FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 PROVISIONAL SPECIFICATION (See section 10, rule 13)
1. Title of the invention"CORROSION INHIBITION FOR OIL AND GAS WELL COMPLETION AND PACKER FLUIDS"
2. Applicant(s)
Name Nationality Address
OIL AND NATURAL GAS CORPORATION LIMITED IN Institute of Engineering & Ocean Technology ONGC Limited, Phase-2 Panvel, Navi Mumbai- 410221


3. Preamble to the description
PROVISIONAL SPECIFICATIONThe following specification particularly describes the invention.

Field of the Invention
The present invention relates to corrosion inhibitor composition to prevent the corrosion of Carbon Steel tubing and casing used in Oil and Gas wells.
Background and Prior art:
The Oil and Gas wells are the only communication channel with the hydrocarbon reservoir in the development of oil and gas fields. Thus it is important to design for optimum well production rate throughout the expected well life.
The well completion tubulars are normally of following carbon steel:-
API 5 CT Grade J-55, API 5 CT Grade L-80 Type I, API 5 CT Grade N-80 Type I, API 5 CT Grade N-80 Type Q, API 5 CT Grade P-110 type e, API 5 CT Grade K-55, API 5 CT Grade N-80 EUE, API 5 CT Grade H-40, API 5 CT Grade C-90 Type 1, API 5 CT Grade C-90 Type 2, API 5 CT Grade C-95, API 5 CT Grade T-95, API 5 CT Grade M-65, API 5 CT Grade Q-125.
The most commonly used tubulars are API 5 CT Grade N-80 Type I and API 5 CT Grade L-80 Type I steels. The well completion design necessitates the use of Sodium Chloride based brines (up to Sp.Gr. 1.26) and Calcium Chloride based brines (up to Sp.Gr.: 2.3) depending on the formation pressure. These brines acts as communicating medium for all work over operations. In the case of gas lift wells, the lift gas having carbon dioxide content in contact with these brines enhances its corrosivity by forming carbonic acid. These brines having high chloride content are already very corrosive to carbon steel. Further during preparation of these brines at the surface, Oxygen from atmosphere gets dissolved in it and enhances corrosivity of brines inside the oil and gas wells. The well tubing and casing are in constant contact with these brines and are susceptible to premature failures leading to loss of well and hence huge loss in oil and gas. Currently treatment of Sodium Dichromate and maintaining high pH of above 9.5 by using Sodium Hydroxide is in practice for the mitigation of tubing and casing corrosion. A minimum dose of 500ppm of Sodium Dichromate is recommended for the above mitigation.

Objects of the present invention
The primary object of the present invention is a non-carcinogenic corrosion inhibitor for down hole tubing and casing in oil and gas wells.
An object of the present invention is to find a substitute for Sodium Dichromate as a corrosion inhibitor for tubing and casing of Oil and Gas wells.
Detailed Description of the invention
In particular the invention relates to corrosion inhibition of carbon steel tubing and casing in down hole conditions in oil and gas wells.
In down hole operating conditions the carbon steel tubings and casings are exposed to brines such as sodium chloride based brines and calcium chloride based brines which leads to corrosion of Carbon Steel. The formulation of the present invention mitigates the corrosion of carbon steel even on exposure to brines like sodium chloride based brines and calcium chloride based brines.
The severity of tubular corrosion increases drastically due to presence of carbon dioxide in lift gas for gas lift operated oil and gas wells. Current practice in Oil and Gas field is to use Sodium Dichromate as corrosion inhibitor for these brines.
As all forms of hexavalent Chromium are recognized by the United States National Institute of Environmental health sciences as a Group I known human Carcinogen, it would be beneficial to eliminate use of hexavalent chromium as corrosion inhibitor for mitigation of corrosion of tubulars in on land and offshore Oil and Gas wells. The environmental agencies limit the amount of Chromium ion tolerance in waste water to
less than 1 ppm. The Environmental Ministry, Govt. of India, has issued notifications on restricting the use of Chromates in oil field.
The invention provides an alternate formulation which is non-carcinogenic hence environmentally friendly and provides corrosion inhibition from Sodium Chloride

