CORROSION INHIBITORS FOR CLEAR, CALCIUM-FREE HIGH DENSITY FLUIDS

This invention relates to corrosion inhibiting compositions and more particularly to corrosion inhibitors for use in calcium-free brines which may be used in formations having high carbonate and high sulfate concentrations.

Aqueous solutions of alkali and alkaline earth metal and zinc halides, and mixtures thereof, are used as drilling, completion, packer and perforating media in oil and gas wells. Depending on the formation pressures encountered in the well, fluids with densities between 1020 and 2400 kg/m³ (8.5 and 20.0 Ib/gal) are designed to provide a hydrostatic head to balance the formation pressure. The advent of the so-called solids-free brines in recent years has greatly improved the completion and workover activities by eliminating formation plugging and solid settling problems previously encountered when water base and oil base drilling fluids were used.

There are oil field uses for clear fluids in which subterranean carbonate and sulfate concentrations cause calcium carbonate and calcium sulfate precipitation from the clear fluids which contain significant calcium ion concentration. In some of these instances, sodium bromide fluid has been used in place of calcium ion-containing fluids in relatively shallow holes where high formation pressures are not encountered.

It has been reported by Shaughnessy, et al., "Workover Fluids for Prudhoe Bay", February-July 1977, that the mixing of calcium chloride workover fluid with formation brine can lead to the precipitation of calcium carbonate within the reservoir rock at Prudhoe Bay (P=₃.₅x₁₀' N/m² (5000 psi); T=104.4°C (220°F)). To minimize formation damage, a nondamaging fluid, sodium bromide, was used. More recently, carbonate and sulfate containing wells have been found requiring fluid densities well above the sodium bromide range, i.e. 1680-2400 kg/m³ (14-20 Ib/gal).

United States Patent Application Serial No. 892,155, filed July 30,1986, discloses new solids-free brine comprising aqueous solutions of zinc bromide, sodium bromide, potassium bromide, and lithium bromide. According to this disclosure, base fluids of zinc bromide/sodium bromide (density=₂₁₀₀ kg/m³ (17.5 Ib/ gal)), zinc bromide/potassium bromide (density=₂160 kg/m³ (18.0 Ib/gal)), and zinc bromide/lithium bromide (density=₂₁₆₀ kg/m³ (18.0 Ib/gal)) were prepared by mixing zinc bromide solutions (60-70 wt.%) with the respective monovalent salts (sodium bromide, potassium bromide, or lithium bromide). Different calcium-free fluids covering the density range from 1380 to 2160 kg/m³ (11.5 to 18.0 Ib/gal) were then formulated by mixing the base fluids with 1500 kg/m³ (12.5 ib/gai) sodium bromide, 1350 kg/m³ (11.3 lb/gal) potassium bromide, and 1600 kg/m³ (13.4 Ib/gal) lithium bromide solutions.

Considering the solubility products of calcium carbonate (3.8×10^-9 at 25°C), zinc carbonate (2.1 ×10^-11 at 25°C), and calcium sulfate (2.4x10-⁵ at 25°C), one would expect that zinc carbonate, calcium carbonate, and calcium sulfate should precipitate when zinc bromide/calcium bromide fluids are mixed with formation brine with high carbonate and sulfate ion concentration. The novelty of the calcium-free fluids is that the absence of calcium ion by the substitution of either sodium bromide, potassium bromide or lithium bromide for calcium bromide, lowers the total concentration of divalent metal ions in calcium-free fluids compared with that of standard zinc bromide/calcium bromide fluids such that, when either 2100 kg/m² (17.5 Ib/gal) zinc bromide/sodium bromide or 2160 kg/m³ (18.0 Ib/gal) zinc bromide/potassium bromide or zinc bromide/lithium bromide are mixed with a 2/8 ratio of formation brine having high carbonate and sulfate ion concentrations, no precipitate will form.

Although solids-free fluids have improved the drilling and completion efficiency, these fluids can be corrosive under operating conditions in drilling, completion, packer and perforating operations. The extent of corrosion is increased at higher temperatures encountered in deeper wells. Any fluid containing zinc halides is more corrosive than those fluids formulated with alkali and alkaline earth metal halides. Therefore it is necessary to use an appropriate corrosion inhibotor when zinc ion is present in the fluid.

