The present invention relates to corrosion inhibition in oil and gas wells. In particular, the present invention relates to a well completion fluid with reduced corrosivity and a process for inhibiting corrosion of carbon steel tubing and casing of high temperature, high pressure oil and gas wells.
Cross-Reference to related applications
[0002] This application is a patent of addition of Indian application Ser. No. 201611011507, filed on March 31, 2016, the disclosure of which is incorporated herein by reference. The application is an improvement of the invention claimed in the specification of the Indian application No. 201611011507.
Background of the invention
[0003] Oil and gas are produced from underground reservoirs in the Earth. A well is bored by drilling a hole up to the target depth and stabilized by placing steel tubes and casing in a telescopic design. Further, drilling is carried out using drilling fluid to replace rock cuttings and stabilizing the drilled hole. Once the target depth is attained, all the drilling fluid is removed and replaced with well completion fluid. The well completion fluids are prepared based on the requirements of the completion such as temperature, pressure and surrounding formations. Further, the well completion fluid is left in the annular region of the oil and gas well between tubing and casing above a packer. The well tubing and casing made up of carbon steel are in constant contact with the well completion fluid and are therefore susceptible to corrosion which may damage the well and lead to loss of oil and gas. Also, damage to casing leads to contamination

of water reservoir and poses a hazard due to gas leak at well head. Therefore, it is important to ensure that well completion fluids meet all the requirements to fulfil their primary functions such as providing pressure control, preventing formation fluid from entering the well bore, maintaining hole stability, minimizing damage of production zone, minimizing corrosion of down hole metals and providing carrying capacity for debris, cuttings and loose sand. Further, the well completion fluids should be free of solids, sufficiently dense to control the reservoir pressure, resistant to viscosity changes over long periods of time and noncorrosive to the well bore and completion components in order to perform the abovementioned functions effectively.
[0004] Conventionally, well completion fluids are prepared by dissolving one or more salts such as zinc bromide and calcium bromide in water, based on the underground reservoir pressure. However, the abovementioned well completion fluids suffer from various disadvantages. For example, single-salt brines have density limitations. Further, brines containing zinc bromide salt have the highest corrosion rates because pH of these brines is low. The above limitation is overcome by using combination of two and more salts to provide adequate density in order to adhere to hydrostatic pressure requirements. However, it has been observed that the brines made using a combination of salts manifests acidic environment. Also, during the preparation of the completion fluids, oxygen from the atmosphere gets dissolved in the brines which increases corrosivity. Corrosivity further increases with the increase in bottom hole temperature, bromide content and salt content.
[0005] In light of the above-mentioned disadvantages, there is a need for a well completion fluid with reduced corrosivity and a process for preparing the same. Further, there is a need for a process that eliminates dissolved oxygen from the well completion

fluids. Furthermore, there is a need for a process which facilitates buffering high density well completion fluids which are highly acidic in nature. In addition, there is a need for well completion fluids that are sufficiently dense and have adequate specific gravity.
Summary of the invention
[0006] In various embodiments of the present invention a process for preparing a well completion fluid having reduced corrosivity for oil and gas wells is provided. The process comprises the steps of adding a pH moderating agent to a predetermined quantity of technical water. Further, the process comprises adding an oxygen scavenger to the solution of technical water and pH moderator. Furthermore, the process comprises adding a predetermined quantity of one or more bromide salts to the solution prepared at previous step, such that the specific gravity of the solution is greater than 2.00 and equal to or less than 2.30. Finally the process comprises adding an acid corrosion inhibitor to the solution.
[0007] In various embodiments of the present invention, a well completion fluid exhibiting reduced corrosion is provided. The well completion fluid comprises technical water as a base fluid, a pH moderating agent, an oxygen scavenger, a combination of bromide salts, and an Acid Corrosion Inhibitor (ACI), wherein quantity of ACI is 2.5%-3% (V/V), further wherein the specific gravity of the well completion fluid is greater than 2.00 and equal to or less than 2.30.
Detailed description of the invention
[0008] The present invention provides for a well completion fluid with reduced corrosivity and a process of preparing the same for inhibiting corrosion in oil and gas wells. In particular, the present invention provides for well completion fluids exhibiting surprisingly low corrosivity, and enhanced stability and higher pressure tolerance at a

