PROCESS FOR CORROSION CONTROL OF CARBON STEEL TUBING
AND CASING OF OIL AND GAS WELLS
Field of the invention
5
[0001] The present invention relates to corrosion inhibition of oil and gas
wells. In particular, the present invention provides a process for corrosion control of
carbon steel tubing and casing of oil and gas wells using corrosion inhibited
completion fluids.
10
Background of the invention
[ O O O 2 ] 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
15 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
20 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
25 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
5 completion fluids should be free of solids, sufficiently dense to control 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 perform the
abovementioned functions effectively.
10 [ 0 0 0 3 ] Conventionally, well completion fluids are prepared by dissolving one
or more salts such as sodium chloride, calcium chloride, zinc bromide and calcium
bromide in water based on the underground reservoir pressure and required specific
gravity. For example, specific gravity of 1.39 is achieved by using a salt such as
calcium chloride. Specific gravity is hrther increased by adding calcium bromide to
15 calcium chloride brine. Furthermore, specific gravity of up to 2.40 is obtained by
adding another salt such as zinc bromide. 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
20 by using combination of two and more salts to attain adequate density for adhering to
required hydrostatic pressure. However, brines made using combination of salts also
suffer from various limitations. The acidic environment is manifested by brines made
up of 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.
[0004] In light of the above-mentioned disadvantages, there is a need for a
5 process for corrosion control of carbon steel tubing and casing of oil and gas wells
using corrosion inhibited completion fluids. Further, there is a need for a process that
eliminates dissolved oxygen from the well completion fluids during preparation
thereby making the well completion fluid less corrosive. Furthermore, there is a need
for a process which facilitates buffering high density well completion fluids that are
10 highly acidic in nature. In addition, there is a need for well completion fluids having
wide range of specific gravities.
Summary of the invention
15 [0005] A process for corrosion control of carbon steel tubing and casing of oil
and gas wells using corrosion inhibited completion fluid is provided. The process
comprises a step of adding a pH moderating agent to water. The process further
comprises a step of adding an oxygen scavenger to the solution containing water and
the pH moderating agent. Furthermore, the process comprises a step of blending one
20 or more salts to the solution after the step of adding the oxygen scavenger, wherein
the one or more salts comprise calcium chloride, calcium bromide and zinc bromide.
Also, the process comprises a step of adding an acid corrosion inhibitor after the step
of blending the one or more salts to obtain the corrosion inhibited completion fluid.
[0006] In an embodiment of the present invention, the pH moderating agent is
sodium bicarbonate in an amount of 1.5%. In an embodiment of the present invention,
the oxygen scavenger is sodium sulphite in an amount of 63 milligrams per litre. In an
embodiment of the present invention, quantity of the one or more salts blended with
5 the solutio~l is based on required specific gravity of the completion fluid. In an
embodiment of the present invention, specific gravity of the completion fluid is 2.00.
In an embodiment of the present invention, 3% acid corrosion inhibitor is added after
the step of blending the one or more salts.
10 [0007] A corrosion inhibited completion fluid 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.
In addition, the completion fluid comprises one or more salts, wherein the one or
more salts comprise calcium chloride, calcium bromide and zinc bromide. Also, the
15 completion fluid comprises an acid corrosion inhibitor.
Detailed description of the invention
[0008] A process for corrosion control of carbon steel tubing and casing of oil
20 and gas wells using corrosion inhibited completion fluids is described herein. The
invention provides for a process that eliminates dissolved oxygen from the well
completion fluids during preparation thereby making the well completion fluid less
corrosive. Further, the invention provides for a process for moderating the pH of the
well completion fluids thereby making the well completion fluids less acidic.
Furthermore, the invention provides for a process for preparing well completion fluids
having adequate specific b~avitya nd wide range of densities.
[0009] The following disclosure is provided in order to enable a person having
5 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
10 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
1 5 to unnecessarily obscure the present invention.
