NO831363L - Corrosion-inhibited liquid mixture that is clear and has a high density - Google Patents

Description

The invention relates to fluids for use in well drilling, and more particularly a corrosion-inhibited fluid with high density that can be used under severe working conditions.

State of the art

Drilling fluids have usually been used to maintain control when perforating, completing or testing oil and gas wells. Drilling fluids that have so far been used for such purposes include mud, salt water,

water or oil. The use of these fluids during drilling operations has generally been satisfactory as such and has actually improved drilling efficiency.

However, the same materials have been used during completion and restoration with undesirable consequences.

For example, the use of drilling mud during well perforating has often led to clogging of the perforations. Solids present in such drilling fluids have caused clogging and have made the completion process unnecessarily complex, expensive and unreliable. In general, the use of drilling mud and other drilling fluids such as etching fluids has led to unwanted settling of solids. Furthermore, drilling media can be somewhat corrosive under long-term, static working conditions, rendering them further unsuitable for applications other than as a temporary drilling aid.

Among the efforts that have been made to overcome the above-mentioned problems, mention may be made of the use of clear salt solutions of high density. Clear fluids comprising solutions of zinc bromide/calcium bromide and possibly calcium chloride with densities of 1.8-2.4 kg/l are described in US patents 4292183 and 3304677, although such solutions are very satisfactory materials for. application when completing, sealing and perforating oil and gas wells, these solutions have so far not been applicable if the temperatures down in the hole are higher than approx. 149°C

due to the high rate of corrosion to which iron and steel are exposed under such conditions. During high-temperature be-

conditions, it has thus far only been possible to use less desirable fluids, such as modified drilling mud etc. At high temperatures, however, the solids in the drilling mud are deposited towards the bottom of the wellbore and solidify, making restoration of the well far more expensive and difficult to carry out .

The zinc ion in a .2.3 kg/l clear fluid can only be held

i. dissolution by maintaining a pH within the range 1.0-1.3, and normally this is done by adding an excess of hydrobromic acid. Although pH values within this range mean that the weighted salt obtains the desired solubility, the resulting fluid is highly corrosive. Even when a 2.3 kg/l zinc bromide/calcium bromide solution is mixed with a calcium bromide/calcium chloride solution to produce a lower density final fluid, the pH stays within the strongly acidic range and never rises above approx. 5.5. The resulting fluids thus remain strongly acidic and strongly corrosive at temperatures above 14 9°C.

US patent 4292183 describes the use of a film-forming, amide-based corrosion inhibitor. The corrosion inhibition system described in this patent document is, however, applicable at temperatures up to approx. 149°C and cannot be used at the higher temperatures with which the present invention is concerned. Within the state of the art, several corrosion-inhibiting systems have been proposed for use in strongly acidic environments. However, only a few have proven satisfactory, and even fewer have been used commercially. Thus, arsenic and/or arsenic compounds have been proposed to provide corrosion protection in strongly acidic solutions used to acidify new wells and producing wells. However, the toxicity of arsenic compounds to humans and the "poisoning" of the catalysts used in refineries have made arsenic compounds undesirable.

A strongly acidic inhibitor is described in US patent 3077454. The inhibitor contains an organic ketone, an aliphatic aldehyde and a fatty acid. It has been suggested that protection up to a temperature of 177°C can be achieved, but only for a shorter time (ie no longer than 16 hours).

A similar material is described in US patent 3634270. A synergistic mixture of organic nitrogen-

and sulfur compounds which prevent the corrosive component in the solution from attacking metal are required. Application in connection with the cleaning of industrial cages and heat exchangers is suggested. Sulfur compounds from the group thi.ourea, allylthiourea, sodium mercaptobenzothiazole, mercaptothiazoline, sodium thiocyanate or mixtures are indicated to be particularly effective. The described working area for the inhibitor systems, however, includes low temperatures not exceeding 14 9°C and short application times not longer; than 16 hours.

In the literature there are references to the inhibition of acid corrosion of steel by means of thioura and its derivatives. T\P. Hoar and R.D. Holliday in J. Appl. Chem. 3 (11): 502-

13 (1953), B. Donnelly, T.C. Downie, G. Grzeskowiak,

HR Hamburg and D. Short in Corrosion Science, 14;597-606

(1974), E. Jackson and M.J. Wilkinson in British Corrosion J., 11:208-11 (1976), and M.B. Lawson in Corrosion, 36:493-7

(1980), are representative examples of the extensive literature.