In order to minimize the corrosion of iron and steel casing, tubing and the equipment exposed to clear brines, special corrosion additives are combined with the brines before they are used. Corrosion inhibitors such as film-forming amine-based inhibitors are used for calcium-based fluids, and are described in US-A-4.292183. According to GB-A-2027686 and DE-A-3316677, corrosion inhibitors such as thiocyanate, thioglycolate or thiourea compounds, alone, or in combination with a quaternary pyridinium, quinolium, or isoquinolinium salts may be used in conventional calcium-ion containing concentrated salt solutions, such as zinc bromide, calcium chloride solutions. However, calcium free solutions are significantly more corrosive than zinc bromide/calcium bromide fluids, and conventional inhibitors are not expected to perform satisfactorily in calcium free fluids.

In US―A―4536302, there is disclosed a method for reducing the corrosion effect of aqueous media such as brine on ferrous metals, by incorporating into the brine a mixture of a sulphur compound and a reducing sugar.

Accordingly, the objective of the present invention is to design corrosion inhibitor systems which can control the corrosion of iron and steel, casing, tubing, and other equipment exposed to high temperature and calcium-free fluids as completion, packer and perforating media. The corrosion inhibitors must be sufficiently soluble in the calcium-free fluids so that they can be formulated in the base fluids.

According to the present invention there is provided a corrosion inhibited clear, high-density fluid for use as a well completion and workover medium comprising:

• a clear aqueous solution of at least about 50 percent zinc bromide by weight of the solution and one or more alkali metal bromides, the solution having a density of 1380 to 2460 kg/m³ (11.5 to 20.5 pounds per gallon) and a pH lying in the range of about 1.5 to 5.5;
• a corrosion inhibiting amount of a corrosion inhibitor selected from the group consisting of sodium, ammonium and calcium thiocyanates and mixtures thereof; and
• an effective amount of an aldose group antioxidant selected from the group consisting of ascorbic acid and salts thereof, gluconic acid and mixtures of ascorbic acid and gluconic acid, said high density fluid being free of calcium except for that of said calcium thiocyanate where calcium thiocyanate is employed.

Ammonium thioglycolate may also be incorporated in the composition as an additional corrosion inhibitor.

Preferably, the corrosion inhibitor and the aldose group antioxidant are supplied in the clear fluid at a level of 0.05-5.00 parts per hundred parts of clear solution.

The abbreviation "gal" used herein means "gallon" (i.e. U.S. gallon). The abbreviations "mpy" and "pmpy" used herein means "mils per year" and "micrometres per year", respectively.

Calcium-free fluids formulated from aqueous solutions of zinc bromide and one or more alkali metal bromides such as lithium bromide, sodium bromide, and potassium bromide have utility in oil and gas wells of high carbonate and sulfate ion concentration.

It is believed that primary causes of corrosion in such calcium-free fluids are dissolved oxygen and zinc ion. Because such fluids may have higher zinc ion concentrations than zinc bromide/calcium bromide solutions of corresponding density, dramatic and unexpected increases in corrosion rate have been observed for such calcium-free fluids. In addition, even trace of oxygen in such fluids cause corrosion problems when they are utilized in well completion or workover.

In order for a compound or a chemical system to be used as an inhibitor or an inhibiting system in the well drilling industries, its desirable properties are to (1) dissolve completely in the fluid without leaving any residue, (2) control corrosion at temperatures up to 204.4°C (400°F) or higher, (3) control corrosion for an extended period of at least 30 days, and (4) not interfere with other chemical additives commonly used in the drilling industries.

It has been discovered that sodium, ammonium, and calcium thiocyanates used alone or in combination with aldose group antioxidants may be successfully employed as corrosion inhibitors in calcium-based fluids. Advantageously, ammonium thioglycolate may also be further incorporated as a corrosion inhibotor in such fluids.

Aldose group antioxidants which may be used in accordance with the invention are ascorbic acid and salts thereof, gluconic acid, mixtures of ascorbic acid and gluconic acid.

