Field of the invention
[0001] The present invention relates, generally, to the field of wellbore fluids used in hydrocarbon bearing subterranean formations. More particularly, the present invention relates to an improved composition of a completion fluid for use in ultra-high pressure and high temperature oil and gas exploration.
Background of the invention
[0002] Oil and gas are obtained from underground reservoirs in the earth by drilling a hole, known as wellbore, up to a target depth. Once the target depth is attained, telescopic steel tubes called casing, are placed to stabilize the wellbore. The wellbore is subsequently subjected to a completion process prior to recovery of oil and gas from the wellbore. Completion process typically comprises cementing the casing, perforating the casing, and placing tubing and pumps to facilitate recovery of oil and gas. To minimize any damage to the permeability of rock formations in wellbore, completion fluids are employed during completion operations. Primary functions of completion fluid are to provide pressure control to maintain wellbore stability, to prevent formation fluids from entering the wellbore, to minimize damage of production zone of wellbore, to minimize corrosion of tubing and pipes within wellbore, and to provide carrying capacity for debris that is formed during drilling of borewell.
[0003] In order to cater to the incessant demands for energy, extensive exploration and production drives are necessary for subterranean reservoirs operating under ultra-high pressure and high temperature (ultra-HPHT) . It has been observed that elevated temperature and pressure limit the use of conventional completion fluids as it institutes tough technical concerns such as requirement of high density, low turbidity, high
2

thermal stability, appropriate flow behavior and low corrosion for use in ultra-HPHT operations.
[0004] Conventionally, completion fluids are typically near saturated brines of bromides or chlorides to achieve a desired density. The corrosive nature of bromides and chlorides due to its low pH combined with high temperature conditions tend to significantly increase corrosion of carbon steel pipes and casing within the wellbore. Also, use of corrosion inhibitors increases cost of operating a wellbore and also, there are limited options for corrosion inhibitors for application in ultra-HPHT conditions. These factors, therefore, restrict the use of bromide or chloride-based completion fluids in ultra-HPHT operations. Further, use of cesium salts (such as formate or acetate) in completion fluid in ultra-HPHT operations is not economically viable.
[0005] Conventionally, alternative to cesium formate salts are, sodium formate and potassium formate which exhibits specific gravity of only 1.3 and 1.57 respectively at saturation, making them unsuitable as completion fluid at ultra-HTPT applications without the use of weighing agents. Therefore, weighting agents, such as, tri manganese tetra oxide
(Mn304) are added along with sodium and potassium formate brines for enhanced density. However, these weighing agents are not completely soluble in the completion fluid, and therefore the undissolved suspended solid particles of different sizes in the completion fluid may interfere with oil and gas recovery operations by blocking reservoir pores. The suspended solid particles may result in reduction of permeability of pores in production zones of wellbore, thereby decreasing overall production of the well and exhibiting unsatisfactory results for minimizing formation damage in wellbore, particularly in ultra-HPHT applications.

[0006] Standard weighing agents that are conventionally added along with brines in the completion fluids are, barite, haematite, lignite, manganese tetroxide, etc. However, these weighing agents remain suspended in the completion fluid and tend to clog pores in the formation. In addition, weighing agents also affect fluid rheology and have an abrasive effect on operational equipment. Certain class of weighing agents, such as, iron oxide exhibits magnetic properties which adversely affects function of equipment such as pipes and tubing within the wellbore. Existing completion fluids, therefore, do not meet the requirements for ultra-HPHT oil and gas applications that function throughout the lifetime of wellbore to enhance productivity and to prevent collapse of wellbore under ultra-HPHT conditions.
[0007] In light of the aforementioned drawbacks, there is a need to formulate a completion fluid for ultra-HPHT oil and gas applications. There is a need for a completion fluid having high density, low turbidity, low corrosion rate, high thermal stability, and long-term storage stability for ultra-HPHT well applications. Further, there is a need for a completion fluid that maintains a desired viscosity value to enhance ability to carry debris and dirt formed during the wellbore drilling operations and resist thermal degradation at ultra-HPHT conditions of the well.
Summary of the Invention
[0008] In various embodiments of the present invention, an improved composition of a completion fluid is provided. The composition of the completion fluid comprises a mixture of an organic salt, an inorganic salt and two additives. In an embodiment of the present invention, the two additives of the completion fluid are a chelating agent and a stabilizer. In an embodiment of the present invention, the composition of