In US patent 4100100 the use of a combination of a quaternary pyridinium salt and an organic thiamide or water-soluble thiocyanate to reduce the corrosion of

iron or steel due to an aqueous acid gas conditioning solution described. The same mixture is described in British patent document 2027686 as a corrosion inhibitor for aqueous salt solutions in a wellbore.

In US Patent 4100100 and British Patent 2027686, it is reported that the addition of a small amount of a water-soluble cobalt salt to the inhibitor combination improves its effectiveness. The sulfur compound in the mixture is preferably made up of ammonium thiocyanate or thiourea. The corrosion tests described in the patent documents apply to moderately high temperatures (i.e. 177°C) and short times (i.e. no longer than 118 hours).

A corrosion inhibitor mixture which is suitable for use in calcium chloride and sodium chloride solutions is described in US patent 3215637. The mixture consists of sodium silicate, zinc chloride and sodium chromate. A mixture of inhibitors is necessary because no single material has proven effective in preventing general and local corrosion. The effectiveness of the inhibitor material was assessed over a longer period of time, i.e. 10 days, but at a low temperature (20°C).

A method for inhibiting corrosion during well drilling operations is described in US patent 4250042. The aqueous drilling fluid is treated with at least one water-soluble ammonium carboxylate salt. The described corrosion tests are carried out in an atmosphere of air and oxygen for 20 hours at 85°C.

The short-term low-temperature protection of the type described above is of little value when completing or restoring oil wells or as a sealing fluid.

In Japanese patent document 7503741 is an oxide film remover

and corrosion inhibitor for copper and copper alloys described comprising 2-20% by volume of an aqueous solution of sodium or ammonium thioglycolate containing bezotriazole, sodium mercaptobenzothiazole and/or imidazole derivatives.l Japanese patent document 7692735 is metal cleaners described for cleaning air conditioning lines or internal combustion radiators in shorter time without the pipelines corroding, by using water-soluble thioglycolate, halogenated alkyl metal or halogenated ammonium thioglycolate. In Japanese patent document 79120007, mixtures of coloring agents, wetting agents, amino acid derivatives and ammonium thioglycolate are described as jet printing inks with good storability and corrosion inhibition.

Thus, within the state of the art, a suitable corrosion-inhibiting system for high-density fluids has not been described which makes it possible to use these under working conditions with high temperatures down the hole.

It is therefore mainly aimed at the invention

to provide a corrosion-inhibiting system for zinc bromide/calcium bromide solutions that possibly contain calcium chloride, to make it possible to use these in oil and

gas wells as well operating fluids at temperatures as high as 204°C.

The invention also aims to provide a mixture of the type described above which can be used with good results in connection with solutions with densities within the range 1/8-2.4 kg/l.

It is a further aim of the invention to provide corrosion protection in such systems for longer periods of up to 90 days or longer.

It is also an aim of the invention to provide a corrosion inhibitor which is sufficiently soluble in high density fluids that it can be supplied in the form of a basic solution which can be mixed to lower densities for applications in the field.

Summary of the invention

The above-described and other objectives, advantages and special features of the invention can be achieved by a corrosion-inhibited, clear agent with a high density that can be used for a longer time as a well operating fluid at an elevated temperature, and the agent is characterized by the fact that it comprises a solution of zinc bromide and calcium bromide and possibly calcium chloride in water and has a density of 1.8-2.4 kg/l, and a corrosion-inhibiting amount of a material from the group ammonium thioglycolate, calcium thioglycolate, thioglycerol and mixtures thereof.

Preferably, the corrosion inhibitor is present in a concentration of 0.01-5.00% by weight of the clear solution.

The invention also provides new methods for producing such corrosion-inhibiting, clear solutions, and the invention also relates to their use for well servicing operations.

Description of the commonly used forms of execution

As noted, professionals have long sought, but without success, to arrive at a corrosion-inhibited zinc bromide/calcium bromide solution that will be able to be used in such

high working temperatures down the hole such as 204°C. By

using a corrosion inhibitor in the agent according to the invention, it is possible to obtain high-density aqueous salt solutions for use as completion, restoration or sealing fluids in deep oil, gas or geothermal wells at high pressure and high temperature. The aims of the invention are achieved by using a corrosion inhibitor which is completely soluble in aqueous solutions of zinc bromide, calcium bromide and calcium chloride within the density range 1.8-2.4 kg/l.