The calcium-free fluids in accordance with this invention are described in copending United States Patent Application Serial No. 892,155, filed July 30, 1986 corresponding to WO 88/01010. These fluids are aqueous solutions containing zinc bromide and one or more alkali metal bromides such as sodium bromide, potassium bromide, and lithium bromide. Such fluids desirably have densities lying in the range of 1380 to 2460 kg/m³ (11.5 to 20.5 Ibs. per gallon ("ppg")), preferably about 1740 to 2160 kg/m³ (14.5 to 18.0 ppg). Fluids of desired densities within this range may be obtained by blending lighter density fluids with a base fluid such as 2160 kg/m³ (18.00 ppg) zinc bromide/potassium bromide solution or 2100 kg/m³ (17.5 ppg) zinc bromide/sodium bromide solution. If densities above the 2160 or 2100 kg/m³ (18.0 or 17.5 ppg) of such base fluids are desired, the densities may be increased by mixing additional alkali metal bromides therewith. Preferably, the calcium-free base fluids of this invention comprise at least about 50% by weight zinc bromide. Clear fluids in accordance with this invention are generally useful at pH's lying in the range of about 1.5-5.5. Such fluids may be formulated in the manner described in the foregoing WO 88/01010.

The corrosion inhibiting agents may be added to the clear fluids in substantially any convenient manner. Thus, the corrosion inhibitors, including optional aldose group antioxidant where desired, may be added directly to the calcium-free fluid either in solid form or in the form of an aqueous corrosion inhibitor composition. Alternatively, the aldose group antioxidant may be separately added to the calcium-free base solution to provide an inhibited base solution, with the optional aldose group antioxidant thereafter being added to the inhibited base solution, or to a solution of modified density prepared therefrom, in either solid or aqueous solution form immediately prior to use. Other methods of addition may be employed so long as the corrosion inhibiting amount of the corrosion inhibitor is provided in the calcium-free fluid at the time of injection into the oil or gas well.

Desirably, the corrosion inhibitor, including optional aldose group antoxidant, is supplied to the calcium-free fluid at a level of about 0.05-5.00, preferably about 0.5 to 2.0, parts per 100 parts of the calcium-free fluid.

Exemplary corrosion inhibited calcium free high density fluids in accordance with the present invention are shown in the following Examples:

Example I

Example II

Example III

Example IV

Example V

Experimental evaluations

The corrosion inhbiiting abilities of the fluids of this invention have been evaluated through extensive corrosion tests at temperatures ranging from 148.9 to 232.2°C (300 to 450°F). Coupons of mild steel (AISI 1010), with dimensions 25.4 mmx50.8 mmx3.2 mm (1"x2"x1/8") (6.4 mm (0.25") diameter hole for mounting) were obtained from Jack Orr Metal Works in Houston and used without further cleaning. All the corrosion tests for the work of the invention were done in Parr's high pressure cells with about 200 ml of fluids contained in a glass liner to prevent electrical conducting between coupon and the bomb.

Extra care was taken to prevent bubbles forming on the surface of the coupons when being immersed in the fluids. The cells are pressurized with nitrogen or oxygen gas to keep the fluid in a liquid state at the test temperature.

The procedure for the tests is summarized as follows: An oven was set at the appropriate temperature level (e.g. 93.3, 121.1, 148.9, 176.7, 204.4 or 232.2°C (200, 250, 300, 350, 400 or 450°F)). The coupons were washed in acetone and toweled dry. The coupons were weighed to the nearest 0.01 milligram using an analytical balance and suspended from a glass hook supporting post without touching the sides or bottom of the glass liner.

After blending the fluid with inhibitor and/or antioxidant, it was filtered and the density and pH were checked. The fluid was then added to the glass liner. The bomb was pressurized to the appropriate pressure with nitrogen gas by using high pressure regulator. After test period is over, the bomb was allowed to cool for at least an hour to allow the internal pressure to drop.

The coupons were then examined. If necessary, qualitative analysis of adherent scale or foreign material was performed. A few new coupons were weighed and immersed along with the test coupons in a suitable hydrocarbon solvent, such as clean xylene or toluene, long enough to remove the oil, oil wet materials, and paraffins. The coupons were dried in a gentle dry air stream. The coupons were then immersed in an inhibited hydrochloric acid just long enough (usually less than 5 minutes) to remove mineral scale and corrosion product. After cleaning, the coupons were immediately immersed in a saturated solution of sodium bicarbonate for one minute to neutralize the acid. The coupons were rinsed with water, rinsed in isopropyl alcohol or acetone, and dried in a stream of air.

