Field of the invention
5 [000 1] The present invention relates to a composition and process of preparation of
perfonnance enhancing oil and gas well completion fluids. In particular, the present
invention provides a well completion fluid for mitigating corrosion in oil and gas wells
and a process of preparation thereof.
1 0 Background of the invention
[0002] 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
15 replace rock cuttings and. stabilizing the drilled hole. Once the target depth is attained
and the drilled hole is cased, 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 fonnations. Further, the
well completion fluid is left in the annular region of the oil and gas well between tubing
20 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
25 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
30 the producing reservoir pressure, resistant to viscosity changes over long periods of
time and noncorrosive to the well bore and completion components in order to perfonn
the abovementioned functions effectively.
2
[0003] 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,
5 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, brines made using combination of salts also suffer from
various limitations. The acidic environment is manifested by brines made up of
10 combination of two or more salts as well. Also, during the process of preparation,
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.
15 [0004] In light of the above-mentioned disadvantages, there is a need for a well
completion fluid for mitigating corrosion in oil and gas wells and a process of
preparation thereof. Further, there is a need for a process that eliminates dissolved
oxygen from the well completion fluids during preparation thereby making the prepared
well completion fluid less corrosive. Furthermore, there is a need for a process which
20 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.
25
Summary of the invention
[0005] A completion fluid for mitigating corrosion in oil and gas wells and a process
of preparation thereof is provided. The completion fluid comprises water as a base fluid.
The completion fluid further comprises a pH moderating agent. Furthermore, the
completion fluid comprises an oxygen scavenger. Also, the completion fluid comprises
30 one or more salts, sodium thiocyanate and an acid corrosion inhibitor.
[0006] In an embodiment of the present invention, the pH moderating agent is sodium
bicarbonate. In an embodiment of the present invention, the pH moderating agent is
present in an amount ranging between 1.25% and 1.75% weight/volume. In an
3
embodiment of the present invention, the oxygen scavenger is sodium sulphite. In an
embodiment of the present invention, the oxygen scavenger is present in an amount
ranging between 50 milligrams per liter and 100 milligrams per liter. In an embodiment
of the present invention, the oxygen scavenger is present in an amount of 63 milligrams
5 per litre.
[0007] In an embodiment of the present invention, quantity of the one or more salts in
the completion fluid is based on required specific gravity of the completion fluid. In an
embodiment of the present invention, specific gravity of the completion fluid is in a
10 range between 1.20 and 2.00. In an embodiment of the present invention, specific
gravity of the completion fluid is 1.85. In an embodiment of the present invention, the
sodium thiocyanate is present in an amount ranging between 1 000 milligrams per liter
and 2000 milligrams per liter. In an embodiment of the present invention, the sodium
thiocyanate is present in an amount of 2000 milligrams per liter. In art embodiment of
15 the present invention, 3% acid corrosion inhibitor is added after addition of the sodium
thiocyanate. In an embodiment of the present invention, the one or more salts comprise
calcium chloride and calcium bromide.
[0008] The process for preparing completion fluids for mitigating corrosion in oil and
20 gas wells comprises adding a pH moderating agent to water. The process further
comprises adding an oxygen scavenger to the solution containing water and the pH
moderating agent. Furthermore, the process comprises blending one or more salts to the
solution after the step of adding the oxygen scavenger. In addition, the process
comprises adding sodium thiocyanate after the step of blending the one or more salts.
25 Also, the process comprises adding an acid corrosion inhibitor after the step of adding
the sodium thiocyanate to obtain the completion fluid.
Detailed description of the invention
30 [0009} A well completion fluid for mitigating corrosion in oil and gas wells and a
process of preparation thereof is described herein. The invention provides for a
composition and process that eliminates dissolved oxygen from the well completion
fluids during preparation thereby making the prepared well completion fluid less
4
5
corrosive. Further, the invention provides for a composition and process for moderating
the pH of the well completion fluids thereby making the well completion fluids less
acidic. Furthermore, the invention provides for well completion fluids having wide
range of specific gravities.
[001 OJ 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
10 embodiments and applications without departing from the spirit and scope of the
invention. Also, the tenninology 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
15 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.
