different ZnBr./NaBr blends in the density range of 13.0-17.5 lb/gal. may be prepared by mixing appropriate volumes of 17.5 lb/gal. ZnBr,/NaBr base fluid with 12.5 lb/gal. NaBr (46.0 wt.% NaBr in water).

Corrosion inhibitors such as thioglycolates and thio- cyanates which effectively control corrosion rates of mild steel may also be added to the completion fluids of the present inven¬ tion to control corrosion of downhole equipment. The reason for the use of corrosion inhibitors is that completion fluids which contain zinc bromide are more corrosive than fluids formulated with alkali and alkaline earth metal bromides (ie., LiBr, NaBr, KBr and CaBr ₂ ). Therefore corrosion inhibitors are generally used when zinc bromide is present. British patent, GB 2 027 687, and German patent, Ger. offen. DE 3 316 677 All, disclose the use of various corrosion inhibitors, such as thioglycolates and thiocynates, in zinc containing fluids. Suitable corrosion inhibitors which are capable of assisting in the control of corrosion with the calcium-free fluids of this invention include alkali metal and ammonium thiocyanates and thioglycolates, calcium thioglycolate and mixtures thereof. In addition, especially preferred corrosion inhibitors for use in the solutions of this invention, most especially those containing 50 wt.% ZnBr. or more, are disclosed in copending United States patent application Serial No. 913,409, filed September 30,_ 1986. The corrosion inhibitors in accordance with the Serial No.

913,409 application include calcium thiocyanate and a mixture of sodium thiocyanate, ammonium thioglycolate, and sodium isoascorbate.

Viscosifiers may also be added to the completion fluids of the "present invention to help increase these fluids' ability to suspend and remove cuttings from the well and to prevent significant loss of fluids to the formation. Natural polymers such as guar gum, xanthan gum, and hydroxyethyl cellulose ("HEC") may be used as viscosifier additives in drilling and completion fluids. Only HEC has been used extensively as a viscosifier for drilling and completion fluids in the density range of 10 to 19.2 lb/gal. HEC polymer, solvated with ethylene glycol or suspended in mineral oil, has been used to viscosify aqueous NaBr, CaCl., CaBr_, and ZnBr_ brines in the density range of 10.0 to 15.0 and from 16.5 to 19.2 lb/gal. This viscosifier however fails to viscosify zinc ion-containing fluids in the density range of 15.0 to 16.5 lb/gal. This failure is believed to be due to the structural changes of solvent and solute caused by the different concentration ratio of halogen to zinc ion. An especially preferred viscosifier system is disclosed in United States Patent application Serial No. 913,415, filed September 29, 1986.

The following Examples are provided for the purpose of further illustration of the preferred embodiment of the present invention and are not intended to be limitations on the disclosed invention.

EXAMPLE I.

Tables 1 through 6, present the weight percents ("wt %" ₎ of the various salt constituents used in preparing drilling and completion fluids having densities in the range of 11.5 to 19.2 lb/gal. The specific gravity of these fluids is also given.

Table 1

Zinc Bromide/Lithium Bromide Fluid Density, Specific Gravity, and Weight Percent

Density at 70°F Sp.Gr. ZnBr ₂ LiBr lb/gal wt.% wt.%

13.5 1.62 2.2 52.4

13.8 1.66 7.0 . 49.0

14 1.69 11.5 45.7

14 1.73 14.5 43.6

14 1.77 20.2 39.5

15 1.80 24.1 36.7

15. 1.84 28.0 34.0 15, 1.87 31.7 31.2

15. 1.91 35.2 28.8 16. 1.95 38.7 26.3 16, 1.98 42.0 23.9

16.8 2.02 45.2 21.6 17.1 2.05 48.2 19.4 17.4 2.09 51.2 17.3 17.7 2.13 54.0 15.3 18.0 2.16 54.8 14.7

