"NONCARCINOGENIC CORROSION INHIBITION FOR OIL AND GAS WELL COMPLETION & PACKER FLUIDS"

Field of the Invention

The present invention relates to composition of Noncarcinogenic Corrosion inhibitor formulation to prevent the corrosion of Carbon Steel Tubing and Casing exposed to high density Sodium Chloride and Calcium Chloride brines as well completion & Packer fluids of Enhanced Oil Recovery (EOR) wells by gas lift process with produced gas rich in acid gas like carbon dioxide.

Background and Prior art: The Oil and Gas wells are the only communication channels 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 configuration of a well after completion can be seen as number of tubular, viz: tubing and casing, isolated from each other by seals at surface and subsurface, with provision for selective intercommunication and safety shut off. The purpose of tubular in a well could be: i. Convey the production from the reservoir to the surface; ii. Convey injection fluids from the surface to the reservoir; iii. Circulate fluids from the surface through the subsurface facility; iv. Contain stationary fluids for well protection or monitoring of subsurface conditions; v. Control production from reservoirs; vi. Protect well from exposure to high pressure, corrosion, erosion etc. There are three basic ways to complete a well: a) Open -hole b) Cased/liner and c) Perforated.

In the open-hole type of completion, casing is set only to the top or slightly into the completion. In the cased and perforated completion, casing is set into or through the producing formation and cemented. The casing is then perforated to provide communication between the well bore and formation.

The well completion tubular is 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-I lO 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 tubular are API 5 CT Grade N-80 Type I and API 5 CT Grade L-80 Type I steels.

In oil and gas wells, produced fluids flow through tubing string which is retrievable and is positioned in a permanently installed casing. The annulus between the tubing and casing is filled with Brine to provide weight and to help seal the packer at the bottom of the annulus. The major function of packer fluids is to stabilize and maintain the entrained material in suspension. The packer fluid should also protect the tubing/casing against burst or collapse and should be reasonably of uniform density throughout the fluid column.

This fluid must be of sufficient density to contain the well pressure in the event of tubing failure. Under long term, static conditions, detrimental changes may take place that can not be rectified easily. Packer fluids must be conditioned to function for years, because no opportunity is afforded for correction without great expense. The packer fluid system contains large area of metal surface in relation to the volume of fluid. The well completion design necessitates the use of high density Sodium Chloride brine (up to Sp.Gr. 1.20) and Calcium Chloride brine (up to Sp.Gr.

1.41) 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 (CO ₂ ) 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 of 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. Sodium dichromate in presence of Sodium hydroxide forms sodium chromate.

Na ₂ Cr ₂ O ₇ + 2 NaOH <→ 2 Na ₂ CKM + H ₂ O

Chromates are passivating inhibitors and offer protection of carbon steel by incorporation into the oxide layer. Chromate initially is adsorbed on the metal followed by oxidation of iron to form a mixture of Fe ₂ θ ₃ and Cr ₂ θ ₃ . Chromate is a powerful oxidizing agent and oxidizes ferrous oxide or ferrous hydroxide to ferric state Fe (OH) ₃ and self-reduction to Cr(OH) ₃ and the hydroxides give rise to Fe ₂ O ₃ and Cr ₂ O ₃ .

6FeO + 2CrO ₄ ^2" + 2H ₂ O <→ 3 Fe ₂ O ₃ + Cr ₂ O ₃ + 4 OH ^"

Anions such as Chloride and Sulfate compete with Chromate and cause pitting corrosion below a critical concentration of Chromate. Normally, 500-1000 ppm of Chromate is considered as critical concentration, below which high pitting corrosion is expected. In the case of well completion brines, Chloride content is very high and always higher critical concentration of Chromate is to be maintained.

The effectiveness of Sodium dichromate as corrosion inhibitor depends on the pH of the brine which is preferably at neutral and higher alkaline range. In the case of enhanced oil recovery (EOR) wells with gas lift operation using produced gas with high content of Carbon dioxide (CO ₂ ), pH of the packer fluid falls slowly to acidic range and hence the Chromate is converted into Dichromate once again. In view of this availability of Chromate is significantly reduced so that inadequate protective oxide layer formation takes place in the event of breakdown of the layer over the years. This lead to inefficient performance of Sodium dichromate as corrosion inhibitor especially in gas lifts wells and hence Tubing and Casing are vulnerable to premature failures by corrosion.

Objects of the Present Invention The primary object of the present invention is a non-carcinogenic corrosion inhibitor formulation for down hole tubing and casing exposed to an acidic medium (pH 4- 4.5) of high density Sodium chloride and Calcium chloride brines as well completion fluids for Enhanced Oil Recovery (EOR) wells with gas lift operation using producing gas rich in Carbon dioxide (CO ₂ ). An object of the present invention is to find a substitute for Sodium

Dichromate as a corrosion inhibitor for tubing and casing exposed to high density Sodium chloride and Calcium chloride brines as well completion fluids of Oil and

Gas wells.