In addition to having the desired solubility, the corrosion inhibitor must be able to provide substantial corrosion inhibition at temperatures up to and including 204°C. The corrosion protection must be maintained for a longer period of at least 30 days, and preferably for 90 days or longer. It is also important that the corrosion inhibition reduces pitting corrosion to a minimum, so that the small corrosion that occurs is general rather than highly specific. Moreover, the corrosion inhibitor must be compatible with other chemical additives that are commonly used together with high-density aqueous salt solutions, such as viscosity-regulating agents, suspending agents or antifoaming agents. According to the invention, it has been shown that ammonium thioglycolate, calcium thioglycolate, thioglycerol or mixtures thereof make it possible to achieve these goals when used in corrosion-inhibiting amounts. According to the present invention, corrosion-inhibited, clear fluids can not only be used with satisfactory results under well operating conditions, but it has also been shown that they can be used for a relatively long time of up to 90 days or longer.

The production of the zinc bromide/calcium bromide solution, which may possibly contain calcium chloride, as such does not form part of the present invention. Satisfactory production methods and mixing methods for such solutions with densities within the range 1.8-2.4 kg/l are described in US patents 4292183 and 4304677.

Such high-density fluids are typically manufactured out of

from. a base fluid with relatively high density (e.g.

a zinc bromide/calcium bromide solution with a density of

approx. 2.3 kg/l and containing approx. 54.7% by weight zinc bromide and approx. 20.6 wt% calcium bromide). Such a stock solution can then be mixed with a calcium bromide solution or a calcium bromide and calcium chloride solution in the field to

achieve the desired density required for a given application.

The corrosion inhibitor used according to the invention can be added to the resulting clear fluid at essentially any stage during the manufacturing cycle. It can thus be present in the basic solution and in one or more of the auxiliary solutions of calcium bromide or calcium bromide and calcium chloride, or the corrosion inhibitor can be added separately to the mixed solution at the scene of the fire.

However, the corrosion inhibitor is preferably present in the basic solution (eg 2.3 kg/l zinc bromide/calcium bromide solution) in a sufficient quantity to give the desired final concentration under all practical conditions of use. A sufficient amount of corrosion inhibitor is thus added so that sufficient protection will be ensured when the basic solution with a density of 2.3 kg/l is mixed to a final density of approx.1»8" kg/l*

The solution to be used for a well maintenance operation should generally contain 0.01-5.0% of one or more of the corrosion inhibitors according to the invention, preferably 0.1-3.0%.

When the corrosion inhibitors are present in the basic solution with a high density (e.g.<2>/<3><kg/l>zinc bromide/calcium bromide solution), the amount of corrosion inhibitor used is preferably 0.1-3.0% by weight of the basic resolution.

As stated, the desired degree of corrosion inhibition can be achieved according to the invention by using ammonium thioglycolate, calcium thioglycolate, thioglycerol or mixtures of these agents. The exact amount of each agent to be used depends on the specific material being used." Thus, when ammonium thioglycolate is used, the preferred range is 0.1-1.0% by weight of the base solution, for calcium thioglycolate the preferred range is 0.3- 3.0% by weight

of the stock solution, and for thioglycerol it is desired

amount 0.1-0.5% by weight of the basic solution,

According to the invention, it is particularly preferred to use ammonium thioglycolate as a corrosion inhibitor.

The corrosion inhibitors used according to the invention can be added in any suitable form. Calcium thioglycolate can be supplied in solid form or as a solution. Thioglycerol is a liquid at room temperature and is normally used in this form. Ammonium thioglycolate is preferably used in the form of a 60% aqueous solution. The indicated amounts of inhibitor refer to amounts of active agent.

Examples of corrosion-inhibited fluids with high density are described in the examples below.