to 16 % w/w, an inorganic salt in a range from 30 to 34 % w/w, a chelating agent in a range from 25 to 34 % w/w and a stabilizer in a range from 0.036 to 0.17 % w/w. The completion fluid composition exhibits strong intermolecular forces of attraction between molecules resulting in a stable complex formation. In an embodiment of the present invention, the ratio of salts to additives in the completion fluid is in a range of 1.5-1.8. In various embodiments of the present invention, the completion fluid exhibits high density, low turbidity, high thermal stability, long term storage stability and low corrosion rate.
[0009] In another embodiment of the present invention, a process for preparation of an improved completion fluid for ultra-high pressure and temperature conditions in wellbore is provided. The process comprises the steps of dissolving a predetermined amount of organic salt in an aqueous base to form a solution. To the solution, a predetermined amount of inorganic salt is added and subjected to stirring till complete dissolution of inorganic salt is achieved to obtain a mixture. Subsequently, a predetermined amount of a chelating agent and a predetermined amount of a stabilizer is added in succession and stirred till a homogenous and clear solution of the completion fluid is obtained. The completion fluid obtained exhibits strong intermolecular forces of attraction between molecules which result in a stable complex formation.
Brief description of the drawings
[0010] The present invention is described by way of embodiments illustrated in the accompanying drawings herein:
[0011] Fig. 1 is a thermogravimetric analysis plot of the completion fluid with and without stabilizer in accordance with an embodiment of the present invention;

[0012] Fig. 2 is a graph depicting flow curve of the completion fluid at 10 bars pressure and at (a) 25°C and (b) 100°C, in accordance with an embodiment of the present invention;
[0013] Fig. 3 is a graph depicting flow curve of the completion fluid over a range of temperatures, in accordance with an embodiment of the present invention;
[0014] Fig. 4 is a graph depicting FTIR spectra of the individual salt solutions and blend, in accordance with an embodiment of the present invention;
[0015] Fig. 5 is a graph depicting FTIR spectra of DTPA, salts blend with DTPA and salts blend with DTPA and HPC in accordance with an embodiment of the present invention;
[0016] Fig. 6 is a graph depicting corrosion rate of mild steel in the completion fluid, in accordance with an embodiment of the present invention; and
[0017] Fig. 7 is a graph depicting corrosion rate of API 5L grade X60 steel in the completion fluid having 2% HPC and 4% HPC, in accordance with an embodiment of the present invention.
Detailed description of the invention
[0018] The present invention discloses an improved composition of a completion fluid for ultra-high pressure and high temperature applications, in accordance with an embodiment of the present invention. The invention provides for a completion fluid that exhibits a surprising effect of high density, low turbidity, high thermal stability, long term storage stability, high viscosity, and low corrosion rate. Further, the invention provides for a process for preparation of the improved completion fluid, in accordance with various

[0019] The disclosure is provided to enable a person having ordinary skill in the art to practice the invention. Exemplary embodiments herein 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. The terminology and phraseology used herein 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 herein. For purposes of clarity, details relating to technical material that is known in the technical fields related to the invention have been briefly described or omitted so as not to unnecessarily obscure the present invention.
[0020] In various embodiments of the present invention, the completion fluid of the present invention, by way of the composition, achieves superior properties for use in high pressure and high temperature oil and gas well exploration.
[0021] In an embodiment of the present invention, the completion fluid of the present invention is an aqueous base comprising a mixture of an organic salt, an inorganic salt, and two additives. In an embodiment of the present invention, the ratio of salts to additives is in a range of 1.5 to 1.8. The salts and additives of the present invention act synergistically to exhibit multiple functions thereby eliminating the need for additional additives such as dispersing agent, viscosifying agent, defoaming agent, tackifier, high temperature protective agent, scale inhibitor, etc. In an exemplary embodiment of the present invention, the

[0022] In an exemplary embodiment of the present invention, a composition of a completion fluid comprises an organic salt in a range from 11 to 16 % w/w, an inorganic salt in a range from 30 to 34 % w/w, a chelating agent in a range from 25 to 34 % w/w and a stabilizer in a range from 0.036 to 0.17 % w/w.
[0023] In an embodiment of the present invention, anions of organic and inorganic salts are selected from a group comprising of formates and iodides. In an embodiment of the present invention, the organic salt is selected from a group comprising of sodium formate, potassium formate, preferably potassium formate. In an embodiment of the present invention, the inorganic salt is selected from a group comprising of sodium iodide, potassium iodide, strontium iodide, barium iodide, zinc iodide, preferably zinc iodide. Zinc iodide is an inorganic compound having a high solid density, high molar mass and high solubility in water. The use of zinc iodide in the preparation of completion fluid acts as an enhancer of density along with HCOOK. The completion fluid is specifically formulated to be free of conventionally used chlorides and bromides of zinc and calcium, which are highly corrosive in nature.
[0024] In an embodiment of the present invention, the chelating agent is an aminopolycarboxylic acid selected from a group comprising of aspartic acid, iminodiacetic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid (DTPA), ethylene glycol-bis
(p-aminoethyl ether)-N, N, N',N'-tetraacetic acid, 1,2-
bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid,
tetraxetan, ethylenediamine-N,N'-bis (2-hydroxyphenylacetic acid), ethylenediamine-N,N'-disuccinic acid, or combinations thereof, preferably diethylenetriaminepentaacetic acid (DTPA).
[0025] In an embodiment of the present invention, the