The coupons were then weighed, making sure that weight loss from cleaning is not significant. If the weight loss of the preweighed blank was significant, a correction was made to compensate for the weight loss for cleaning. Any abnormal conditions of the coupons (such as color, pitting, streaking, film coating, or crystal formation) were noted.

Corrosion in mils per year (mpy) was then calculated using the following formula:where

• -weight loss is in milligrams
• -coupon density is in gram/cm³
• -coupon area is in inch²
• -time is in hours

Corrosion in micrometres per year (pmpy) can then be calculated.

Comparative example

According to the disclosure of DE-A-3316677, ammonium thioglycolate at a concentration of 0.6 wt.% can control the corrosion rate of mild steel in zinc bromide/calcium bromide fluids. In order to evaluate the effectiveness of this agent in calcium-free fluids, zinc bromide/potassium bromide fluids were prepared by blending 77 wt.% zinc bromide (density=2430 kg/m³ (20.3 Ib/gal)) with 38 wt.% potassium bromide (density=₁₃₄₀ kg/m³ (11.2 Ib/gal)). Seven day, 204.4°C (400°F) corrosion rates of mild steel in both uninhibited and inhibited ZnBr₂/KBr were measured. These results, along with corrosion data for ZnBr₂/ CaBr₂ fluids, are presented in Table I.

The corrosion rate in uninhibited 1740 kg/m³ (14.5 Ib/gal) ZnBr₂/KBr fluid is five times that of the 2100 kg/m³ (17.5 Ib/gal) ZnBr₂/CaBr₂ fluid, which contains about the same concentration of ZnBr₂. Thus, in addition to zinc ion concentration, the type of second salt (CaBr₂ or KBr) greatly influences the corrosion rate. Although ammonium thioglycolate reduces the corrosion rate in 1740 kg/m³ (14.5 Ib/gal) ZnBr₂/KBr somewhat, this reduction is still not good enough to be considered effective. The corrosion data for 1920 kg/m³ (16.0 Ib/gal) ZnBr₂/KBr when compared with those for 2220 kg/m³ (18.5 Ib/gal) ZnBr₂/CaBr₂ are even more pronounced.

Example VI

Zinc bromide/potassium bromide base fluid having ZnBr₂ concentration similar to that of 2300 kg/m³ (19.2 lb/gal) ZnBr₂/CaBr₂ (i.e., 54 wt% ZnBr₂) was prepared by first diluting the 77 wt.% ZnBr₂ with water and then adding dry KBrto have a final composition of 56.2 wt.% ZnBr₂ and 17.3 wt.% KBr. Table II presents some corrosion data for mild steel in 2160 kg/m³ (18.0 Ib/gal) ZnBr₂/KBr using different corrosion inhibitors. Although the concentration of ZnBr₂ in 2160 kg/m³ (18.0 Ib/gal) ZnBr₂/KBr and 2300 kg/m³ (19.2 Ib/gal) ZnBr₂/CaBr₂ fluids is about the same, corrosion rates are much higher in ZnBr₂/KBr fluid. The corrosion rate of mild steel in an uninhibited 2300 kg/m³ (19.2 Ib/gal) ZnBr₂/CaBr₂ is 3760 pmpy (148 mpy), much lower than the value of 15500 pmpy (610 mpy) mesaured for the uninhibited 2160 kg/m³ (18.0 Ib/gal) ZnBr₂/KBr fluid.