Example 1

A 2.3 kg/l zinc bromide/calcium bromide solution was prepared as follows: The pH of an uninhibited 2.3 kg/l zinc bromide/calcium bromide stock solution was regulated by using a slurry of zinc oxide, calcium bromide and water or by using 54% hydrogen bromide to 1.2-0.1. The density was adjusted to 2.3 io.006 kg/l with 1.7 kg/l calcium bromide. The solution was filtered through a medium sintered glass funnel coated with filter aid. The filtered density- and pH-regulated fluid was heated to 95-5°C. The desired amount of corrosion inhibitor was added to the hot fluid. After stirring for 30 minutes, the solution was hot filtered through a medium sintered glass funnel coated with a filter aid. The solution was cooled to room temperature.

Example 2

A 2.16 kg/l uninhibited zinc bromide/calcium bromide stock solution was prepared by dissolving 102.0 kg zinc bromide and 138.8 kg calcium bromide in 102.5 kg water. This gave one barrel (115.6 L) of 2.16 kg/l zinc bromide/calcium bromide fluid. The pH was regulated with a slurry of zinc oxide, calcium oxide and water or with 54% hydrogen bromide to 2.3-0.1. The density was regulated to 2.16^0.006 kg/l with 1.7 kg/l of calcium bromide or with solid zinc bromide. The solution was filtered through a medium sintered glass funnel coated with filter aid. The filtered, density-regulated fluid was heated to 95-5°C. The desired amount of corrosion inhibitor was added to the hot fluid. After stirring for 30 minutes, the solution was hot filtered through a medium sintered glass funnel coated with a filter aid. The solution was cooled to room temperature.

Example 3

A 1.74 kg/l uninhibited zinc bromide/calcium bromide stock solution was prepared by dissolving 13.3 kg zinc bromide and 135.6 kg calcium bromide in 127.3 kg water. This produced a barrel of 1.74 kg/l • zinc bromide/calcium bromide stock solution. Its pH was adjusted with a slurry of zinc oxide, calcium oxide and water to 6.0-0.1. Its density was adjusted to 1.74-0.006 kg/l with 1.7 kg/l calcium bromide or solid zinc bromide. The solution was filtered through a medium sintered glass funnel coated with filter aid. The filtered, density-regulated fluid was heated to 95-5°C. The desired amount of corrosion inhibitor was added to the hot fluid. After stirring for 30 minutes, the solution was hot filtered through a medium sintered glass funnel coated with a filter aid. The solution was cooled to room temperature.

Example 4

A 2.3 kg/l zinc bromide/calcium bromide solution was prepared containing 1.0 wt% ammonium thioglycolate by the method of Example 1. One barrel of a 2.1 kg/l mixture of uninhibited 2.3 kg/l zinc bromide/calcium bromide was prepared by to mix 0.66 barrels (241.4 kg) of inhibited 2.3 kg/l zinc bromide, calcium bromide and 0.34 barrels (92.0 kg) of 1.7 kg/l calcium bromide. After stirring for 30 minutes, the fluid was filtered through a medium sintered glass funnel coated with a filtration aid.

Example 5

A 2.3 kg/L zinc bromide/calcium bromide solution was prepared containing 1.0% by weight calcium thioglycolate by the method of Example 1. One barrel of 1.86 kg/L zinc bromide/calcium bromide, calcium chloride solution was prepared by mixing 0.119 barrels (43.5 kg) inhibited 2.3 kg/l< zinc bromide/calcium bromide solution with 0.881 barrel (251.7 kg) 1.8 kg/l calcium bromide, calcium chloride solution. After stirring for 30 minutes, the fluid was filtered through a medium sintered glass funnel coated with a filter aid.

Example 6

A 2.3 kg/l zinc bromide/calcium bromide solution was prepared containing 2.0 wt% thioglycerol by the method of Example 1. One barrel of 1.98 kg/L zinc bromide/calcium bromide was prepared by mixing 0.46 barrels (168, 2 kg) inhibited 2.3 kg/l zinc bromide/calcium bromide solution with 0.54 barrels (14 6.1 kg) of 1.7 kg/l calcium bromide solution. After stirring for 30 minutes, the fluid was filtered through a medium sintered glass funnel coated with a filter aid.