having an average molecular weight in a range from 100000 to 500000 to form a clear solution in an aqueous base, for improving thermal stability and long-term stability of the completion fluid. In another embodiment of the present invention, the stabilizer is selected from a group comprising of polyethylene glycol, polyacrylamides, polyacrylic acid copolymer, polyvinyl alcohol, hydroxyethyl cellulose, hydroxypropyl cellulose (HPC), carboxymethyl cellulose, or combinations thereof, preferably hydroxypropyl cellulose (HPC) .
[0026] Advantageously, the completion fluid in accordance with an embodiment of the present invention exhibits high density and high solubility of ingredients and can be formulated to a desired density at a low cost. Further, the completion fluid prevents formation damage, has low water activity and osmotic effect on shales thereby eliminating formation of precipitate and is capable of withstanding a significant amount of CO2 influx within the wellbore. The completion fluid also forms a protective carbonate layer in the presence of CO2 on the steel tubing within the wellbore, thereby reducing the high initial corrosion rates.
[0027] Further, advantageously, the completion fluid of the present invention does not employ weighting agents to enhance density. The density of completion fluid of the present invention is enhanced by addition of a chelating agent in the preparation of the completion fluid, preferably diethylenetriaminepentaacetic acid (DTPA) in accordance with an embodiment of the present invention. The addition of chelating agent increases the density by exceeding the solubility of salts, so that more salts can be added to increase density.
[0028] In an embodiment of the present invention, a stable

completion fluid which makes the completion fluid solid-free, as they remain in a dissolved state. In an exemplary embodiment of the present invention, the stability constant of DTPA with Zn2+ ions is as high as 18.75 for 1:2 (metal: chelate), thereby exceeding solubility limit of the salts required to build density. DTPA enhances the solubility of salts and stabilizes metal ions in a solubilized form in the completion fluid for a prolonged period. In addition, DTPA also renders the completion fluid of the present invention free of suspended solids, thereby reducing the turbidity. In an exemplary embodiment of the present invention, the completion fluid exhibits a low turbidity of about 4 NTU (Nephelometric Turbidity Units), thereby reducing damage to the production zone of the wellbore. The low turbidity is an indication that all the compounds in the completion fluid remain in a dissolved state. Advantageously, the completion fluid does not employ any other additives like dispersing agent, defoaming agent etc.
[0029] In an embodiment of the present invention, by way of the composition, chelating agent enhances the solubility of the salts, and the stabilizer enhances the stability of the completion fluid in ultra-HPHT applications. In an exemplary embodiment of the present invention, the stabilizer is hydroxypropyl cellulose (HPC) having a molecular weight in a range from 1,00,000 and 3,70,000. HPC binds the iodide ions thereby forming a polymeric iodophor (polymer-iodine complex). The stabilizer imparts properties of thermal stability, corrosion resistance, desirable rheological behavior and long-term stability to the completion fluid. The stabilizer imparts shear thinning behavior to the completion fluid over a wide range of shear rates as it would experience a range of shear rates during mixing, pouring, pumping and pipe flow. The stabilizer also imparts shear thinning behavior to the completion fluid over a wide range of temperatures as it would experience a gradient of temperature while circulating in a

[0030] In an embodiment of the present invention, the additives in the completion fluid, exhibits strong intermolecular forces of attraction with metal ions, thereby achieving high viscosity for ultra-HPHT oil and gas operations. The additive also lowers the crystallization temperature of the completion fluid, further enhancing the stability of the completion fluid.
[0031] In accordance with an embodiment of the present invention, the completion fluid, comprises of only two salts and two additives, yet attains superior properties to satisfy the requirements of high pressure and high temperature well applications. The completion fluid surprisingly achieves high density, low turbidity, low corrosion and high thermal stability as well as long term storage stability, by way of selection of salts and additives in the specified amount in accordance with various embodiments of the present invention.
[0032] Advantageously, the completion fluid is devoid of weighting agents and corrosion inhibitors and yet exhibits a high density of 2.069 - 2.23 g/cm3 and a very low steady state corrosion rate. Additionally, the components of the completion fluid are in a dissolved state, thereby rendering the completion fluid solid free. Low turbidity of the completion fluid decreases damage to formation in the wellbore. Further, advantageously, the completion fluid of the present invention exhibits low corrosion rate without the use of additional corrosion inhibitor. The components of completion fluid of the present invention serve multiple functions by way of functional interactions therebetween, and use of additives like dispersing agent, viscosifying agent, defoaming agent, tackifier, high temperature protective agent, scale inhibitor, etc. are eliminated.