Although some corrosion protection is obtained using ammonium thioglycolate, calcium thioglycolate and a combination of ammonium thioglycolate and sodium isoascorbate, the seven day corrosion rates (4600, 4750 and 3100 pmpy (181, 187 and 122 mpy)) are far above the acceptable range of 381-762 µmpy (15-30 mpy). The combination of sodium thiocyanate, ammonium thioglycolate, and sodium isoascorbate of the present invention is an excellent corrosion inhibitor system for the 2160 kg/m³ (18.0 Ib/gal) ZnBr₂/KBr fluid. The calcium thiocyanate agent of this invention at a concentration of 0.44 wt.% can also effectively reduce the corrosion rate of this fluid. It is believed that in this particular fluid (2160 kg/m³) (18.0 lb/gal) ZnBr₂/KBr), the presence of small concentrations of calcium ion demonstrates a synergistic effect with thiocyanate ion leading to unexpectedly good corrosion inhibition.

Example VII

This example involves a 2040 kg/m³ (17.0 Ib/gal) ZnBr₂/KBr fluid which, on the basis of ZnBr₂ concentration alone, should behave like the 2220 kg/m³ (18.5 Ib/gal) ZnBr₂/CaBr₂ fluid. Seven day, 176.7°C (350°F) corrosion rate of mild steel in 2220 kg/m³ (18.5 lb/gal) zinc bromide/calcium bromide fluid inhibited with either 0.6 wt.% ammonium thioglycolate or 0.4 wt.% sodium thiocyanate is 1020 pmpy (40 mpy). In contrast, as shown in Table III, the corrosion rate in 2040 kg/m³ (17.0 Ib/gal) zinc bromide/potassium bromide fluid inhibited with the same inhibitors is an unacceptably high 1850 µmpy (73 mpy). Thus, in addition to zinc ion concentration, the nature of the additional salts involved is responsible for the corrosion rate. The addition of 0.2 wt.% sodium isoascorbate to 0.4 wt.% NaSCN increases the corrosion protection somewhat over the sodium thiocyanate inhibition. Increasing both sodium thiocyanate and sodium isoascorbate concentrations (0.6 wt.% NaSCN and 0.6 wt.% Na-IA) increased the corrosion protection to satisfactory levels. Finally, a corrosion inhibitor system comprised of 0.4 wt.% sodium thiocyanate, 0.6 wt.% ammonium thioglycolate, and 0.8 wt.% sodium isoascorbate gives the lowest corrosion rate for mild steel in 2040 kg/m³ (17.0 ppg) zinc bromide/potassium bromide.

Example VIII

Corrosion test results in ZnBr₂/NaBr with varying concentrations of ZnBr₂ and inhibited with different inhibitors are presented in Table IV. The corrosion results for 2100 kg/m³ (17.5 Ib/gal) fluid (52 wt.% ZnBr₂) indicate that for this calcium-free fluid, calcium thiocyanate and a corrosion inhibitor system ("CIS") comprised of 0.4 wt.% sodium thiocyanate, 0.6 wt.% ammonium thioglycolate, and 0.8 wt.% sodium isoascorbate are the only effective corrosion inhibitor systems. Calcium thioglycolate at 1.0 wt.% concentration reduces the corrosion rate but is not as effective as other inhibitors used. In the case of 1740 kg/m³ (14.5 Ib/gal) ZnBr₂/NaBr fluid (25 wt.% ZnBr₂), the results indicate that the CIS gives the best corrosion protection. For 2160 kg/m³ (18.0 Ib/gal) ZnBr₂/NaBr with a concentration of 61 wt.% ZnBr₂, the corrosion inhibitor CIS gives a high level of corrosion protection.

Example IX

In order to show the long term effectiveness of the CIS corrosion inhibitor system described above (i.e.., 0.4 wt.% sodium thiocyanate, 0.6 wt.% ammonium thioglycolate, and 0.8 wt.% sodium isoascorbate) in calcium-free fluids containing high zinc bromide concentrations, thirty day corrosion tests were performed. The results are presented in Table V. The concentrations of zinc bromide in 2100 kg/m³ (17.5 Ib/ gal) and 2160 kg/m³ (18.0 lb/gal) ZnBr₂/NaBr fluids are 53 wt.% and 61 wt.%, respectively. The 2160 kg/m³ (18.5 Ib/gal) ZnBr₂/KBr fluid contains 58 wt.% ZnBr₂. The corrosion test results in Table V clearly indicate that even for calcium-free fluids with high concentrations of ZnBr₂, the corrosion rate can be satisfactorily controlled by using increased concentrations of corrosion inhibitors.