[0011] The invention provides for well completion fluids for mitigating corrosion of
20 carbon steel tubing and casing in oil and gas wells and a process of preparation thereof.
The carbon steel tubing and casing are exposed to well completion fluids comprising
high density brines at 17 5 oc temperature.
[0012] The well completion fluids of the present invention use technical water
25 (hereinafter referred to as water) as a base fluid. Sodium bicarbonate is added to the
water. Sodium bicarbonate is used as a pH moderating agent and raises the pH of the
solution. In an embodiment of the present invention, 1.25 to 1.75% weight/volume
(weight/volume is also referred to as w/v hereinafter) sodium bicarbonate is added to
water. In an embodiment of the present invention, any other suitable pH moderating
30 agent may be used to raise the pH of the solution in an amount ranging between 1.25%
and 1.75% (w/v). On dissolution of sodium bicarbonate in water, an oxygen scavenger
is added to the solution. In an embodiment of the present invention, sodium sulphite is
added to the above solution to eliminate dissolved oxygen in the solution. In an
exemplary embodiment of the present invention, the oxygen scavenger is added to the
5
5
above solution in an amount ranging between 50 milligrams per liter w/v and 100
milligrams per liter w/v or 50 parts per million and 100 ppm (parts per millions is also
referred to as ppm hereinafter). In a preferred embodiment of the present invention, 63
milligrams per liter of oxygen scavenger is added to the solution.
[0013] The above solution is then blended with one or more salts such as, but not
limited to, calcium chloride, calcium bromide or a combination of calcium chloride and
calcium bromide. In an embodiment of the present invention, the quantity of the salt or
combination of salts added to the above solution is based on the requirement of specific
10 gravity to be maintained for a particular oil and gas well. Further, well completion fluids
having wide ranges of densities and specific gravity are obtained by varying and
monitoring the amount of salt or combination of salts added to the solution. In an
embodiment ofthe present invention, the specific gravity ofthe solution after blending
the salt or combination of salts ranges between 1.20 and 2.00 depending on the quantity
15 and type of salt or combination of salts dissolved/blended. In a preferred embodiment.
of the present invention, the specific gravity of the solution after blending a
combination of calcium bromide and calcium chloride salts is 1.85.
[0014] Once the one or more salts are dissolved in the solution, sodium thiocyanate is
20 added in an optimum quantity to the solution. In an embodiment of the present
invention, sodium thiocyanate is added in an amount ranging between 1 000 milligrams
per litre (w/v) and 2000 milligrams per litre (w/v) or 1000 ppm and 2000 ppm. In a
preferred embodiment of the present invention, 2000 ppm or 2000 milligrams per litre
(w/v) of sodium thiocyanate is added to the solution after dissolution of the one or more
25 salts.
[0015] Once sodium thiocyanate is added, an Acid Corrosion Inhibitor (ACI) is added
to the above solution. The ACI is a free flowing and homogenous organic liquid, at 24
± 2°C, which is free from visible impurities. Further, the qualitative test for the presence
30 of Arsenic is negative for the ACI. Furthermore, the pour point of the ACI is not more
than l2°C. In an embodiment of the present invention, 0.5-3% volume/volume
(volume/volume is referred to as v/v hereinafter) dosing of the ACI is added to the
above solution. In an embodiment of the present invention, any ACI formulation
adhering to the above-mentioned properties is used.
6
[0016] The corrosion rates of steel in presence ofthe well completion fluids obtained
using the above-mentioned process were calculated based on test parameters simulating
the high temperature and high pressure oil and gas wells. The test parameters are
5 mentioned below:
Test Temperature: 175 ± 2°C
Test Pressure: 1000 psi
Condition ofTest: Static
Duration ofTest: 6 hours
10 Acid Volume/Test Coupon Surface Area: 75 ml /sq. inch
Type of Steel: N- 80
15
Type of Acid: Hydrochloric (15% w/w)
Concentration of ACI: 30 ml/1 or 3%
Corrosion without pitting: 0.044g/cm2 (Maximum)
[0017] During testing, coupons ofL-80 steel were fabricated using API 5 CT L-80 steel
casing pipe material. The coupons were wet ground to a surface finish of 400 grit. The
coupons were then degreased with xylene, washed with distilled water and rinsed with
acetone and dried in dry hot air. The surface area and the initial weight of the coupons
20 was then determined. Subsequently, the coupons were immersed in PARR 4571 HPHT
Autoclave filled with the well completion fluids prepared using the above mentioned
process. The immersed coupons were exposed to the simulated test conditions at 175°C.