[0033] The properties of the completion fluid are provided in table 1, in accordance with an embodiment of the present invention.
Table 1

Composition Density (g/cm3) Turbidity (NTU) Thermal Stability Corrosion Rate
HCOOK Znl2
DTPA HPC 2.069 2.23 4 (±0.3) Upto 250°C < 2 mpy
[0034] As shown in table 1, the completion fluid of the present invention has a high density in a range from 2.082-2.23 g/cm3, low turbidity of about 4 NTU, high thermal stability up to 250°C and long-term storage stability with low steady state corrosion rate of <2 mpy (mils per year penetration) for carbon and API grade steel. The completion fluid exhibits a long-term storage stability of over 200 days. The completion fluid of the present invention also exhibits a shear thinning behavior over a wide range of shear rates under high temperature high pressure conditions.
[0035] In an embodiment of the present invention, a process for preparation of completion fluid is provided. The process comprises the steps of dissolving a predetermined amount of potassium formate to an aqueous base, preferably water. To the potassium formate solution, a predetermined amount of zinc iodide is added and stirred for complete dissolution. Further, a predetermined amount of diethylenetriaminepentaacetic acid
(DTPA) is added to the solution mixture and stirred till a complete dissolution is achieved. A measured volume from an already prepared hydroxypropyl cellulose (HPC) solution in water is added to the solution mixture and stirred till a homogenous and clear solution is obtained, which is the completion fluid. In an embodiment of the present invention, the process is carried out at room temperature and atmospheric pressure. The stirring of the solution is carried out using

techniques, but are not limited to sonicator, high shear mixer, agitator with double propeller or helical impeller, etc.
[0036] The disclosure herein provides for examples illustrating the process for preparing the completion fluid in accordance with an embodiment of the present invention. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments may be practiced and to fulther enable those of skill in the art to practice the embodiments. Accordingly, following examples should not be construed as limiting the scope of the embodiments herein.
Working Examples Example 1
[0037] A measured amount of water was poured into a high shear mixer, to which 15.97 wt. % of potassium formate was added. After dissolution of potassium formate, 31.95 wt.% of zinc iodide was added to the solution and agitated until a complete dissolution to obtain a mixture. A total of 33.73 wt.% of DTPA was added to the mixture in small quantities in succession and agitated till it was completely dissolved. A measured volume from an already prepared 2% hydroxypropyl cellulose (HPC) in water was added such that the concentration of HPC was 0.036 wt.% and agitated till a homogenous and clear solution was obtained (which is the completion fluid). The measured density of the formulated completion fluid was 2.082 g/cm3. The completion fluid showed a turbidity of 4.03 NTU and a long-term stability of over 200 days. In this example, density was measured using a density bottle and a density meter. Turbidity was measured using a turbidity meter.
[0038] The completion fluid prepared in accordance to Example 1 was evaluated for thermal stability, viscosity, corrosion

rate and extent of interaction between the compounds, as demonstrated below.
Thermal Stability:
[0039] Thermogravimetric analysis (TGA) was conducted to determine the thermal stability of the completion fluid. The study was conducted in air atmosphere with a heating rate of 20°C min"1 to a temperature of 900°C. Thermogravimetric analysis was performed in air to investigate the oxidative degradation of material and to observe the impact of additives on the thermal stability of completion fluid was observed. As shown in Fig. 1, curve-I indicates a high-density formulation without the stabilizer, while curve-II indicates a high-density composition with a low molecular weight polymer stabilizer. Addition of the stabilizer clearly shifted the initial thermal degradation at around 100°C, thereby making the completion fluid thermally stable over high temperature. Table-2 provides data on the weight loss at various temperatures, up to 250°C.
Table 2