Example 7

A 2.3 kg/l. zinc bromide/calcium bromide solution was prepared containing 0.5% by weight anrnium thy.bg lycolate and 0.5% by weight thioglycerol using the method of Example 1. One drum of 2.22 kg/l zinc bromide/calcium bromide/calcium chloride fluid was prepared by mixing 0.833 barrels (304.7 kg) of inhibited 2.3 kg/l zinc bromide, calcium bromide and 0.167 barrels (47.7 kg) of 1.8 kg/l calcium bromide/calcium chloride solution. After stirring for 30 minutes, the fluid was filtered through a medium sintered glass funnel coated with a filter aid.

The effectiveness of the corrosion inhibitors used according to the invention has been demonstrated experimentally using the following test method. The density of a pH-regulated, unrestrained base fluid was first determined. The solution was then heated to 95<i>5°C, and the desired amount of corrosion inhibitor was added to the hot, uninhibited base fluid. The solution was hot filtered through a medium sintered glass funnel coated with filtration aid. and the solution was allowed to cool to room temperature. The base fluid was then mixed to the desired density in accordance with mixing tables, and the mixed fluid was filtered through a medium sintered glass funnel coated with filter aid. The density of the fluid at 15.6°C was determined, and the pH of the fluid was measured using a pH meter.

The test fluid was then added to a high temperature aging cell so that a test chip was covered. The test cells were pressurized with nitrogen and placed in an oven for the desired time and at the desired temperature. At the end of the test, the test cells were removed from the eye and allowed to cool for at least 2 hours. The pieces were weighed before and after the test, and the weight loss is an indication of corrosion.

The corrosion rate in yum per year was calculated using

where AV = weight loss in mg

MD = the metal density in g/cm<3>

OA = surface area of the piece in cm<2>

T = time in hours

To obtain baseline data, the corrosion rate for mild steel after 7 days of exposure to mixtures of uninhibited zinc bromide and calcium bromide solutions with densities within the range 1.86 kg/l up to 2.22 kg/l was determined and is reproduced in Table I. These data show that corrosion rates vary from a relatively low rate at low temperatures and densities to extremely high rates at high temperatures and densities.

Corrosion rates after 7 days were obtained at 204°C using a 2.1 kg/l zinc bromide/calcium bromide mixture (3 9.6% zinc bromide, 29.5% calcium bromide) containing 0.3% by weight of the inhibitors in the mixtures according to the invention. These data are reproduced in table II.

The corrosion rates after 7 days for mild steel were measured using different solution mixtures containing 0.6% ammonium thioglycolate and .1.0% calcium thioglycolate, and the results are reproduced in Tables III and IV. These data show significantly reduced corrosion rates compared to the use of the uninhibited solutions used according to Table I.

Thirty-day corrosion tests give even better results. 1 Tables V and VI are the corrosion rate after 30 days for, respectively, mixtures of 2.3 kg/l zinc bromide and calcium bromide (containing respectively 0.6<y>ect% ammonium thioglycolate and calcium thioglycolate) with 1.7 kg/l calcium bromide was reproduced. In all cases, corrosion rates of less than 254 µm per year were observed when using solutions with densities below approximately 2.16 kg/l. The corrosion rate of 254 µm per .year on the basis of a test period of 30 days is generally accepted as an industry standard for use in determining the availability of corrosion inhibition.

Even if the corrosion levels after a standard test time of 38 days at a concentration of 2.22 kg/l are slightly above the desired level, they are nevertheless a significant improvement compared to the use of the unrestricted solution (see Table I). In addition, high density solutions from a concentration of 2.22 kg/l and upwards are typically mixed into lower density ranges in actual use.

The corrosion inhibitors used according to the invention

Thus works effectively under practically all commercially applicable conditions.

Table VII shows corrosion rates for mild steel after 7 days and 30 days using different high-density fluid mixtures with different densities between 2.16 and 2.22 kg/l, and the results show the rapid increase in corrosion that occurs when the densities increase to over 2.16 kg/l.

The effect of changing the inhibitor concentration can be seen from the data reproduced in Table VIII which show corrosion rates for mild steel after 30 days using mixtures with a density of 2.1 kg/l inhibited with ammonium thioglycolate, calcium thioglycolate and thio-. glycerol.

This data shows the preferred and applicable areas

for the inhibitors used according to the invention.