After the completion of exposure tests, retrieved coupons were washed with water,
rubbed against filter paper, rinsed in Clark's solution, followed by thorough washing in
25 water. Finally, the coupons were washed in distilled water and rinsed in xylene and
acetone and dried in dry hot air. The coupons were then weighed to detennine the
weight loss in the coupons and the corrosion rate in millimetre per year and milli inch
per year.
30 [0018] Based on the weight loss in the coupons, corrosion rate in milli inch per year
(mpy) was calculated using the following formula:
Corrosion Rate, (mpy) = (3.45 X 106 x W) I (Ax T x D)
wherein,
W: Coupon Weight Loss in grams
5
A: Surface area in centimetre2
T: Exposure Time in hours
D: Density of carbon steel in grams/centimeter3
Corrosion rate in millimetre per year= 0.0254 x Corrosion rate in milli inch per year
[0019] The experiments were repeated with brines prepared using the process of the
invention and effectiveness of corrosion inhibition process was detennined by
calculating percentage inhibition as per the formula given below:
Inhibitor Efficiency in % = (A- B) x 100 I A
10 wherein,
A = Corrosion rate for blank
B = Corrosion rate with Inhibitor
[0020] The results of the experiments/test are provided in the table below:
15
S.
Medium
Corrosion Rate
Inhibition %
No. mpy
1
Brine containing combination of Calcium Chloride 165.50/ 172.29
0.00
and Calcium Bromide with specific gravity 1.85 Avg=l68.90
Brine containing combination of Calcium Chloride
158.30/ 163.46
2 and Calcium Bromide with specific gravity 1.85 +
Avg=160.88
4.75
1000 ppm NaSCN
Brine containing combination of Calcium Chloride
152.36/ 156.68
3 and Calcium Bromide with specific gravity 1.85 +
Avg=154.52
7.24
1500 ppm NaSCN
Brine containing combination of Calcium Chloride
127.92/ 114.33
4 and Calcium Bromide with specific gravity 1.85 +
Avg=121.12
28.29
2000 ppm NaSCN
Brine containing combination of Calcium Chloride
120.49/ 115.29
5 and Calcium Bromide with specific gravity 1.85 +
Avg=117.89
30.20
2500 ppm NaSCN
Brine containing combination of Calcium Chloride
6
and Calcium Bromide with specific gravity 1.85 + 66.75/ 111.53
47.22
1.5% NaHC03+ 63ppm Sodium sulphite+ 2000 Avg=89.14
ppm NaSCN
Brine containing combination of Calcium Chloride
154.78/ 160.46
7 and Calcium Bromide with specific gravity 1.85 + 1%
Avg=157.62
6.67
ACI(v/v)
8
5
Brine containing combination of Calcium Chloride
152.34/ 145.82
8 and Calcium Bromide with specific gravity 1.85 +
Avg=149.08
11.73
2% ACI(v/v)
Brine containing combination of Calcium Chloride
142.71/ 134.32
9 and Calcium Bromide with specific gravity 1.85 +
Avg=138.52
17.99
3%ACI(v/v)
Brine containing combination of Calcium Chloride
10
and Calcium Bromide with specific gravity 1.85 + 94.74/ 103.53
41.30
1.5% NaHC03+ 63ppm Sodium sulphite+ 3% Avg=99.14
ACI(v/v)
Brine containing combination of Calcium Chloride
11
and Calcium Bromide with specific gravity 1.85 + 39.17/45.97
74.80
1.5% NaHC03+ 63ppm Sodium sulphite+ 2000 Avg=42.57
ppm NaSCN + 2% ACI(v/v)
Brine containing combination of Calcium Chloride
12
and Calcium Bromide with specific gravity 1.85 + 40.37/ 38.37
76.70
1.5% NaHC03+ 63ppm Sodium sulphite+ 2000 Avg=39.37
ppm Sodium thiocyanate+ J.% ACI(v/v)
[0021] Results for corrosion rate studies enumerated in table above show significant
decrease in corrosion rate and increase in corrosion inhibition percentage when well
completion fluids of the present invention are used.