An object of the present invention is to find a suitable formulation as a corrosion inhibitor for tubing and casing exposed to high density Sodium chloride and Calcium chloride brines as well completion fluids of Oil and Gas wells. An object of the present invention is to find a suitable formulation as a corrosion inhibitor for tubing and casing in CO ₂ rich gas lift operated Enhanced Oil Recovery (EOR) wells.

Summary of the invention

Accordingly, the present disclosure provides a non chromate corrosion inhibitor composition capable of forming a thin film on a metallic surface wherein said metallic surface is exposed to medium of pH 4 - 4.5. The disclosure also provides a composition comprising at least a cathodic corrosion inhibitor, an anodic corrosion inhibitor, and a metal complexing ligand, in the ratio of 40 to 80: 10 to 30: 20 to 40 respectively. Also the present disclosure provides a process for the preparation of formulation of a non chromate corrosion inhibitor composition.

In addition, the disclosure provides a process of inhibiting corrosion of metal exposed to high density sodium chloride or calcium chloride brine medium.

These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Features of Invention The Invention provides an alternate formulation which is non-carcinogenic hence environment friendly and provides inhibition of corrosion from Sodium Chloride and Calcium Chloride brines used as well completion and packer fluids in On-land and Off-shore 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.

It is a substitute to Sodium Dichromate, and efficiently inhibits the corrosion of

tubing and casing in CO ₂ rich gas lift operated Enhanced Oil Recovery (EOR) wells. Detailed Description of the Invention

The invention relates to corrosion inhibition of carbon steel Tubing and Casing in down-hole conditions in oil and gas wells.

5 In down-hole operating conditions the carbon steel tubing and casing are exposed to high density Sodium chloride or Calcium chloride brines which lead to corrosion of Carbon Steel. The formulation of the present invention mitigates the corrosion of carbon steel exposed to high density Sodium chloride and Calcium chloride brines. o The production of hydrocarbon by gas lift process is one of the Enhanced oil

Recovery (EOR) technique, in which produced gas under pressure injected into the well tubing at desired locations through gas lift valves to bring down the hydrostatic head in the tubing column and hence facilitate enhanced flow of hydrocarbons. The gas used for this purpose is the produced gas rich in Carbon dioxide, (CO ₂ ). The lift s gas rich in carbon dioxide in contact with high density brines reduces the pH to 4.0 - 4.5 and nullifies inhibition characteristics of Sodium dichromate.

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 0 corrosion inhibitor for mitigation of corrosion of tubular 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 philosophy of formulation design was aimed at mitigating the corrosion 5 mechanisms taking place on carbon steel exposed to these brines under down-hole operating conditions of Oil and Gas wells. The corrosion mechanism involves: i. General corrosion, ii. Localized pitting corrosion. o The present disclosure relates to a non chromate corrosion inhibitor composition capable of forming a thin film on a metallic surface exposed to medium of pH 4 - 4.5, said composition comprising at least a cathodic corrosion inhibitor, an anodic corrosion

inhibitor, and a metal complexing ligand, in the ratio of 40 to 70: 10 to 30: 20 to 40 respectively.

The constituents of the composition 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 present disclosure also relates to a non chromate corrosion inhibitor composition wherein the anodic corrosion inhibitor is a salt of a Group VB or a VIB metal.

The present disclosure further relates to a non chromate corrosion inhibitor composition wherein the cathodic corrosion inhibitor is a compound of element of Group IHB or a rare earth metal preferably chlorides of Cerium, Neodymium, Praseodymium, Samarium, Lanthanum and Yttrium. An embodiment of the present disclosure is a non chromate corrosion inhibitor composition wherein the metal complexing ligand is a water soluble salt of an organic hydroxyl acid.

Another embodiment of the present disclosure is a non chromate corrosion inhibitor composition wherein the anodic corrosion inhibitor is a compound of molybdenum or vanadium.

Yet another embodiment of the disclosure is a non chromate corrosion inhibitor composition wherein the anodic corrosion inhibitor is selected from ammonium heptamolybdate or sodium ortho vanadate.

Yet another embodiment of the disclosure is a non chromate corrosion inhibitor composition wherein the cathodic corrosion inhibitor is a rare earth chloride.

Yet another embodiment of the disclosure is a non chromate corrosion inhibitor composition wherein the cathodic corrosion inhibitor chloride is Cerium

chloride.

Yet another embodiment of the disclosure is a non chromate corrosion inhibitor composition wherein the organic hydroxyl acid is selected from glycolic acid, maleic acid, tartaric acid or citric acid. Yet another embodiment of the disclosure is a non chromate corrosion inhibitor composition wherein the- metal complexing ligand is tri sodium citrates- hydrate.

Yet another embodiment of the disclosure is a non chromate corrosion inhibitor composition comprising of Cerium chloride, Ammonium heptamolybdate, Tri-Sodium Citrate-2-hydrate in the ratio of 40 to 80: 10 to 30: 20 to 40 respectively.