Sample Weight loss (%) at 100°C Weight loss (%) at 200°C Weight loss (%) at 250°C
I 10.561 22.257 28.707
II 4.166 19.7 24.092
[0040] The temperatures listed in the table were selected to be 100°C,200°C and 250°C. The temperature 100°C is the boiling point of water and the temperatures 200°C and 250°C are the lower and higher limits of operating conditions for ultra-HPHT. From table 2, it is clear that, the completion fluid of the present invention (Fig. 1-Curve-II) is thermally stable, indicating strong intermolecular forces of attraction between the components as it shows multi step degradation instead of

indicate that the completion fluid exhibits strong intermolecular forces of attraction between molecules which resulted in a stable complex formation, thus minimizing weight losses at harsh conditions such as a high heating rate and air environment. With the polymer-iodine complex formation, the stability of curve-II was improved showing reduced weight losses than curve-I, as shown in Fig. 1.
Viscosity
[0041] Figs. 2 and 3 depict flow behavior of the completion fluid obtained in a concentric cylinder geometry rheometer at high pressure and at various high temperatures. Fig. 2 and 3 clearly indicate that the completion fluid demonstrates shear thinning over a wide range of shear rates as well as temperatures. No sharp increase in viscosity was observed due to the effect of high pressure and temperature which indicates that the completion fluid is thermally stable and did not undergo crystallization with the gradient of temperature experienced in the wellbore, providing an appropriate thermo-rheological behavior for real-time applications during circulation in a well.
Fourier-transform Infrared Spectroscopy (FTIR)
[0042] Figs. 4 and 5 show the FTIR spectra of completion fluid. The analysis demonstrates interactions between the functional groups of chemical compounds and mechanism behind the high density, thermal stability as well as long-term stability in the completion fluid.
[0043] Fig. 4 (a) and (b) illustrate individual salt solutions while 4(c) indicates the blend of potassium formate and zinc iodide. Referring to Fig. 4(c), a sharp peak at 418 cm"1 indicates the Zn-0 bond formation thereby confirming

exist as polyiodides. The interaction between Zni2 and HCOOK resulted in a compatible blend thereby preventing precipitation.
[0044] From Fig. 5, FTIR spectrum of DTPA (a) and two salt solutions with DTPA (b) and salt solutions with DTPA and HPC
(c) were compared. Substantial changes in the spectra clearly indicate that there have been interactions among the major functional groups of the additives DTPA and HPC with the salts. This indicates the stability of fluid by complex formation from the spectral shift in infrared. In addition, the shifts to lower wave numbers indicate an increase in mass and therefore an improvement in density. Table 3 provides data on assignment of characteristic peaks and their shift during the sequential addition of compounds. Vibrations in -COOH such as C=0 and OH stretch as well as C-N vibrations in DTPA ascertain their participation in the complex formation. Here, DTPA binds to the zinc ions confirming that oxygen and nitrogen form the centers of coordination. Comparing the graphs (b) and (c) , the peak shifts confirm the polymer-iodide complex formation which supports improvement in density, thermal stability as well as the long-term stability of compounds. Table 3 shows the shift in characteristic peaks in FTIR as illustrated in Fig. 5.
Table 3

DTPA With DTPA With HPC
C-N 1200-1250 1109 1105
C=0 1635-1740 1620 1604
OH stretch 3082,3018 2960 2916
Corrosion Rate
[0045] From Fig. 6, it can be observed that mild steel shows highest corrosion rate of 5.88 mpy after 24 hours of exposure with the completion fluid of 2.08 g/cm3. The corrosion rate

the completion fluid. Subsequently, the corrosion rate becomes almost steady with further time of exposure up to 30 days.
[0046] Specific experiments were conducted with a low-carbon steel and API 5L X60 grade of steel. Table 4 illustrates data for corrosion rate vs time of exposure of API 5L X60 grade steel coupons in completion fluid of Example 1 with formate-based fluid having 2% HPC, in accordance with an embodiment of the present invention.
Table 4

Exposure Time Corrosion Rate (mpy)
12 hrs 1.59
24 hrs 2.03
3 days 1.19
5 days 0.99
7 days 0.89
11 days 0.83
21 days 0.84
[0047] Table 5 illustrates data for corrosion rate vs time of exposure of API 5L X60 grade steel coupons in completion fluid of Example 1 with formate-based fluid having 4% HPC, in accordance with an embodiment of the present invention.
Table 5

Exposure Time Corrosion Rate (mpy)
12 hrs 1.39
24 hrs 1.81
3 days 0.93
5 days 0.82
7 days 0.77
11 days 0.79
21 days 0.74