specific gravity greater than 2.0 and less than or equal to 2.3. The higher specific gravity supports hydrostatic pressure requirements. The present invention provides for elimination of dissolved oxygen from the well completion fluid thereby making the well completion fluid less corrosive. Further, the invention provides for moderating pH of the well completion fluids thereby making the well completion fluid less acidic.
[0009] The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Exemplary embodiments are provided only for illustrative purposes and various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
[0010] The present invention would now be discussed in context of following embodiments.
[0011] In accordance with various embodiments of the present invention, a well completion fluid for high temperature and high pressure (HTHP) oil and gas wells is provided. The well completion fluid comprises technical water as a base fluid, a pH moderating agent, an oxygen scavenger, a combination of bromide salts and an Acid Corrosion Inhibitor (hereinafter referred as ACI). In an embodiment of the present

invention, the pH moderating agent is sodium bicarbonate (NaHCCh). In an exemplary embodiment of the present invention, the well completion fluid comprises 1.5%-l .75% (W/V) sodium bicarbonate. In a preferred embodiment of the present invention, the well completion fluid comprises 1.5% (W/V) sodium bicarbonate.
[0012] In an embodiment of the present invention, the well completion fluid comprises sodium sulphite as the oxygen scavenger. In an embodiment of the present invention, the well completion fluid comprises 60-65 (ppm parts per million) (W/V) of sodium sulphite. In a preferred embodiment of the present invention, the well completion fluid comprises 63 (ppm parts per million) (W/V) of sodium sulphite.
[0013] In an embodiment of the present invention, the combination of bromide salts comprises calcium bromide and zinc bromide. In an embodiment of the present invention, the quantity of calcium bromide and zinc bromide is selected such that the specific gravity of the completion fluid is greater than 2.00 and equal to or less than 2.30. In a preferred embodiment of the present invention, the specific gravity of the completion fluid is 2.30.
[0014] In an embodiment of the present invention, the completion fluid comprises 2.5%-3% ACI (V/V). In a preferred embodiment of the present invention, the completion fluid comprises 3% ACI (V/V). In a preferred embodiment of the present invention, the liquid selected as the acid corrosion inhibitor is a free flowing and homogenous organic liquid at a temperature ranging between 22°C - 26°C. Further, the material is free from visible impurities and provides negative result for the presence of Arsenic. Furthermore, the pour point temperature of the selected liquid is less than or equal to 12°C. Finally, the corrosion result for the selected liquid is less than or equal to 0.024g/cm2 using the following test parameters Test Temperature: 160°C; Test

Pressure: 1000 psi; Condition of Test: Static; Duration of Test: 6 hours; Acid Volume/Test Coupon Surface Area: 75 ml/sq. inch; Type of Steel: N-80; Type of Acid: Hydrochloric (15% w/w); Concentration of ACI: 30 ml/1.
[0015] Advantageously, in accordance with various embodiments of the present invention, the well completion fluid exhibits high density and specific gravity with reduced corrosivity. High density and high specific gravity of the completion fluid provides stability and supports high hydrostatic pressure. Further, the completion fluid of the present invention exhibits an unexpected result of 99.41% efficiency as shown in an exemplary experiment later in the specification. Further, the well completion fluid of the present invention does not form any deposits on application in the HTHP oil and gas wells which is a significant advantage over existing fluids.
[0016] In accordance with various embodiments of the present invention, a process for preparing a well completion fluid exhibiting reduced corrosivity for inhibiting corrosion in HTHP oil and gas well is provided. The process comprises the steps as explained below.
[0017] Firstly, a predetermined quantity of technical water (hereinafter referred to as water) is used as a base fluid. Subsequently a pH moderating agent is added to the water. In a preferred embodiment of the present invention, the pH moderating agent is Sodium bicarbonate (NaHCCb). In an exemplary embodiment of the present invention, 1.5%-1.75% (W/V) sodium bicarbonate is added to water. In a preferred embodiment of the present invention, 1.5% (W/V) sodium bicarbonate is added to water to raise the pH of the solution. In an embodiment of the present invention, any other suitable pH moderating agent may be used to raise the pH of the solution.