based brines and Calcium Chloride based brines used as well completion and packer fluids in Onland and Offshore oilfields.
It is a substitute for Sodium Dichromate and hence it complies with the requirement of notification issued by Environmental Ministry, Govt, of India to discontinue the use of Chromates in oilfields.
The philosophy of design was aimed at mitigating the corrosion mechanisms taking place on carbon steel exposed to these brines under down hole operating conditions of Oil and Gas wells. The corrosion mechanisms involve
• general corrosion; and
• localized pitting corrosion.
The formulations are blended as mixed corrosion inhibitor i.e. combination of anodic and cathodic corrosion inhibitors. Anodic corrosion inhibitor component mitigate general metal loss corrosion by its activity at anodic sites while the cathodic corrosion inhibitor component mitigate localized pitting corrosion by its activity at sites of cathodic reactions. The metal complexing agent (ligands) optimally facilitates the chemical activities of both anodic and cathodic components. The identified inhibitor components form a thin film on the metallic surface and mitigate both general and localized pitting corrosions, taking place in down hole operating conditions. The ligand assists in the mobility of required ions to form film on metal and to reform whenever there is peeling of film.
The anodic corrosion inhibitor component comprises of salts of Transition elements from Group VB & VI B of periodic table, viz: salts of Molybdenum, Vanadium, and Tungsten.
The cathodic corrosion inhibitor component comprises of compounds of rare earths elements from Group IIIB of periodic table viz: Cerium, Neodymium, Praseodymium,

Samarium, Lanthanum and Yttrium.
The metal complexing ligands are water soluble salts of organic hydroxyl acids like Glycolic acid, Malic acid, Tartaric acid and Citric acid.
The components of the formulations mentioned above are not listed as carcinogenic chemicals by United States National Institute of Environmental Health Sciences and are indigenously available in India. Examples
Formulation 1: For Sodium Chloride based brines
(Sodium Chloride {Specific gravity: 1.20} and Sodium Chloride + Soda ash {sp.gr.: 1.26}
One formulation can be prepared by blending Rare earth chloride, preferably Cerium chloride, CeCl3.6H20, (Mol.wt.: 354.48), transition element salt preferably Ammonium heptamolybdate ((NH4)6Mo7O24.4H20, (MoLwt 1235.86), and hydroxyl acid salts preferably Tri-Sodium Citrate-2-hydrate((C6HsNa307 .2H2O) (Mol.wt: 294.10) at the composition ratio of 40 to 70: 10 to 30: 20 to 40 and dissolving the blended product in water as 2.5% solution. Requisite quantity of formulation is to be treated to give following final dosing rate: Cathodic: 1.5mM to 3mM. Anodic: 0.15mM to 0.5mM Ligand: 1.4mM to 2mM
Formulation II for Calcium Chloride based brines
(Sodium Chloride + Calcium Chloride{Sp.Gr.: 1.40}, Calcium Chloride(Sp.GR.:1.41}, Calcium Chloride + Calcium Bromide{Sp.GR.: 1.82}, and Calcium Chloride + Calcium Bromide+ Zinc Bromide{Sp.Gr.:2.3}
Another formulation can be prepared by blending Rare earth chloride preferably Cerium chloride, CeCl3.6H20, (Mol.wt.: 354.48), transition element salt preferably Sodium Ortho Vanadate, Na3V04.12H20) (Mol.wt 399.94), salt of hydroxyl acid