[0048] Fig. 7 is a graphical representation of data illustrated in Table 4 and Table 5, in accordance with an embodiment of the present invention.
Example 2
[0049] A measured amount of water was poured into a high shear mixer, to which 15.34 wt. % of potassium formate was added. After dissolution of potassium formate, 32.39 wt.% of zinc iodide was added to the solution and agitated for complete dissolution. A total of 26.66 wt.% of DTPA was added to the mixture in small quantities in succession and agitated till it was completely dissolved. A measured volume from an already prepared 2% hydroxypropyl cellulose (HPC) in water was added such that the concentration of HPC was 0.0368 wt.% and agitated till a homogenous and clear solution was obtained (completion fluid). The measured density of the formulated completion fluid was 2.109 g/cm3. The completion fluid showed a turbidity of 4.09 NTU and a long-term stability of over 200 days.
Example 3
[0050] A measured amount of water was poured into a high shear mixer, to which 11.16 wt.% of potassium formate was added. After potassium formate was dissolved, 33.47 wt.% of zinc iodide was added to the solution and agitated for complete dissolution. A total of 27.49 wt.% of DTPA was added to the mixture in small quantities in succession and agitated till it was completely dissolved. A measured volume from an already prepared 2% hydroxypropyl cellulose (HPC) in water was added such that the concentration of HPC was 0.0357 wt.% and agitated till a homogenous and clear solution was obtained (completion fluid). The measured density of the formulated completion fluid was 2.233 g/cm3. The completion fluid showed a turbidity of 4.33 NTU and a long-term stability of over 200 days.

Example 4
[0051] A measured amount of water was poured into a high shear mixer, to which 15.23 wt. % of potassium formate was added. After dissolution of potassium formate, 30.46 wt.% of zinc iodide was added to the solution and agitated for complete dissolution. A total of 25.38 wt.% of DTPA was added to the mixture in small quantities in succession and agitated till it was completely dissolved. A measured volume from already prepared 4% hydroxypropyl cellulose (HPC) in water was added such that the concentration of HPC was 0.17 wt.% and agitated till a homogenous and clear solution was obtained (completion fluid). The measured density of the formulated completion fluid was 2.0 69 g/cm3. The completion fluid showed a turbidity of 3.9 NTU and a long-term stability of over 200 days.
[0052] Table 6 below provides data on thermal stability, turbidity, long term stability and corrosion rate, in comparison with parameters that were measurable for existing completion fluids in accordance with various embodiments of the present invention.
Table 6

Lsting impositions Density
(g/c3) Thermal stability (°C) Turbidity (NTU) Long Term Stability Corrosion Rate
Chloride sed with cite and reral iitives 1.5- 2.4 210 - - -
Potassium cmate with bassium cophosphate 1.58-1.70

bh corrosion libitor and reral iitives
Potassium
cmate,
bassium
lydrogen
Dsphate,
sium formate
bh additives 1.7-1.9
Potassium cmate,
Lghting agent i other iitives 1.7-2.3 180
Potassium
cmate with
Dxidant,
-kifier,
ale inhibitor
i other
iitives 1.82 180 0.0007
mm/a at
80°C for
N80 steel
sheets
"hloride sed with Lghting snt,
erosion libitor and ier additives 1.2-1.68 90 - - 0.54-0.77
g.m"2. h"1
for N80
steel
formate brine bh weighting snt and iitives 1.8-2.5 200

Potassium cmate with iium citrate i other iitives 1. 65
"hloride and Dmide based 1.03-2.2 200 <50
npletion lid of Bsent
rention 2.069 -2.23 250 4(±0.3) >2 0 0 days <2 mpy
[0053] In the existing completion fluids of the above table 6, it was observed that existing completion fluid 1 comprises multiple additives or modifiers such as dispersing agent, a stabilizer, salts (six salts including chlorides), a high temperature protective agent, a high-temperature flow pattern regulator and a weighting agent. It also comprises weighting agent barite with other additives. The existing completion fluid 1 demonstrated a low density of 1.5-2.4 g/cm3. The thermal stability of existing completion fluid 1 was up to 210°C. Surprisingly, in comparison, the completion fluid in accordance with various embodiment of the present invention, exhibited higher density range of 2.069-2.23 g/cm3. The completion fluid demonstrated long term storage stability at room temperature over 200 days. The completion fluid of present invention also demonstrated a higher thermal stability, up to 250°C (ultra-high temperature wells range). Further, surprisingly, the completion fluid of the present invention demonstrated showed low corrosion effect on metallic parts of <2 mpy and a very long room temperature stability of over 200 days .
[0054] Existing completion fluid 2 discloses a strong-inhibition low corrosion brine completion fluid comprising of