[0018] Subsequent to dissolution of sodium bicarbonate in water, an oxygen scavenger is added to the prepared solution. In an embodiment of the present invention, sodium sulphite is added as the oxygen scavenger to the prepared solution to eliminate dissolved oxygen. In an embodiment of the present invention, 60-65 ppm of sodium sulphite is added to the solution comprising water and sodium bicarbonate. In a preferred embodiment of the present invention, 63 ppm of sodium sulphite is added to the solution comprising water and sodium bicarbonate.
[0019] Further, the above solution is blended with one or more bromide salts to achieve high density and high specific gravity greater than 2.00 and equal to or less than 2.30. In an embodiment of the present invention, the one or more bromide salts include calcium bromide and zinc bromide. In a preferred embodiment of the present invention, a combination of bromide salts is added to the solution to achieve a specific gravity greater than 2.00 and equal to or less than 2.30. In a preferred embodiment of the present invention, a predetermined quantity of calcium chloride is added to the solution to achieve a specific gravity of 1.85. Subsequently a predetermined quantity of zinc bromide is added to raise the specific gravity of the solution up to 2.30. The specific gravity of the solution is achieved based on the predetermined quantity of the bromide salts.
[0020] Finally, a well completion fluid is formed by adding an Acid Corrosion Inhibitor (ACI) to the above solution. In a preferred embodiment of the present invention, 2.5%-3% (V/V) ACI is added to the above prepared salt solution. In a preferred embodiment of the present invention, 3% (V/V) ACI is added to the above prepared salt solution.
[0021] In a preferred embodiment of the present invention, the liquid selected as the acid corrosion inhibitor is a free flowing and homogenous organic liquid at a

temperature ranging between 22°C - 26°C. Further, the selected liquid is free from visible impurities and provides negative result for the presence of arsenic. Furthermore, the pour point temperature of the selected liquid is less than or equal to 12°C. Finally, the corrosion result for the selected liquid is less than or equal to 0.024g/cm2 using the following test parameters: Test Temperature: 160°C; Test Pressure: lOOOpsi; Condition of Test: Static; Duration of Test: 6 hours; Acid Volume/Test Coupon Surface Area: 75 ml/sq. inch; Type of Steel: N-80; Type of Acid: Hydrochloric (15% w/w); Concentration of ACI: 30 ml/1. It should be noted that the quantity and nature of ACI cannot be extrapolated to any other combination of salts without laboratory evaluation.
[0022] In an exemplary embodiment of the present invention, an HTHP corrosion cell made up of Hastelloy C276 is used for conducting experiments on the completion fluid of the present invention. The HTHP corrosion cell made up of Hastelloy C276 is sustainable to highly corrosive environment with acidic pH.
[0023] Effective corrosion inhibition abilities of the well completion fluid of the present invention were evaluated by conducting various tests in an environment simulating high temperature and high pressure oil and gas wells. A PARR 4571 HPHT Autoclave reactor was selected and the test temperature was set to 160°C for simulating High Temperature High Pressure (HTHP) oil and gas wells.
[0024] Steel coupons were fabricated from API 5 CT L-80 steel casing pipe material. Further, the coupons were ground wetted to achieve a surface finish of 400 grit. The coupons were then degreased with xylene, washed with distilled water and rinsed with acetone. Furthermore, the coupons were dried in dry hot air, and surface area and initial weight of the coupons was determined.

[0025] Subsequently, the coupons were immersed in PARR 4571 HPHT Autoclave reactor filled with the well completion fluid prepared in accordance with the process of the present invention. The immersed coupons were exposed to simulated test conditions at 160°C. After completion of exposure tests, retrieved coupons were washed with water, rubbed against filter paper, rinsed in Clark's solution, followed by thorough washing in water. Yet further, the coupons were washed in distilled water, rinsed in xylene and acetone and dried in dry hot air. The coupons were weighed to determine the weight loss in the coupons. Finally, the corrosion rate for the coupons in millimetre per year (mm/yr) and milli inch per year (mpy) was calculated based on the determined weight loss using the formula below.
[0026] Corrosion Rate in milli inch per year (mpy) = (3.45 x 106 x W) / (A x T x D) wherein, W is Coupon Weight Loss in gram; A is Surface area of coupon in cm2; T is Exposure time in hours; D is Density of carbon steel in g/cm3. Further, Corrosion rate in millimetre per year (mm/y) = 0.0254 x Corrosion rate in milli inch per year (mpy).
[0027] The above test was repeated by preparing multiple completion fluids (hereinafter refered to as brines) having respective compositions and effectiveness of corrosion inhibition process was determined. In particular, the quantity of calcium bromide and zinc bromide was varied. Effectiveness of corrosion inhibition process was determined by calculating percentage inhibition using the formula below:
[0028] Inhibitor Efficiency (% )= (A - B) x 100 / A, wherein A = Corrosion rate for blank; B = Corrosion rate with Inhibitor
[0029] The results of the above described tests conducted on API 5CT L-80 steel coupon exposed to completion fluids having specific gravity upto 2.30 and comprising

different combinations of Calcium Bromide and Zinc Bromide are exemplified in Table 1 below:

S.No Brine Corrosion Rate mpy Efficiency %
1 Brine with specific gravity 2.30 comprising Zinc Bromide alone 14040.07
2 Brine with specific gravity 2.30 comprising a predetermined quantity of Calcium Bromide to raise the specific gravity to 1.50 and a predetermined quantity of Zinc Bromide to raise the specific gravity to 2.30 2556.30
3 Brine with specific gravity 2.30 comprising 1.5% NaHCCh + 63 ppm of Sodium Sulphite + a predetermined quantity of Calcium Bromide to raise the specific gravity to 1.50 + a predetermined quantity of Zinc Bromide to raise the specific gravity to 2.30 +3%ACI (v/v) 70.79 97.23%
4 Brine with specific gravity 2.30 comprising a predetermined quantity of Calcium Bromide to raise the specific gravity to 1.70 and a predetermined quantity of Zinc Bromide to raise the specific gravity to 2.30 2159.26
5 Brine with specific gravity 2.30 comprising 1.5% NaHCCb + 63 ppm of Sodium Sulphite + a predetermined quantity of Calcium Bromide to raise the specific gravity to 1.70 + a predetermined quantity of Zinc Bromide 7.08 99.67%

to raise the specific gravity to 2.30 +3%ACI (v/v)
6 Brine with specific gravity 2.30 comprising 1.75% NaHCC>3 + 63 ppm of Sodium Sulphite + a predetermined quantity of Calcium Bromide to raise the specific gravity to 1.70 + a predetermined quantity of Zinc Bromide to raise the specific gravity to 2.30 +3%ACI (v/v) 10.61 99.50%
7 Brine with specific gravity 2.30 comprising 1.75% NaHCCb + 63 ppm of Sodium Sulphite + a predetermined quantity of Calcium Bromide to raise the specific gravity to 1.70 + a predetermined quantity of Zinc Bromide to raise the specific gravity to 2.30 +2.5%ACI (v/v) 13.26 99.38%
8 Brine with specific gravity 2.30 comprising a predetermined quantity of Calcium Bromide to raise the specific gravity to 1.85 and a predetermined quantity of Zinc Bromide to raise the specific gravity to 2.30 416.50
9 Brine with specific gravity 2.30 comprising 1.5% NaHCCb + 63 ppm of Sodium Sulphite + a predetermined quantity of Calcium Bromide to raise the specific gravity to 1.85 + a predetermined quantity of Zinc Bromide to raise the specific gravity to 2.30 +3%ACI (v/v) 2.43 99.41%

[0030] As illustrated in Table 1 above, it was observed that the corrosion rate for the completion fluid prepared in accordance with present invention exhibited a surprisingly minimum value of around 2.43 mpy. Further, an unexpected result of an efficiency percentage of a maximum of around 99.41% was achieved. Further, the completion fluid prepared in accordance with the present invention exhibited a surprising effect of enhanced stability and high tolerance at high pressure.
The process for preparing completion fluid exhibiting reduced corrosivity is most effective when the various constituents of the well completion fluid are added in the above mentioned sequence with a preferred quantity of each constituent. Further, effective corrosion inhibition can be achieved only when the completion fluid of the present invention are used while tubing and casing are commissioned inside the HTHP oil and gas wells and are engaged in the process of oil and gas production. Mere application of coating of corrosion inhibiting fluids on tubular surfaces does not provide corrosion inhibition. The process of the present invention involves extensive application of interdisciplinary knowledge in oil and gas industry.
[0031] While the exemplary embodiments of the present invention are described and illustrated herein, it will be appreciated that they are merely illustrative. It will be understood by those skilled in the art that various modifications in form and detail may be made therein without departing from or offending the spirit and scope of the invention as defined by the appended claims.

We claim:

1.A process for preparing a well completion fluid having reduced
corrosivity for oil and gas wells, the process comprising the steps of:
a. adding a pH moderating agent to a predetermined
quantity of technical water;
b. adding an oxygen scavenger to the solution of technical
water and pH moderator;
c. adding a predetermined quantity of one or more bromide
salts to the solution prepared at step b, such that the specific gravity of
the solution is greater than 2.00 and equal to or less than 2.30;
d. adding an acid corrosion inhibitor to the solution
prepared at step c.
2. The process as claimed in claim 1, wherein the pH moderating agent is sodium bicarbonate NaHCCh, further wherein the quantity of pH moderator is in the range of 1.5%-l .75% (W/V);
3. The process as claimed in claim 1, wherein the quantity of pH moderator added to technical water is 1.5% (W/V).
4. The process as claimed in claim 1, wherein the oxygen scavenger is sodium sulphite; wherein the quantity of sodium sulphite is in the range of 60-65ppm (parts per million) (W/V).
5. The process as claimed in claim 1, wherein 63 ppm of sodium sulphite is added as the oxygen scavenger.