[OOlO] The invention provides for a process for corrosion control of carbon
steel tubing and casing of oil and gas wells using corrosion inhibiting completion
fluids.
2 0
[OOll] The well completion fluids of the present invention use technical water
(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, 1.5% sodium bicarbonate is
25 added to water. In an embodiment of the present invention, any other suitable pH
6
moderating agent may be used to raise the pH of the solution. 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. Eliminating the dissolved oxygen
5 facilitates in reducing corrosion caused by the dissolved oxygen in the well
completion fluid. In an exemplary embodiment of the present invention, dosing of 63
ppm of sodium sulphite is added to the above solution to remove dissolved oxygen.
[0012] The above solution is then blended with one or more salts such as, but
10 not limited to, calcium chloride, zinc chloride, calcium bromide and zinc bromide. In
an embodiment of the present invention, the quantity of the one or more salts added to
the above solution is based on the requirement of specific 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
15 the one or more salts added to the solution. In an embodiment of the present
invention, the specific gsavity of the solution after dissolving the one or more salts
may range from 1.20-2.00 depending on the quantity and types of the one or more
dissolved salts.
20 [0013] Once the one or more salts are dissolved, an Acid Corrosion Inhibitor
(ACI) is added to the above solution which facilitates in hrther reducing the corrosive
effect of the well completion fluids of the present invention. 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 of Arsenic is negative for the ACI.
25 Furthermore, the pour point of the ACI is not more than 12°C. In an embodiment of
the present invention, 3% dosing of the ACI is added to the above solution. In an
embodiment of the present invention, any ACI formulation adhering to the abovementioned
properties is used. Once the ACI is added to the above solution, the
corrosion inhibiting well completion fluids are obtained. The obtained corrosion
5 inhibiting well conlpletion fluids are capable of mitigating corrosion of carbon steel
tubing and casing of oil and gas wells at high temperature of at least 175OC.
[0014] The corrosion rate of steel in presence of completion fluids obtained
using the abovementioned process were calculated based on test parameters
10 simulating the high temperature and high pressure oil and gas wells. The test
parameters are mentioned below:
Test Temperature: 175 * 2OC
Test Pressure: 1000 psi
Condition of Test: Static
15 Duration of Test: 6 hours
Acid VolumeITest Coupon Surface Area: 75 ml Isq. inch
Type of Steel: N - 80
Type of Acid: Hydrochloric (1 5% wlw)
Concentration of ACI: 30 mlll or 3%
20 Required results - Corrosion without Pitting: 0.044 g/cm2 (Maximum)
[0015] During testing, coupons of N-80 steel were fabricated using API 5 CT
N-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
25 and rinsed with acetone and dried in dry hot air. The surface area and the initial
8
weight of the coupons was then determined. Subsequently, the coupons were
immersed in PARR 4571 HPHT Autoclave filled with the well completion fluids
prepared using the abovementioned process. The immersed coupons were exposed to
the simulated test conditions at a temperature of 175°C. After the completion of
5 exposure tests, retrieved coupons were washed with water, rubbed against filter paper,
rinsed in Clark's solution, followed by thorough washing in water with 1 % non-ionic
detergent. Finally, the coupons were washed in distilled water and rinsed in xylene
and acetone and then dried in dry hot air. The coupons were then weighed to
determine the weight loss in the coupons and the corrosion rate in millimetre per year
10 and milli inch per year.