Table 2

Zinc Bromide/Sodium Bromide Fluid

Density, Specific Gravity, and Weight Percent

Density at 70°F Sp.Gr. ZnBr, NaBr lb/gal wt.% wt.%

13.0 1.56 6.3 43.0

13.5 1.62 12.1 40.2

14.0 1-68 17.6 37.6

14.5 1.74 22.6 35.2

15.0 1.80 27.3 33.0

15.5 1.86 31.7 30.9

16.0 1.92 35.9 28.9

16.5 1.98 39.7 27.1

17.0 2.04 43.4 25.3

17.5 2.10 46.8 23.7

TABLE 3 Zinc Bromide/Potassium Bromide Fluid Density, Specific Gravity, and Weight Percent

Density at 70°F So.Gr ZnBr _a ______ lb/gal wt.; wt.%

11.5 1.38 2.6 37. 5 12.0 1.44 8.7 35.3

12 1.50 14.4 33.1

13 1.56 19.7 31.1

13 1.62 24.5 29.2

14 1.68 29.4 27.3

14 1.74 33.2 25.9

15 1.80 37.1 24.4

15 1.86 40.8 23.0

16.0 1.92 44.2 21.7

16.5 1.98 47.4 20.5

17.0 2.04 50.5 19.3

17.5 2.10 53.3 18.2

18.0 2.16 56.0 17.2

TABLE 4

Zinc Bromide/Sodium Bromide/Potassium Bromide Fluid

Density, Specific Gravity, and Weight Percent

Density at 70°F Sp.Gr. ZnBr, KBr NaBr lb/gal wt.% wt.% wt.%

13.0 1.56 7.1 2.2 40.0

13.5 1.62 13.6 4.2 34.7

14.0 1.68 19.7 6.0 29.7

14.5 1.74 25.3 7.8 25.1

15.0 1.80 30.6 9.4 20.8

15.5 1.86 35.5 10.9 16.8

16.0 1.92 40.1 12.3 13.0

16.5 1.98 44.4 13.6 9.5

17.0 2.04 48.5 14.9 6.1

17.5 2.10 52.4 16.1 3.0

18.0 2.16 56.0 17.2 0.0

Table 5 Zinc Bromide/Potassium Bromide Fluid Density, Specific Gravity and Weight Percent Density at 70°F Sp.Gr. ZnBr ₂ KBr lb/gal wt.% wt.%

11.5 1.38 3.0 37.0 12.0 1.44 10.0 33.5 12.5 1.50 16.5 30.3 13.0 1.56 22.5 27.3 13.5 1.62 28.0 24.3 14.0 1.68 33.1 21.9 14.5 1.74 37.9 19.5 15.0 1.80 42.3 17.3 15.5 1.86 42.5 20.0 16.0 1.92 42.7 22.5 16.5 1.98 42.9 24.9

17, 0 2.04 43.1 27.1 17, 5 2.10 43.3 29.3 18, 0 2.16 43.4 31.2 18, 5 2.22 - 43.6 33.1 19.0 2.28 43.7 34.9 19.2 2.31 43.8- 35.6

Table 6 Zinc Bromide/Potassium Bromide Fluid Density, Specific Gravity and Weight Percent Density at 70°F Sp.Gr. ZnBr ₂ KBr lb/gal wt.% wt.%

13 .0 1.56 7.7 41.4

13 .5 1.62 14.9 37.1

14 .0 1.68 21.5 33.2

14 .5 1.74 27.7 29.5

15, .0 1.80 33.5 26.0

15. .5 1.86 35.1 27.3

16. .0 1.92 36.5 28.5

16, ,5 1.98 37.9 29.6

17. .0 2.04 39.2 30.7

17. ,5 2.10 40.4 31.7

18. 0 2.16 41.5 32.6

18. 5 2.22 42.6 33.5

19. 0 2.28 43.6 34.4

19. 2 2.31 44.0 34.7

All of the calcium-free fluids in the density range of 11.5 to 19.2 lb/gal-described above may be prepared by mixing a two salt base fluid (e.g., 18.0 lb/gal ZnBr ₂ /KBr or 17.5 lb/gal ZnBr ₂ /NaBr) with single- or two-salt solutions having a lower density than the base fluid (e.g., 12.5 lb/gal NaBr or 15.0 lb/gal ZnBr./KBr) . These fluids may also be formulated by mixing solutions of ZnBr and LiBr, NaBr, or KBr and dry salts (e.g., 77 wt.% ZnBr ₂ , 54 wt.% LiBr, 46 wt.% NaBr, 38.5 wt.% KBr, and dry salts) .

