FORM-2 THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
COMPLETE
Specification
(See Section 10 and Rule 13)


COMPOSITION AND METHOD FOR DISSOLUTION OF STRONTIUM SULFATE SCALES
OIL & NATURAL GAS CORPORATION LIMITED
an Indian Company.
Mumbai Region, IOGPT, MM Department Panvel 410220, Dist.-Raigad, Maharashtra (India)

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THfe MANNER IN WHICH IT IS TO BE PERFORMED

Field of the Invention
This invention relates to a chemical composition for dissolving alkaline earth metal scales.
More particularly, the present invention relates to the chemical composition for dissolving the specific scales from oilfield equipment and oil/gas wells.
Background of the Invention
An oil well is a term for any perforation made through the earth's surface designed to find and release both crude oil and natural gas (hydrocarbons). The oil well is created by drilling a hole into the earth with a drilling rig. After drilling and casing the well, the well is enabled to produce oil and gas by making necessary modifications.
Oil wells are classified by the produced fluid such as wells that produce oil, wells that produce oil and natural gas, or wells that only produce natural gas. Another way to classify the oil wells is by onshore (land) or offshore wells (drilled in sea).
In the production of oil and gas from oil and gas wells, ordinarily salt water is also produced. The salt water very often contains high levels of scale-forming ions which tend to precipitate and form scales, whenever there is an abrupt change in physical conditions and or in chemical compositions. This results in the accumulation of large quantities of scales and precipitated salts which plug the inner tube and restrict the flow of oil and gas there from. Furthermore, scales are also formed on other components which include well casing, tubing and surface storage facilities and processing
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equipment such as pipes, valves, heating coils and tubes, separators and the likes. Scale- formation and accumulation of the precipitated salts adversely affects the production rate. Furthermore, at times it also stops the whole operation and therefore is always associated with severe cost implications.
The problem of scale deposition is well known in the oil industry, in oil field operations, processing and desalination.
Scales begin to form when the state of any natural fluid is disturbed in such a way that the solubility limit for one or more components is exceeded. The solubility limits change as a result of various conditional changes including temperature variation, pressure variation, change in pH, release of gases from formation fluids like C02 and contact with incompatible water. The temperature variation is the main cause of the scale deposition. The pressure has a less effect on scale solubility. But the pressure changes in the tubing are responsible for scale formations due to release of dissolved gases.
In order for a scale to form it must grow from a solution and the process is called nucleation. Nucleation can be classified as homogeneous nucleation and heterogeneous nucleation. In homogeneous nucleation, unstable clusters of atoms are formed within a saturated fluid. The atom clusters form small seed crystals triggered by local fluctuations in the equilibrium ion concentration in supersaturated solutions. On the other hand, when the crystal growth tends to initiate on a pre-existing fluid-boundary surface, it results in the onset of heterogeneous nucleation. Heterogeneous nucleation sites include surface defects such as pipe surface roughness or perforations in production liners, or even joints and seams in tubing and pipelines.
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Heterogeneous crystallization is a major cause of scale formation in oilfield. The scale in the production tubing may occur as a thick layer which adheres to the inside of the tubing. It has a thickness of a few centimeters and has crystals one cm or larger. The primary effect of scale growth on tubing is to lower the production rate by increasing the surface roughness of the pipe and reducing the flowing area. This results in differential pressure between reservoir and the tubing pressure going down resulting in decline in production. If mineral growth increases, then access to the lower sections of the well becomes impossible, and ultimately the mineral growth blocks production flowing through the tubing.
Tubing scales vary in chemical composition, which includes asphaltene or wax layers, and the layers of scales that are closest to the tubing may contain iron sulfides, carbonates or corrosion products.
The carbonate or sulfate scales of calcium near the well bore region have a finer particle size than Ba/Sr scales, in microns rather than centimeters. It blocks gravel packs and screens as well as matrix pores. The scales near the well bore are commonly formed after long periods of well shut-in because cross flow mixes incompatible waters from different layers.
Scale deposits in producing oil and gas wells are generally of four common types namely calcium carbonate, calcium sulfate, barium sulfate and strontium sulfate. Calcium carbonate scale is formed when the pressure decreases, releasing carbon dioxide and changing bicarbonates to carbonates. Calcium sulfate on the other hand is a relatively soluble salt. However, when close to its solubility limit, it can form hard deposits. Pressure decrease in the tubing lead
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to partial evaporation of water and result in super-saturation and rapid precipitation of the salt.
In general, deposits of barium sulfate and strontium sulfate result from water incompatibility. Once formed, these scales are nearly impossible to remove from the tubing, valves and surface equipment due to their low solubility. Different conventional methods have been known for removal of strontium scales such as scale inhibitors have been designed to poison nucleation that use chemicals and reduce the rate of scale formation to almost zero. Similarly, magnetic treatment of produced water, Sulfate reduction technology (nanofiltration), mechanical methods are also known.