preferably Tri-Sodium Citrate-2-hydrate ((C6H5Na307 .2H20) (Mol.wt: 294.10) at the composition ratio of 50 to 80: 15 to 25: 20 to 40 respectively and dissolving the blended product in water as 2.5% solution. Formulation II is the same as the Formulation I with the exception of Molybdenum salts as anodic corrosion inhibitor. Molybdenum salt is not effective in Calcium Chloride based brines due to the formation of Calcium Molybdate precipitate. Requisite quantity of formulation is to be treated to give following final dosing rate: Cathodic: 2mM to 4mM Anodic: 0.5mMtolmM Ligand: 1.2mM to 2.5mM
An experimental study was carried out the details of which are as follows:
EXPERIMENTAL STUDY DATA
Test Conditions:
Simulation of down hole operating conditions of oil and gas wells of onshore and offshore fields. Following worst condition was simulated: Partial pressure of carbon dioxide: 30 psi
Temperature: 140° C
Equipment used:
High Pressure High Temperature Autoclave, PARR model 4571 sl.no.219 of PARR
Instrument Company, USA.
Test medium:
1. Sodium Chloride based brine (Sp.Gr. 1.20)
2. Calcium Chloride brine (Sp.Gr. 1.35) Coupon Specimen:
API 5 CT L-80 steel type I
Procedure:
Experiments of gravimetric corrosion rate studies were performed on specimen
coupons fabricated from API 5 CT L-80 tubing, which are exposed to Sodium
Chloride based brine (Sp.Gr. 1.20) and Calcium Chloride brine (Sp.Gr. 1.35). Down

hole well conditions of temperature and corrosive environment of carbon dioxide partial pressure were simulated inside a PARR model 4571 sl.no.219 PARR INSTRUMENT COMPANY, USA. The coupons were wet ground to a surface finish of 400 grit. The coupons were degreased with benzene, washed with distilled water and rinsed with acetone and dried in a vacuum desiccator. The surface area and the initial weight of coupons were determined. Then the coupons were immersed in PARR HPHT Autoclave filled with high density brines viz: Sodium Chloride based brine (Sp.Gr.1.20) and Calcium Chloride based brines (Sp.Gr. 1.35). The worst case down hole conditions of 140°C temperature and 30 psi partial pressure of carbon dioxide are simulated. After the completion of exposure tests, the retrieved coupons were washed with water, rubbed against filter paper, rinsed in Clarke solution for one minute, followed by thorough washing in water with 1% non ionic detergent. Finally the coupons were washed in distilled water and rinsed in acetone and dried in vacuum desiccators. Then the coupons were weighed to determine the weight loss in the coupons and the corrosion rate in milli inch per year (mpy) was determined using the following formula:
3.45 X 108 X W
Corrosion Rate =
mpy A X T X D
W : Coupon Weight loss, g A : Surface (area,cm2) T: Exposure Time (hours) D: Density(g/cm3)
The experiments were repeated with brines treated with formulations designed in this invention and corrosion inhibition percentage was calculated to assess the inhibitor performance. As per the formula given below:
Cor. rate for Blank - Cor. rate with Inhibitor
Inhibitor Efficiency, % = X 100
Cor. rate for Blank

The results of the study are given below:
I. Coupon: L-80, Medium: Sodium Chloride (NaCl) based Brine(Sp.Gr. 1.20)
Formulation I

No. Inhibitor Dosing Temperature Deg.C PartialPressureC02psi Corrosion ratempy (%Inhibition)
AnodicInhitormg/1 Cathodic Inhibitormg/1 Ligand mg/1 SodiumDichromateppm
1 0 0 0 0 140 30 271.31(0)
2 0 0 0 1000 140 30 78.1(71.21)
3 250 750 500 0 140 30 12.65(95.33)
II. Coupon: L-80, Medium: Calcium Chloride based brine (Sp.Gr. 1.35) Formulation II

No. Inhibitor Dosing Temperature Deg.C Partial Pressureco2psi Corrosionratempy(%Inhibition)
AnodicInhibitormg/1 CathodicInhibitormg/1 Ligand mg/1 SodiumDichromatemg/1
1 0 0 0 0 140 30 377.10(0)
2 0 0 0 1000 140 30 116.50(69.10)
3 400 900 400 0 140 30 31.41(91.67)

Dated this 11th day of January 2007

T. Srinivasan Agent for the Applicant

Abstract
A Non-carcinogenic corrosion mitigation inhibitor to prevent corrosion of carbon steel in down hole well tubing and well casing. The said corrosion mitigation inhibitor comprising an anodic corrosion inhibitor, a cathodic corrosion inhibitor and metal complexing ligands.