EXAMPLE II.

A NaBr solution having a density of 12.5 lb/gal was prepared by mixing 46.0 wt. % solid NaBr and 54.0 wt.% water. A ZnBr ₂ NaBr base fluid having a density of 17.5 lb/gal was pre¬ pared by combining 46.8 wt. % solid ZnBr ₂ , 23.7 wt. % solid NaBr, and 29.5 wt. % water. Varying amounts of ZnBr ₂ /NaBr base fluid

(density, 17.5 lb/gal) were then mixed with different amounts of NaBr solution (density, 12.5 lb/gal) in order to prepare differ¬ ent completion fluids in the density range of 13.0 to 17.5 lb/gal. The volumes of the base fluid and NaBr solution required to formulate these different completion fluids, along with the respective densities and thermodynamic crystallization tempera¬ tures of the completion fluids, are given in Table 7.

TABLE 7

Blending Procedure-ZnBr ₂ NaBr Fluid

Using 17.5 lb/gal ZnBr ₂ /NaBr and 12.5 lb/gal NaBr

Density at 70°F ZnBr ₂ /NaBr NaBr Cryst.Pt. lb/gal bol bbl (TCP) °F

12.5 0.000 1.000 21

13.0 0.100 0.900 18

13.5 0.200 0.800 15

14.0 0.300 0.700 10

14.5 0.400 0.600 1

15.0 0.500 0.500 -16

15.5 0.600 0.400 -3

16.0 0.700 0.300 9

16.5 0.800 0.200 ^" 23

17.0 0.900 0.100 37

17.5 1.000 0.000 47

The low pH and relatively low concentrations of the divalent salt in these fluids (compared with ZnBr ₂ /CaBr ₂ /CaCl ₂ fluids) make them particularly suitable for use in formations with high carbonate and/or high sulfate concentrations.

EXAMPLE III.

Another calcium-free completion fluid was prepared by mixing a ZnBr ₂ /LiBr base fluid with a LiBr solution. The 18.0 lb/gal ZnBr ₂ /LiBr base fluid was prepared by combining an appro¬ priate volume of aqueous 77 wt % ZnBr ₂ solution (density, 20.3

lb/gal) with the requisite amount of aqueous 54 wt % LiBr solu¬ tion (density, 13.4 lb/gal). Different ZnBr ₂ /LiBr fluids with densities in the range of 14.0 to 18.0 lb/gal were then formulat¬ ed by combining varying amounts of the 18.0 lb/gal ZnBr ₂ /LiBr base fluid with different volumes of the aqueous 13.4 lb/gal LiBr solution. Table 8 provides the different volumes of base fluid and LiBr solution required to formulate these completion fluids and the thermodynamic crystallization temperatures of the fluids.

Table 8

Mixing Procedure-ZnBr ₂ /LiBr Fluid

Using 18.0 lb/gal ZnBr ₂ /LiBr and 13.4 lb/gal LiBr

Composition for 1 bbl (42 gal)

Density of 70°F ZnBr,/LiBr LiBr Cryst.Pt lb/gal bbl bbl (TCP)°F

15.3 0.428 0.552 -60°F

EXAMPLE IV.

A different calcium free completion fluid, ZnBr ₂ /KBr, was prepared in two ways. The 18.0 lb/gal base fluid was pre¬ pared by combining the appropriate amount of the aqueous 77 wt % ZnBr ₂ solution with the requisite volume of the aqueous 38.5 wt % KBr solution (density, 11.3 lb/gal). This method of preparation was not preferred however because of the low density (i.e., 11.3 lb/gal) of the aqueous 38.5 wt % KBr solution. Mixing of the low density KBr solution with the ZnBr ₂ solution resulted in a base fluid with an inordinately high ZnBr ₂ concentration. The pre¬ ferred method was to dilute the aqueous 77 wt % ΣnBr ₂ with water and then add the required weight of solid KBr to achieve a 18.0 lb/gal base fluid. This base fluid can then be mixed with the 11.3 lb/gal aqueous KBr solution to prepare different completion fluids having densities in the range of 11.5 to 18.0 lb/gal. Table 9 presents the various mixtures of base fluids and KBr solutions used ^* to make the completion fluids of Example ^* IV along with the thermodynamic crystallization temperatures for these completion fluids.