Existing knowledge
A limited number of solvents are known in the prior art which are used for dissolving barium and strontium sulfates. However, the prior art processes require the use of a chelating agent, such as amino poly-carboxylic acids such as ethylene diamine tetra acetic acid (EDTA) and similar reagents. Other compounds suggested for dissolving strontium sulfate include crown ethers, cryptands and other exotic and expensive materials. Although EDTA has been frequently used for removing oilfield scales, its field experience has not been wholly successful. The rate of dissolution, especially for producing wells, is far too slow and is therefore not economically feasible. Even with the help of common solvents or surfactant flushes, EDTA type materials have difficulty in attacking oil-and asphaltene containing scales. Hence, due to the high cost of operations, there is a need for a solvent composition that completes the dissolution of scales at a faster rate and inexpensive during the operations in offshore fields.
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US patent 5,458,860 which is also published as W093241993 and European patent 0772696 which is also published as WO09607763 disclose a method for removing alkaline earth metal scales by contacting the scale with a scale removing solvent system while simultaneously transmitting sonic energy in the range of 1.5 KHz to 6.5 KHz through the solvent. However, the aforesaid method disclosed in the above patent documents is not feasible since directing the low frequency sonic energy to all parts of the well bore to remove alkaline earth metal scales is not easy.
US patents 5,200,117 and 5,084,105 disclose a process to remove barium sulfate scales and other sulfate scales by using a combination of polyaminopolycarboxylic acid like EDTA or DTP A along with monocarboxylic acids such as acetic acid, hydroxyacetic acid, mercaptoacetic acid and salicylic acid as a catalyst or synergist for the dissolution from wells, well stream processing equipment, pipelines and tubular goods used to produce oil from subterranean formation.
US patent 5,049,297 discloses a composition for dissolving alkaline earth metal sulfate scales by using polyaminopolycarboxylic acid like EDTA or DTPA as a chelating agent and thiosulfate or nitriloacetate anions as a catalyst or synergist from wells, well stream processing equipment, pipelines and tubes used to produce oil from subterranean formations. However, the aforesaid mentioned synergist interacts with the solvent extraction process making the process more complex.
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Objects of the Invention
It is an object of the present invention to provide a composition for removal of an alkaline earth metal sulfate scales.
Still another object of the present invention is to provide a chemical composition which is more effective in solubilizing sulfate scales, deposits or crystals.
Another object of the present invention is to provide a composition for effectively removing and dissolving sulfate scales from well bore, valves, tubing or pipeline, chemical equipment, boilers and heat exchangers.
Yet another object of the present invention is to provide a composition for removal of sulfate scales at a substantially faster rate,
A further object of the present invention is to provide an alkaline earth metal sulfate scale dissolution composition at a substantially reduced cost.
Definitions
As used in the present specification, the following words and phrases are generally intended to have the meanings as set forth below, except to the extent that the context in which they are used indicates otherwise.
"Celestite" means a mineral consisting of strontium sulphate. "DTPA" means Diethylene Triamine Penta-Acetic acid.
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"Tubing" means oil well production strings.
Summary of the Invention
In accordance with this invention there is provided a chemical composition for dissolving Strontium Sulphate scales comprising an intimate mixture of:
a. diethylene Triamine Penta-acetic Acid (DTPA) as a chelating
agent in the range of about 25% to 35% of the mass of the
composition;
b. oxalic acid as a synergist in the range of about 10% to 15% of the
mass of the composition; and
c. high alkaline pH maintaining agent in the range of about 65 to
75% of the mass of the composition.
Typically, the chelating agent is in the range of 0.3 to 0.6 M.
Typically, the oxalic acid is in the range of 0.1 to 0.7M.
Typically, the pH maintaining agent is one selected from a group consisting of sodium hydroxide, potassium hydroxide, cesium hydroxide and lithium hydroxide.
Preferably, the pH maintaining agent is potassium hydroxide.
Typically, the chemical composition is adapted to operatively maintain a pH in the range of 10.5 to 12 at the time of dissolution of the scales.
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In accordance with this invention there is also provided a method for dissolving Strontium Sulphate scales from oil wells deposited at various locations comprising following steps:
• mixing a predetermined quantity of Diethylene Triamine Penta-acetic Acid (DTPA) with an Oxalic acid;
• adding Potassium Hydroxide as a high alkaline pH maintaining agent; and
• introducing the composition into oil wells having Strontium Sulphate scales at a temperature in the range of 25 to 80 C to dissolve the various scales.
Brief Description of the accompanying Drawings
The invention will be described in detail with reference to the accompanying
drawings.