[0016] Based on the weight loss in the coupons, corrosion rate in milli inch
per year (mpy) was calculated using the following formula:
15 Corrosion Rate = (3.45 X lo6 x W) / (AX T x D)
wherein,
W: Coupon Weight Loss in grams
A: Surface area in centimetre2
T: Exposure time in hours
20 D: Density in grams/centimeter3
Corrosion rate in millimetre per year = 0.0254 x Corrosion rate in milli inch per year
[0017] The experiments were repeated with brines prepared using the
referenced invention and effectiveness of corrosion inhibition process was determined
25 by calculating percentage inhibition as per the formula given below:
9
Inhibitor Efficiency in % = (A - B) x I00 1 A
wherein,
A = Corrosion rate for blank
B = Corrosion rate with Inhibitor
5
[0018] In an exemplary embodiment of the present invention, results of
corrosion rate studies for API 5CT N-80 steel coupon exposed to well completion
fluids of the present invention, comprising calcium chloride, calcium bromide and
zinc bromide (specific gravity 2.0), at a temperature of 175°C are illustrated in the
10 table below:
S. Medium Corrosion Inhibition
No. Rate %
mPY
1 Brine containing combination of Calcium 1 83.48 0.00
Chloride, Calcium Bromide and Zinc
Bromide with specific gravity 2.0
2 Brine containing combination of Calcium 28.38 84.53
Chloride, Calcium Bromide and Zinc
Bromide with specific gravity 2.0 + 3% ACI
(v/v>
3 Brine containing combination of Calcium 24.78 86.49
Chloride, Calcium Bromide and Zinc
Bromide with specific gravity 2.0 + 3% ACI
(v/v) +1.5% NaHC03 + 3% ACI(v/v)
4 Brine containing combination of Calcium 17.99 90.19
Chloride, Calcium Bromide and Zinc
Bromide with specific gravity 2.0 + 3% ACI
(v/v) + 1.5% NaHC03 + 63ppm Sodium
sulphite + 3% ACI(v/v)
[0019] Results for corrosion rate studies enumerated in the table above show
15 significant decrease in corrosion rate of steel and increase in corrosion inhibition
percentage when well completion fluids prepared using the process of the present
invention are used.
[0020] While the exemplary embodiments of the present invention are
5 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 tnodifications 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 corrosion control of carbon steel tubing and casing of oil and gas
wells using corrosion inhibited completion fluid, the process comprising the steps of:
5
adding a pH moderating agent to water;
adding an oxygen scavenger to the solution containing water and the pH
moderating agent;
10
blending one or more salts to the solution after the step of adding the oxygen
scavenger, wherein the one or more salts comprise calcium chloride, calcium
bromide and zinc bromide; and
15 adding an acid corrosion inhibitor after the step of blending the one or more
salts to obtain the corrosion inhibited completion fluid.
2. The process of claim 1, wherein the pH moderating agent is sodium bicarbonate in
an amount of 1.5%.
2 0
3. The process of claim 1, wherein the oxygen scavenger is sodium sulphite in an
amount of 63 milligrams per litre.
4. The process of claim 1, wherein quantity of the one or more salts blended with the
25 solution is based on required specific gravity of the completion fluid.
5. The process of claim 1, wherein specific gravity of the completion fluid is 2.00.
6. The process of claim 1, wherein 3% acid corrosion inhibitor is added after the step
5 of blending the one or more salts.
7. A corrosion inhibited completion fluid comprising:
water as a base fluid;
a pH moderating agent;
10 an oxygen scavenger;
one or more salts, wherein the one or more salts comprise calcium chloride, calcium
bromide and zinc bromide; and
an acid corrosion inhibitor.
15 8. The corrosion inhibited completion fluid of claim 7, wherein the pH moderating
agent is sodium bicarbonate in an amount of 1.5%.
9. The corrosion inhibited completion fluid of claim 7, wherein the oxygen scavenger
is sodium sulphite in an amount of 63 milligrams per litre.
20
10. The corrosion inhibited completion fluid of claim 7, wherein quantity of the one
or more salts in the completion fluid is based on required specific gravity of the
completion fluid.
11 . The corrosion inhibited completion fluid of claim 7, wherein specific gravity of
the corrosion inhibited con~pletionfl uid is 2.00.
12. The corrosion inhibited completion fluid of claim 7, wherein the completion fluid
5 comprise 3% acid corrosion inhibitor.