TABLE 9 Blending Procedure - ZnBr ₂ /KBr Fluid Using 18.0 lb/gal ZnBr ₂ /KBr and 11.3 lb/gal KBr Composition for 1 bbl (42 gal)

Density at 70°F ZnBr ₂ /KBr KBr Cryst.Pt. lb/gal bbl bbl (TCP) °F

12.0 0.104 0.896 6

12.5 0.179 0.821

13.0 0.254 0.746 -8

13.5 0.328 0.672

14.0 0.403 0.579 -12

14.5 0.478 0.522

15.0 0.552 0.448

15.5 0.627 0.373 -35

16.0 0.701 0.299

16.5 0.776 0.224 -64

17.0 0.851 0.149

17.5 0.925 0.075 -23

18.0 1.000 0.000

Because of their low crystallization 'temperatures (6 to -64°F), these completion fluids can be used during the winter months without danger of solidification.

EXAMPLE V.

Another calcium-free fluid may be prepared by dissolv¬ ing ZnBr ₂ , NaBr, and KBr salts in water. As an example of the numerous ways of preparing completion fluids, a 18.0 lb/gal ZnBr ₂ /KBr base fluid (prepared according to Example IV) was mixed with a an aqueous solution of NaBr having a density 12.5 lb/gal to formulate ZnBr ₂ /NaBr/KBr fluids having densities in the range of 13.0 to 18.0 lb/gal. The various volumes of the base fluid and NaBr solution used in these completion fluids are given in Table 10.

TABLE 10 Blending Procedure-ZnBr ₂ /KBr/NaBr Fluid Using 18.0 lb/gal ZnBr ₂ /KBr and 12.5 lb/gal NaBr Composition for 1 bbl (42 gal)

Density at 70 ^β F ZnBr_/KBr NaBr lb/gal bbl bbl

13.0 0.091 0.909

13.5 0.182 0.818

14.0 0.273 0.727

14.5 0.364 0.636

15.0 0.455 0.545

15.5 0.546 0.454

16.0 0.636 0.364

16.5 0.727 0.273

17.0 0.818 0.182

17.5 0.909 0.091

18.0 1.000 0.000

EXAMPLE VI.

Calcium-free ^' completion fluids having densities greater than 18.0 lb/gal. may be prepared by dissolving a greater amount of solid monovalent salt (i.e., NaBr, KBr, or LiBr) into the base fluids than in the previous examples. For example, a 20.5 lb/gal. ZnBr ₂ /NaBr completion fluid was prepared by dissolving solid NaBr in a 17.5 lb/gal. ZnBr,/NaBr base fluid. It has also been discovered that 19.2 lb/gal. calcium-free base fluids having low compp-sition of ZnBr_ (42-44 wt.%) by dissolving solid monovalent salt, into the zinc bromide solution. Owing to the high composi¬ tion- of monovalent salt in these base fluids, if they are blended down with lower density base fluids (i.e. 11.3 lb/gal. KBr, 12.5 lb/gal. NaBr, or 13.4 lb/gal. LiBr), solid monovalent salt will precipitate out of the solution (salting out). The problem was resolved by formulating an intermediate density base fluid (15.0 lb/gal.) by blending the 77 wt.% ZnBr, solution with the lower density base fluids. These new base fluids were then used with

19.2 lb/gal. fluids to blend up and with lower density base fluids to blend down.

The composition of ZnBr ₂ in the new 19.2 lb/gal. base fluids is about 42.0 - 44.0 wt.%. Because of lower concentra¬ tions of ZnBr ₂ KBr in these base fluids compared with those used in Examples I-V, they are less corrosive to metal equipment than the base fluids used in the previous examples.

EXAMPLE VII

A 19.2 lb/gal ZnBr ₂ /KBr base fluid was prepared by adding 7.1g water to 56.9g 77 wt.% ZnBr ₂ and then dissolving 36.0 solid 99 wt.% KBr into the resulting solution. The composition of this fluid is therefore 43.8 wt.% ZnBr_/KBr, 35.6 wt.% KBr and 20.6 wt.% water.