potassium pyrophosphate), hydrophobic modified sodium alginate, clay anti-swelling agent, filtrate reducer, corrosion inhibitor and the balance of water. The density of the existing completion fluid 2 was reported in a range of 1.58-1.70 g/cm3. Advantageously, the completion fluid in accordance with various embodiment present invention, exhibited a higher density range of 2.069-2.23 g/cm3. The thermal stability of completion fluid of the present invention was high, that is, upto 250°C (ultra-high temperature wells range). Further, surprisingly, the completion fluid of the present invention demonstrated a low corrosion effect on metallic parts of <2 mpy and a very long room temperature stability of over 200 days.
[0055] Existing completion fluid 3 discloses a solid-phase-free high density well completion fluid, which is prepared with potassium hydroxide, potassium dihydrogen phosphate, potassium monohydrogen phosphate, potassium formate, potassium pyrophosphate, cesium formate, tackifier, filtrate-loss reducer and the balance water, with the density of the well completion fluid in a range of 1.7-1.9 g/cm3. Advantageously, the completion fluid of the present invention exhibited a higher density range of 2.069-2.23 g/cm3. The thermal stability of the completion fluid of the present invention was high, that is upto 250°C (ultra-high temperature wells range). Further surprisingly, the completion fluid of the present invention demonstrated a low corrosion effect on metallic parts of <2 mpy and a very long room temperature stability of over 200 days.
[0056] Existing completion fluid 4 discloses a high-density well drilling/completing working fluid comprising water, dispersant, potassium or sodium formate, high-temperature-resistant salt-resistant composite sulfonate copolymer, high-temperature-resistant lifted cutting agent, compound plugging

the completion fluid 4 was in a range of 1.7-2.3 g/cm3 and had a temperature resistance of 180°C. Surprisingly, the completion fluid of the present invention, exhibited a higher density, i.e., in a range of 2.069-2.23 g/cm3. The thermal stability of completion fluid of the present invention was high, that is upto 250°C (ultra-high temperature wells range). Further, surprisingly, the completion fluid of the present invention demonstrated showed a low corrosion effect on metallic parts of <2 mpy and a very long room temperature stability of over 200 days.
[0057] Existing completion fluid 5 discloses a temperature-resisting, low corrosive high-density solid-free test fluid comprising deoxidant, phosphate, tackifier, viscosity increasing agent, scale inhibitor and soluble formate
(potassium or sodium). The density of the completion fluid 5 was up to 1.82 g/cm3 and demonstrated a thermal resistance of up to 180°C. Surprisingly, the completion fluid of the present invention exhibited a higher density range of 2.069-2.23 g/cm3. The thermal stability of completion fluid of present invention was high, that is, upto 250°C (ultra-high temperature wells range). Further, surprisingly, the completion fluid of the present invention demonstrated a low corrosion effect on metallic parts of <2 mpy and a very long room temperature stability of over 200 days.
[0058] Existing completion fluid 6 discloses a solid-free well-completion confining liquid comprising several additives such as weighting agent, clay stabilizer, corrosion inhibitor, oxygen scavenger, and pH value regulator. The density of the well-completion confining liquid was low in a of range 1.20g/cm3-l.68g/cm3. Surprisingly, the completion fluid of the present invention exhibited a higher density range of 2.069-2.23 g/cm3. The thermal stability of completion fluid of the present invention was high, that is, upto 250°C (ultra-high

completion fluid of the present invention demonstrated a low corrosion effect on metallic parts of <2 mpy and a very long room temperature stability of over 200 days.
[0059] Existing completion fluid 7 discloses a well completion fluid comprising sodium formate, potassium formate and/or cesium formate, fluid loss additive, plugging agent, buffer agent, dispersing agent, defoaming agent, sepiolite and weighting agent adjusted according to density. The density of the completion fluid 7 was 1.8-2.5 g/cm3 and was found to be suitable for only well completion operation at the downhole temperature of 160-200°C. Surprisingly, the completion fluid of the present invention, exhibited a higher density range of 2.069-2.23 g/cm3. The thermal stability of completion fluid of the present invention was high, that is, upto 250°C (ultra¬high temperature wells range). Further, surprisingly, the completion fluid of the present invention demonstrated low corrosion effect on metallic parts of <2 mpy and a very long room temperature stability of over 200 days.
[0060] Existing completion fluid 8 discloses use of highly concentrated liquids based on organic and inorganic salts for drilling and completion of wells. Composition of completion fluid 8 includes aqueous solution of potassium formate, lithium citrate and/or sodium citrate, sodium or potassium amide sulfonate, amido sulfonate lithium, sodium formate, lithium formate and lithium/sodium amido sulfonate mixture wherein the density of the solution was about 1.65 g/cm3. Surprisingly, the completion fluid of the present invention exhibited higher density range of 2.069-2.23 g/cm3. The thermal stability of completion fluid of the present invention was high, that is, upto 250°C (ultra-high temperature wells range). Further, surprisingly, the completion fluid of the present invention demonstrated a low corrosion effect on metallic parts of <2 mpy and a very long room temperature stability of over 200