When the corrosion inhibited compounds according to the invention are used, it is possible to achieve very low corrosion during longer operating times. In table IX, corrosion data for mild steel after 90 days are given using mixtures of 2.3 kg/l zinc bromide/calcium bromide (containing ammonium thioglycolate) with 1.7 kg/l calcium bromide at four densities within the range 1.86-2.22 kg/l at 204°C. These data show that at densities below 2.16 kg/l, extremely low corrosion rates are obtained. Only at densities above 2.16 kg/l does the corrosion rate increase to above the set limit value of 254 µm per year. Although reduced protection is obtained at densities o<y>er 2.16 kg/l, a significant improvement is obtained compared to uninhibited fluids with a density of 2.28 kg/l.

Use of the mixture according to the invention for operating well boreholes is reproduced in the example below.

Example 8

When a zinc bromide/calcium bromide fluid is used as sealing fluid in an oil well, this aims to reduce the pressure drop across the packing in the wellbore to a minimum. The packing serves to isolate the hydrocarbon-producing section from the rest of the wellbore. The advantage of using a clear fluid as sealing fluid is that a minimum of solids are introduced into the well space. Solids will settle to the bottom over time and build up

on the gasket and make it difficult to remove the gasket. Cleaning the wellbore and all surface equipment before the solids-free fluid is filled in the wellbore is therefore important. All equipment, pumps, lines and storage tanks must be clean. The borehole must be scraped and flushed with water. A fluid density is chosen which satisfies the requirements of the well (in a number of cases a fluid density is used which gives a hydrostatic pressure in the wellbore at the packing level of 14.1 kg/cm above the formation pressure). The fluid is prepared by mixing the base fluids (which preferably include a 2.3 kg/l zinc bromide/calcium bromide fluid and a 1.8 kg/l calcium bromide/calcium chloride or a 1.7 kg/l calcium bromide fluid) containing corrosion inhibitors to obtain the mixture according to the invention, and these are filled in suitable storage tanks at the drilling rig site. At the right time, the fluid in the well space is filled by displacement of the fluid in the well space. The fluid will remain in the well until auxiliary work is necessary to stimulate oil production from the well.

The invention provides clear fluids with high density that can be safely and effectively used as means for completing, sealing and perforating wells. They do not corrode equipment and personnel, remain stable and can be satisfactorily used for a longer period of time. The solution should remedy a long-felt need within the well operation area for clear solutions with high densities that are capable of being used at high working temperatures in the wellbore.

Claims (10)

1. Corrosion inhibited,. clear mixtures with high density and able to be used for a long time as well operating fluid at elevated temperature, characterized in that they comprise a solution of zinc bromide and calcium bromide in water with a density of 1.8^-2.4 kg/litre and a corrosion-inhibiting amount of ammonium thioglycolate, calcium thioglycolate, thioglycerol or mixtures thereof. 2. Mixture according to claim 1, characterized in that the solution also comprises calcium chloride. 3. Mixture according to claim 1 or 2, characterized in that the solution comprises 0.01-5.0% by weight of the corrosion-inhibiting component, based on the weight of the solution. 4. Mixture according to claims 1-3, characterized in that the corrosion-inhibiting component is ammonium thioglycolate. 5. Procedure for operating a well borehole, where a high density fluid comprising a solution of zinc bromide and calcium bromide in water and with a density of 1.8-2.4 kg/litre is injected into the well to exert a sufficient hydrostatic pressure to control the well, characterized in that a corrosion-inhibiting quantity of ammonium thioglycolate, calcium thioglycolate, thioglycerol or mixtures thereof is introduced into the high-density fluid. 6. Method according to claim 5, characterized in that a solution is used which also contains calcium chloride. 7. Method according to claim 5 or 6, characterized in that a solution is used which contains 0.01-5.0% by weight of the corrosion-inhibiting component, based on the weight of the solution. 8. Method according to claims 5-7, characterized by that as the corrosion inhibitory component ammonium thioglycolate is used. 9. Method for inhibiting the corrosion potential at high temperature for a clear high density solution of zinc bromide and calcium bromide in water with a density of 1.8-2.4 kg/litre, characterized in that a corrosion-inhibiting amount of ammonium thioglycolate, calcium thioglycolate, thioglycerol or mixtures is introduced into the solution hence. 10. Method according to claim 9, characterized in that the solution also contains calcium chloride, that the solution contains 0.01-5.0% by weight of the corrosion-inhibiting component, based on the weight of the solution, and that the corrosion-inhibiting component consists of ammonium thioglycolate.