Yet another embodiment of the disclosure is a non chromate corrosion inhibitor composition comprising of Cerium chloride, Sodium Ortho Vanadate, Tri- Sodium Citrate-2-hydrate in the ratio of 50 to 80: 15 to 25: 20 to 40 respectively. Yet another embodiment of the disclosure is a process for the preparation of formulation of a non chromate corrosion inhibitor composition comprising blending at least a cathodic corrosion inhibitor, an anodic corrosion inhibitor, and a metal complexing ligand, in the ratio of 40 to 80: 10 to 30: 20 to 40 respectively and dissolving the blended product in water as 2.5% solution. Yet another embodiment of the disclosure is a process of inhibiting corrosion of metal exposed to high density sodium chloride or calcium chloride brine medium comprising injecting requisite quantity of formulation of the non chromate corrosion inhibitor composition, to the brine medium to give the final cathodic concentration in brine in the range of 800 mg/1 to 1200mg/l, anodic concentration in brine in the range of 200mg/l to 400 mg/1 and ligand concentration in brine in the range of 400 mg/1 to 500 mg/1.

Yet another embodiment of the disclosure is a process of inhibiting corrosion of metal exposed to high density sodium chloride brine mediums comprising maintaining in the medium the formulation, wherein the requisite quantity of formulation to be injected to Sodium chloride brine to give the final cathodic concentration in brine in the range of 800 mg/1 to lOOOmg/1, anodic concentration in brine in the range of 200mg/l to 300 mg/1 and ligand concentration in brine is in the range of 400 mg/1 to 500 mg/1.

Yet another embodiment of the disclosure is a process of inhibiting corrosion of metal exposed to high density calcium chloride brine medium comprising maintaining in the medium the formulation, wherein the requisite quantity of formulation to be injected to Calcium chloride brine to give the final cathodic concentration in brine is in the range of l OOOmg/1 to 1200mg/l, anodic concentration in brine in the range of 300mg/l to 400mg/l and ligand concentration in brine in the range of 400mg/l to 500mg/l.

The present disclosure also relates to a composite comprising a metal surface coated with the non chromate corrosion inhibitor composition wherein the metal is carbon steel.

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 brines

Sodium Chloride {Specific gravity: 1.20}

The formulation can be prepared by blending Rare earth chloride, preferably Cerium chloride, CeCb-OH ₂ O, (MoI. wt: 354.48), transition element salt preferably Ammonium heptamolybdate ((NH4) ₆ Mo ₇ O ₂₄ .4H ₂ 0, (Mol.wt 1235.86), and hydroxyl acid salts preferably Tri-Sodium Citrate-2-hydrate((CeH5Na3O7 .2H ₂ O) (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 injected to Sodium chloride brine to give following final dosing rate: Cathodic concentration in brine: 800 mg/1 to lOOOmg/1

Anodic concentration in brine: 200mg/l to 300 mg/1

Ligand concentration in brine: 400 mg/1 to 500 mg/1 Formulation II: for Calcium Chloride brines

(Sodium Chloride + Calcium Chloride {Sp.Gr. 1.40}, Calcium Chloride (Sp.GR.1.41)

The formulation can be prepared by blending Rare earth chloride preferably Cerium chloride, CeCl ₃ -OH ₂ O, (Mol.wt.: 354.48), transition element salt preferably

Sodium Ortho Vanadate, Na ₃ VO ₄ .12H ₂ O) (Mol.wt 399.94), salt of hydroxyl acid preferably Tri-Sodium Citrate-2-hydrate ((C ₆ H ₅ Na ₃ O ₇ .2H ₂ O) (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 injected to Calcium chloride brine to give following final dosing rate:

Cathodic concentration in brine: lOOOmg/1 to 1200mg/l Anodic concentration in brine: 300mg/l to 400mg/l

Ligand concentration in brine: 400mg/l to 500mg/l 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 ⁰ 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 brine and Calcium Chloride brine. 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 400grit. 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 brine (Sp.Gr.1.20) and Calcium Chloride brines (Sp.Gr. 1.40). The worst case down-hole conditions of 14O ⁰ 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 lO ⁸ X W

Corrosion Rate = mpy A X T X D

wherein W : Coupon Weight loss, g

A : Surface (area,cm ² ) T: Exposure Time (hours) D: Density (g/cm ³ )

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 corrosion rates of L-80 coupon are given in tables 1, 2 & 3 below for (I) Sodium chloride brine, Sp.Gr.1.20, (II) Calcium Chloride + Sodium

Chloride brine, Sp.Gr. 1.40, and (III) Calcium Chloride brine, Sp.Gr. 1.41, respectively.

(I). For Sodium chloride brine, Sp.Gr.1.20; Test temperature: 140 ⁰ C

Table 1 Results of Corrosion rate for L-80 steel coupons in Sodium chloride brine

(II) For Calcium Chloride + Sodium Chloride brine, Sp.Gr. 1.40, Test temperature: 14O ⁰ C

Table 2 Results of Corrosion rate for L-80 steel coupons in Calcium Chloride + Sodium Chloride brine

(III), for Calcium Chloride brine, Sp.Gr. 1.41, Test temperature: 140 ⁰ C

Table 3 Results of Corrosion rate for L-80 steel coupons in Calcium Chloride brine