6. The process as claimed in claim 1, wherein adding one or more bromide salts to the solution prepared at step c comprises adding a first bromide salt to raise the specific gravity of the solution upto 1.85; subsequently adding a second bromide salts to raise the specific gravity to 2.30, further wherein the first bromide salt is calcium bromide and the second bromide salt is zinc bromide.
7. The process as claimed in claim 1, wherein the one or more bromide salts are selected from a group comprising of calcium bromide and zinc bromide.
8. The process as claimed in claim 1, wherein the quantity of acid corrosion inhibitor is in the range of 2.5% -3 % (V/V).
9. The process as claimed in claim 1, wherein the quantity of acid corrosion inhibitor is 3% (V/V).
10. The process as claimed in claim 1, wherein the liquid selected as the acid corrosion inhibitor is a free flowing and homogenous organic liquid at a temperature ranging between 22°C - 26°C, wherein further the liquid is free from visible impurities and provides negative result for the presence of arsenic.
11. The process as claimed in claim 10, wherein the pour point temperature of the selected liquid is less than or equal to 12°C and the corrosion result for the selected liquid is less than or equal to 0.024g/cm2.
12. A well completion fluid exhibiting reduced corrosion, said well completion fluid comprising:

technical water as a base fluid;
a pH moderating agent;
an oxygen scavenger;
a combination of bromide salts; and an Acid Corrosion Inhibitor (ACI), wherein quantity of ACI is 2.5%-3% (V/V), further wherein the specific gravity of the well completion fluid is greater than 2.00 and equal to or less than 2.30.
13. The well completion fluid as claimed in claim 12, wherein the pH moderating agent is sodium bicarbonate (NaHCOa) and the quantity of sodium bicarbonate is 1.5%-1.75% (W/V).
14. The well completion fluid as claimed in claim 12, wherein the pH moderating agent is sodium bicarbonate (NaHCCb) and the quantity of sodium bicarbonate is 1.5% (W/V).
15. The well completion fluid as claimed in claim 12, wherein the quantity of oxygen scavenger is 60-65 (ppm parts per million) (W/V);
16. The well completion fluid as claimed in claim 12, wherein the oxygen scavenger is sodium sulphite and the quantity of sodium sulphite is 63 ppm (W/V).
17. The well completion fluid as claimed in claim 12, wherein the combination of bromide salts include calcium bromide and zinc bromide, further wherein a quantity of calcium bromide and a quantity of zinc bromide is

selected such that the specific gravity of the well completion fluid is greater than 2.00 and equal to or less than 2.30.
18. The well completion fluid as claimed in claim 12, wherein the
well completion fluid comprises:
technical water as the base fluid;
the pH moderating agent is sodium bicarbonate;
the oxygen scavenger is sodium sulphite, wherein the quantity of oxygen scavenger is 63 ppm (parts per million) (W/V);
the combination of bromide salts, wherein the combination of bromide salts include calcium bromide and zinc bromide, further wherein the predetermined quantity of calcium bromide and a quantity of zinc bromide is selected such that the specific gravity of the well completion fluid is 2.30.
and the Acid Corrosion Inhibitor (ACI), wherein quantity of ACI is 3% (V/V), further wherein the specific gravity of the well completion fluid is 2.30.
19. The well completion fluid as claimed in claim 12, wherein the
liquid selected as the acid corrosion inhibitor is a free flowing and homogenous
organic liquid at a temperature ranging between 22°C - 26°C, wherein further
the liquid is free from visible impurities and provides negative result for the
presence of arsenic.

20. The well completion fluid as claimed in claim 19, wherein the pour point temperature of the selected liquid is less than or equal to 12°C and the corrosion result for the selected liquid is less than or equal to 0.024g/cm2.
21. The well completion fluid as claimed in claim 18, wherein the inhibitor efficiency of the completion fluid is approximately 99.41% and a corrosion rate of approximately 2.43 milli inch per year (mpy) is obtained for a specified steel coupon using the completion fluid.