A 15.0 lb/gal ZnBr ₂ /KBr base fluid was prepared by mixing 140.7 ml of 77 wt.% ZnBr ₂ solution (d = 20.3 lb/gal) with 209.8 ml of 38.5 wt.% KBr solution (d = 11.3 lb/gal). The resulting fluid contained 42.3 wt.% ZnBr ₂ , 17.3 wt.% KBr and 40.4 wt.% water.

Tables 11 and 12 present the blending procedures for ZnBr ₂ /KBr fluids using 19.2 lb/gal ZnBr ₂ /KBr, 15.0 lb/gal ZnBr ₂ /KBr and 11.3 lb/gal KBr. The thermodynamic crystallization temperatures are also given in Tables 11 and 12.

Table 11

Blending Procedure for ZnBr ₂ /KBr Fluid Using 15.0 lb/gal ZnBr ₂ /KBr and 11.3 lb/gal KBr

Density at 70°F 15. 0 lb/gal ZnBr _/KBr 11. .3 lb/gal KBr Cryst.Pt lb/gal bbl bbl (TCP) °F

11. 5 0.054 0.946 12.0 0.189 0.811 6.8 12.5 0.324 0.676 13, 0 0.460 0.540 - 0.4 13.5 0.595 0.405 14.0 0.730 0.270 -11.6 14.5 0.865 0.135 15.0 1.000 0.000 -36.0

Table 12

Blending Procedure for ZnBr ₂ /KBr Fluid Using 19.2 lb/gal and 15.0 lb/gal ZnBr ₂ /KBr

Density at 70°F 19.2 lb/gal 15.0 lb/gal Cryst.Pt lb/gal bbl _* bbl (TCP) ² F

15.5 0.119 0.881 -

16.0 0.238 0.762 -27.9

16.5 0.357 0.643

17.0 0.476 0.524 -24.5

17.5 0.595 0.405

18.0 0.714 0.286 -12.6

18.5 0.833 0.167

19.0 0.952 0.048

19.2 1.000 0.000 3.0

Example VIII

A 19.2 lb/gal ZnBr ₂ /NaBr base fluid was prepared by adding 8.2g water to 57.Ig 77 wt.% ZnBr ₂ solution and then dissolving 34.7g 97 wt.% dry NaBr into the resulting solution. The composition of this fluid was therefore 44.0 wt.% ZnBr ₂ , 34.7 wt.% NaBr and 21.3 wt.% water. Another zinc bromide/sodium bromide base fluid (15.0

lb/gal) was prepared by mixing 112.4 ml of 77 wt.% ZnBr ₂ solution (d = 20.3 lb/gal) with 237.6 ml of 46.0 wt.% NaBr solution (d = 12.5 lb/gal). The resulting fluid contained 33.5 wt.% ZnBr ₂ , 26.0 wt.% NaBr and 40.5 wt.% water.

Tables 13 and 14 present the blending procedures for ZnBr ₂ /NaBr fluids using 19.2 lb/gal and 15.0 lb/gal ZnBr ₂ /NaBr, and 12.5 lb/gal NaBr. The thermodynamic crystallization temperatures for these fluids are also given in Tables 13 and 14.

Table 13

Blending Procedure for ZnBr ₂ /NaBr Fluid Using 15.0 lb/gal ZnBr_/NaBr and 12.5 lb/gal NaBr

at 70°F 15, .0 lb/gal ZnBr ₂ /NaBr 12, 5 lb/gal NaBr Cryst.Pt lb/gal bbl bbl (TCP) °F

13.0 0.200 0.800 5.6

13.5 0.400 0.600

14.0 0.600 - 0.400 -35.3

14.5 0.800 0.200

15.0 1.000 0.000 -27.7

Table 14

Blending Procedure for ZnBr ₂ /NaBr Fluid Using 19.2 lb/gal and 15.0 lb/gal ZnBr ₂ /NaBr