[0061] Existing completion fluid 9 discloses a composition for a solids-free drilling fluid comprising a simple or binary brine of divalent metals, a synthetic polymer, a biopolymer, an alkalizing agent, a bridging agent, a clays hydration inhibitor and a demulsifying surfactant. The density of the fluid of completion fluid 9 was 1.03 to 2.2 g/cm3 and the thermal stability of the fluid 200°C. Surprisingly, the completion fluid of the present invention exhibited a higher density range of 2.069-2.23 g/cm3. The thermal stability of completion fluid of the present invention was high, that is, upto 250°C (ultra-high temperature wells range). Further, surprisingly, the completion fluid of the present invention demonstrated a low corrosion effect on metallic parts of <2 mpy and a very long room temperature stability of over 200 days .
[0062] As demonstrated in table 6, the completion fluid in accordance with various embodiments of the present invention, exhibited superior properties as compared to already existing completion fluids.
[0063] 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 the scope of the invention.

We Claim:

1) An improved composition of completion fluid for ultra¬
high pressure and temperature conditions in wellbore,
comprising:
an organic salt in a range from 11-16 %w/w;
an inorganic salt in a range from 30-34 %w/w;
a chelating agent in a range from 25-34 %w/w; and
a stabilizer in a range from 0.036-0.17 %w/w, wherein the
completion fluid composition exhibits strong
intermolecular forces of attraction between molecules
which result in a stable complex formation.
2) The composition as claimed in claim 1, wherein ratio of the organic salt and the inorganic salt to the chelating agent and the stabilizer is 1.5-1.8.
3) The composition as claimed in claim 1, wherein the organic salt is selected from a group comprising of sodium formate, potassium formate.
4) The composition as claimed in claim 1, wherein the inorganic salt is selected from a group comprising of sodium iodide, potassium iodide, strontium iodide, barium iodide, zinc iodide.
5) The composition as claimed in claim 1, wherein the chelating agent is an aminopolycarboxylic acid selected from a group comprising of aspartic acid, iminodiacetic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid (DTPA), ethylene glycol-bis (p-aminoethyl ether)-N, N,N',N'-tetraacetic acid, 1,2-bis (o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid, tetraxetan, ethylenediamine-N,N'-bis (2-hydroxyphenylacetic acid), or ethylenediamine-N,N'-disuccinic acid.

The composition as claimed in claim 1, wherein the stabilizer is selected from a group comprising of polyethylene glycol, polyacrylamides, polyacrylic acid copolymer, polyvinyl alcohol, hydroxyethyl cellulose, hydroxypropyl cellulose (HPC) or carboxymethyl cellulose.
The composition as claimed in claim 6, wherein molecular weight of the stabilizer is in a range of Mw 1,00,000 to 5,00,000.
The composition as claimed in claim 1, wherein density of the completion fluid is in a range from 2-2.23 g/cm3.
The composition as claimed in claim 1, wherein turbidity of the completion fluid is less than 4 NTU (Nephelometric Turbidity Unit).
The composition as claimed in claim 1, wherein thermal stability of the completion fluid is upto 250°C and long-term stability is over 200 days.
The composition as claimed in claim 1, wherein corrosion rate of the completion fluid is less than 5 mpy (mils per year penetration) for carbon steel and less than 2 mpy for API grade steel.
A process for preparation of an improved completion fluid for ultra-high pressure and temperature conditions in wellbore, comprising the steps of:
dissolving a predetermined amount of organic salt in an aqueous base to form a solution;
adding a predetermined amount of an inorganic salt to the organic salt solution and stirring the solution to obtain a mixture;

adding a predetermined amount of a chelating agent to the mixture and stirring the mixture; and
adding a predetermined amount of a stabilizer to the mixture and stirring the mixture to obtain a homogenous clear solution of the completion fluid, wherein the completion fluid exhibits strong intermolecular forces of attraction between molecules which result in a stable complex formation.
13) The process as claimed in claim 12, wherein the process is carried out at room temperature under ambient atmospheric pressure.
14) The process as claimed in claim 12, wherein the aqueous base is water, the organic salt is potassium formate, the inorganic salt is zinc iodide, the chelating agent is diethylenetriaminepentaacetic acid and the stabilizer is hydroxypropyl cellulose.
15) The process as claimed in claim 14, wherein the predetermined amount of potassium formate is in a range from 11-16 %W/w, the predetermined amount of zinc iodide is in a range from 30-34 %W/w, the predetermined amount of diethylenetriaminepentaacetic acid is in a range from 25-34 %W/w, the predetermined amount of the stabilizer is in a range from 0.036-0.17 %W/w.