[0022] 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
1 0 invention as defined by the appended claims.

We claim:
1. A completion fluid for mitigating corrosion in oil and gas wells comprising:
water as a base fluid;
5 a pH moderating agent;
10
an oxygen scavenger;
one or more salts;
sodium thiocyanate; and
an acid corrosion inhibitor.
2. The completion fluid of claim i, wherein the pH moderating agent 1s sodium
bicarbonate.
3. The completion fluid of claim 1, wherein the pH moderating agent is present in an
15 ·amount ranging between 1.25% and 1.75% weight/volume.
4. The completion fluid of claim 1, wherein the oxygen scavenger is sodium sulphite.
5. The completion fluid of claim 1, wherein the oxygen scavenger is present in an
20 amount ranging between 50 milligrams per liter and 1 00 milligrams per liter.
6. The completion fluid of claim 1, wherein the oxygen scavenger is present in an
amount of 63 milligrams per litre.
25 7. The completion fluid of claim 1, wherein quantity of the one or more salts in the
completion fluid is based on required specific gravity of the completion fluid.
30
8. The completion fluid of claim 1, wherein specific gravity of the completion fluid is
in a range between 1.20 and 2.00.
9. The completion fluid of claim 1, wherein specific gravity of the completion fluid is
1.85.
10
10. The completion fluid of claim 1, wherein the sodium thiocyanate is present in an
amount ranging between 1000 milligrams per liter and 2000 milligrams per liter.
11. The completion fluid of claim 1, wherein the sodium thiocyanate is present in an
5 amount of 2000 milligrams per liter.
12. The completion fluid of claim 1, wherein 3% acid corrosion inhibitor is added after
addition of the sodium thiocyanate.
10 13. The completion fluid of claim 1, wherein the one or more salts comprise calcium
chloride and calcium bromide.
15
20
25
14. A process for preparing completion fluids for mitigating corrosion in oil and gas
wells, the process comprising the steps of:
adding a pH moderating agent to water;
adding an oxygen scavenger to the solution containing water and the pH
moderating agent;
blending one or more salts to the solution after the step of adding the oxygen
scavenger;
adding sodium thiocyanate after the step of blending the one or more salts; and
adding an acid corrosiOn inhibitor after the step of adding the sodium
thiocyanate to obtain the completion fluid.
15. The process as claimed in claim 14, wherein the pH moderating agent is sodium
30 bicarbonate.
16. The process as claimed in claim 14, wherein the pH moderating agent is present in
an amount ranging between 1.25% and 1. 7 5% weight/volume.
11
17. The process as claimed in claim 14, wherein the oxygen scavenger is sodium
sulphite.
18. The process as claimed in claim 14, wherein the oxygen scavenger is present in an
5 amount ranging between 50 milligrams per liter and 100 milligrams per liter.
19. The process as claimed in claim 14, wherein the oxygen scavenger is present in an
amount of 63 milligrams per litre.
10 20. The process as claimed in claim 14, wherein quantity of the one or more salts
blended with the solution is based on required specific gravity of the completion fluid.
15
21. The process as claimed in claim 14, wherein specific gravity of the completion fluid
is in a range between 1.20 and 2.00.
22. The process as claimed in claim 14, wherein specific gravity of the completion fluid
is 1.85.
23. The process as claimed in claim 14, wherein the sodium thiocyanate is present in
20 an amount ranging between 1000 milligrams per liter and 2000 milligrams per liter.
24. The process as claimed in claim 14, wherein the sodium thiocyanate is present in
an amount of 2000 milligrams per liter
25 25. The process as claimed in claim 14, wherein 3% acid corrosion inhibitor is added
after the step of adding the sodium thiocyanate.
30
35
26. The process as claimed in claim 14, wherein the one or more salts comprise calcium
chloride and calcium bromide.