Density at 70°F 19.2 lb/gal 15 .0 lb/gal Cryst.Pt lb/gal bbl bbl (TCP) °F

15.5 0.119 0.881

16.0 0.238 0.762 2.8

16.5 0.357 0.643

17.0 0.476 0.524 24.4

17.5 0.595 0.405

18.0 0.714 0.286 39.7

18.5 0.833 0.167

19.0 0.952 0.048

19.2 1.000 0.000 48.6

FORMATION DAMAGE EXPERIMENTS

Formation damage experiments have shown that, when 18.0 lb/gal ZnBr ₂ /KBr or 17.5 lb/gal ZnBr ₂ /NaBr completion fluids were mixed with a 2/8 ratio of formation brine having a high carbonate and/or high sulfate concentration, no precipitate was formed. However, when the same experiments were performed with a 18.0 lb/gal ZnBr ₂ /CaBr ₂ completion fluid, a white precipitate was formed. In other experiments, 14.5 lb/gal ZnBr ₂ /NaBr and ZnBr ₂ /KBr-completion fluids were mixed separately with a 3/7 ratio of formation brine and no precipitate was formed. However, when the same test was performed with a 14.5 lb/gal ZnBr ₂ /CaBr ₂ completion fluid, a white precipitate was formed.

Considering the solubility products for calcium carbon- ate (i.e., 3.8 x 10 —9 at 25 ^β C) and zinc carbonate (i.e., 2.1 x

10 ^-11 at 25°C), it would be expected that zinc carbonate and calcium carbonate would precipitate when ZnBr ₂ /NaBr or ZnBr ₂ /KBr completion fluids were mixed with formation brine. However, no precipitates formed with the solutions of this invention. The novelty of the present invention lies in the discovery that the

substitution of either sodium bromide or potassium bromide for calcium bromide alters the expected reaction between zinc and carbonate ions such that no insoluble zinc carbonate precipitate is formed. Without being limited to the correctness of any particular theory this unusual effect may be due to a lower pH and a lower concentration of divalent metal ions in the calcium-free completion fluids than in the standard ZnBr ₂ /CaBr ₂ completion fluids. Another possible explanation is that zinc bromide, sodium bromide, potassium bromide and lithium bromide may form double salts in aqueous solution, preventing zinc carbonate precipitation. Still another possible explanation is the formation of complex ions between zinc ions and bromide ions,

+ «_ a i.e. , ZnBr, ZnBr-, ZnBr ₄ which may prevent the carbonate precipitation. Also, the reported solubility products for CaC0 ₃ and ZnC0 ₃ are those at infinite dilution or when the activity coefficient of the ions involved approach unity. In the con¬ centrated salt solutions of the present invention, the activity ^• coefficients of calcium, zinc and carbonate ions may, due to high ionic strength, be different than unity, and hence the reported values for solubility products of ZnCO, and CaCO- cannot be used as a criteria for predicting the formation of precipitates. Whatever the explanation, it is clear that, most unexpectedly, the calcium-free fluids of this invention may quite successfully be employed with carbonate and sulfate containing formation brines without precipitation of insoluble zinc salts.

VISCOSIFICATION EXPERIMENTS

The calcium-free completion fluids of the present invention can be easily viscosified with any HEC-based liquid viscosifier. Tables 15 and 16 presents the funnel viscosities and rheology data for different ZnBr ₂ /NaBr and ZnBr ₂ /KBr fluids viscosified with HEC-based liquid viscosifier.

Table 15

Funnel Viscosity and Rheology Data for Calcium-Free Fluids (ZnBr./NaBr)

Viscosified With 15 lb/bbl HEC-Based Liquid Viscosifier One Hour Mixing

Fluid

Density Funnel Fann Fann Apparent Plastic Yield at 70°F Viscosity RPM RPM Viscosity Viscosity Point lb/gal (sec) 600 300 CP cp lb/100 εσft

15.0 366 269 213 135 86 157

15.5 419 285 230 143 55 175

16.0 409 OS 241 OS OS OS

16.5 383 OS 240 OS OS OS

17.5 595 OS 279 OS OS OS

19.0 1195 OS OS OS OS OS

OS = off scale, greater than 300

Table 16

Funnel Viscosity and Rheology Date for Calcium-Free Fluids (ZnBr ₂ KBr) Viscosified with 15 ^* lb/bbl . HEC-Based Liquid Viscosifier

Fluid

Density Funnel Fann Fann Apparent Plastic Yield at 70°F Viscosity RPM RPM Viscosity Viscosity Point lb/gal (sec) 600 300 CP cp lb/100 soft

15.0 210 226 183 113 43 140

15.5 310 260 214 130 46 168

16.0 350 259 207 130 52 155

16.5 320 276 224 138 52 172

17.5 605 OS 289 OS OS OS

18.5 530 OS 286 OS OS OS

19.0 471 OS 281 OS OS OS

OS = off-scale, greater than 300

These data show that the 15 lb/bbl HEC-based liquid viscosifier was effective as a viscosifier for zinc ion-containing fluids in the density range of 15.5 to 19.0 lb/gal. The funnel viscosity

measurements, which cannot be manipulated mathematically, are presented with the measurements obtained from the viscometer for purposes of permitting comparison of these completion fluids fluid viscosities. A viscosified fluid used as a "pill" should exhibit a funnel viscosity of about 200 sec. The data in Tables

15 and 16 indicate that concentrations of 10 to 15 lb/bbl of the

HEC-based liquid viscosifier are sufficient to generated funnel viscosities of 200 sec.

TOXICITY EXPERIMENTS

Toxicity data for the calcium-free fluids of the present invention indicates that these fluids may be safely employed. While zinc bromide solution has been found to be a primary eye irritant, neither zinc bromide nor any of the mon¬ ovalent salt solutions (LiBr, NaBr, and KBr) has been considered primary skin irritants. Table 10 contains LD _S0 (i.e., the lethal dosage at which 50% of the test animals die) toxicity data from the 1981-82 Registry of Toxic Effects of Chemical Substances from the United States Department of Health, Education and Welfare. See also Sax, Dangerous Properties of Industrial Materials, 6th ed. , or the Merck Index, 10th ed.

CORROSION INHIBITION

Seven day corrosion rates were determined in a manner known to those skilled in the art for calcium-free completion fluids in accordance with this invention using thioglycolate/ thiocy nate-based corrosion inhibitors. Specific corrosion inhibitors tested included a mixture of sodium thiocyante, ammonium thioglycolate and sodium isoascorbate ("C.I.A."); calcium thiocyanate ("C.I.B."); and sodium thiocyanate _.

("C.I.C.").

Table 17

Seven Day Corrosion Rates of Mild Steel Coupons in Calcium-Free Fluids

Fluid Density at 70°F. Temp. Inhibitor Corrosion Rate

(lb/gal) (°F) (mpy)

18.0 ^a ZnBr ₂ /KBR 300 Blank 610

18.0 ZnBr^/KBR 300 C.I.A. 15

18.0 ZnBr ₂ /KBR 300 C.I.B. 20 17.0 ^b ZnBr ₂ /KBr 350 Blank 350

17.0 ZnBr ₂ /KBr 350 C.I.A. 7

17.5 ZnBr-/NaBr 300 Blank 456

17.5 ZnBr ₂ /NaBr 300 C.I.A. 14

17.5 ZnBr ₂ /NaBr 300 C.I.B. 19

14.5 ZnBr ₂ /NaBr 300 Blank 52

14.5 ZnBr ₂ /NaBr 300 C.I.A. 8 19.0 ^d ZnBr ₂ /KBr 350 Blank 112

19.0 ZnBr ₂ /KBr 350 C.I.A. 9

19.0 ZnBr ₂ /KBr 350 C.I.B. 8

19.0 ZnBr ₂ /KBr 350 C.I.C. 8

19.0 ^e ZnBr ₂ /NaBr 350 Blank 53 ^*

19.0 ZnBr ₂ /NaBr 350 C.I.A. 6

19.0 ZnBr ₂ /NaBr 350 C.I.B. 7

19.0 ZnBr ₂ /NaBr 350 C.I.C. 9

a 56.2 wt.% ZnBr,/17.3 wt.% KBr b 50.3 wt.% ZnBr,/19.4 wt.% KBr *c"- 52.0 wt.% ZnBr,/18.0 wt.% NaBr d 43.2 wt.%._ ZnBr_,/35.6 wt.% KBr e 42.5 wt.% ZnBr ₂ /34.5 wt.% NaBr

These data show that thioglycolate and thiocyanate group contain¬ ing corrosion inhibitors act as effective corrosion inhibitors for zinc containing solutions of the present invention.