In the accompanying drawing, Figure 1 is a graphical representation showing
the comparison of amount of EDTA and DTPA dissolved on Y-axis with time
on X-axis.
Figure 2 is a graphical representation showing the effect of synergist on the celestite dissolution rate of DTPA, DTPA-Salicylate and DTPA-oxalate on Y-axis along with time on X-axis.
Figure 3 is a graphical representation showing the effect of temperature on the dissolution rate of celestite of DTPA-oxalic acid-KOH on Y-axis along with time on X-axis.
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Figure 4 is a graphical representation showing the effect of concentration on the
dissolution rate of celestite of DTPA-oxalic acid-KOH on Y-axis along with on
X-axis.
Fig-5 - A view of deposited scales in oil well tubing.
Detailed description of the Invention
In -the last few years strontium sulfate scale formations have impaired production in the oil wells thereby reducing the diameter and restricting the fluid flow and well productivity. The specific strontium sulfate scales are highly crystalline and are toughest to be handled once formed under critical conditions of temperature, pressure, water composition and oil water ratio. Out of few options available for removal of the scales like mechanically under reaming technology conveyed on heavy wall coiled tubing, chemical route is safer and less time consuming.
The present invention envisages a solvent composition for a treatment of dissolving alkaline earth metal scales formed in the reservoir, well bore, production tubing and flow lines comprising a chelating agent, a synergist or a catalyst and a pH maintaining agent.
Furthermore, the present invention also provides a process for dissolution of alkaline earth metal scales such as strontium/strontium sulfate from well bore, valves, tubing or pipeline, chemical equipment, boilers and heat exchangers at a faster rate.
In accordance with this invention, the process employs a solvent composition having high specific activity for strontium and strontium sulfate scales. The alkaline aqueous solvent system is injected in the well bore, tubing or pipeline
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in which the scale deposition has taken place. The chelating agent present in the solvent forms a complex ion with strontium ion which passes into the solution. This allows conversion of Strontium / strontium Sulfate to Strontium Oxalate which is easier to dissolve.
The solvent composition for dissolving and removing the stronium/strontium sulfate scales formed in the well bore, production tubing or surface pipelines comprises DiethyleneTriaminePentaacetic acid (DTPA) as a chelating agent or a chelant which is designed to form a stable complex with the cation of the alkaline earth metal scales forming material. DTPA is the preferred chelating agent since it forms most soluble complexes at greater reaction rate.
DTPA (Diethylene 'friamine Pentaacetic Acid) is a metal ion chelating agent with eight active metal-completing sites on each molecule and so can provide up to 8-fold coordination around a free metal ion. With increasing pH, DTPA becomes progressively de-protonated and the active ionic species in solution become negatively charged. At pH 12, where the Sr-DTPA complex has the greatest stability, all of the DTPA in solution exists as the penta-negative ion DTPA. A consequence of oil stability of the Sr-DTPA complex is that when a crystalline Strontium salt is immersed in DTPA solution at high pH, the DTPA5 anions have a strong tendency to complex with Sr++ ions exposed on Ore crystal surface. This result in the formation of a Sr-DTPA surface complex in which the Sr ions are part bonded to DTPA and part bonded to the sulfate ions in the celestite structure. The strong bonding between Sr++ and DTPA5 weakens the bonding to the sulfate ions and eventually the surface complex passes into solution, in solution the shape of a DTPA molecule has considerable degrees of freedom but when it complexes with a Sr2+ ion it forms a metal chelate in which the DTPA molecule wraps around" the metal ion providing 8-fold co-
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ordination to the Sr with 3 Nitrogen and 5 Carbon atoms. Thus DTPA is an
octo- dentate chelating ligand.
The concentration of the chelant in the solvent is preferably in the range of 0.3
to 0.6 M to result in an acceptable degree of scale removal. Chelant
concentrations in excess are usually not necessary because the efficiency of the
chelant utilization becomes lower at excess chelant concentrations.
Figure 1 shows the results of the dissolution experiments in terms of the
percentage of the 1 gram Strontium sulfate sample dissolved as a function of
time at 25 C. This plot shows that celestite dissolves at faster rate using DTPA
solvent composition as compared to the EDTA.
Oxalic acid is used as a synergist or catalyst to DTPA. There is an increase in the dissolution rate, although its specific role is not understood. It is believed that, the adsorption of the synergist or catalyst on the strontium sulfate surface modifies the surface crystal structure in such a way that the strontium in the modified crystal is easily removed by the chelating agent.
The concentration of the synergist in the solvent composition is in the range of 0.1M to 0.7M in order to achieve an increased efficiency of scale dissolution. Figure 2 shows that the presence of oxalic acid in the said composition clearly enhances the rate of dissolution and it is the most effective composition for dissolving the celestite scales at faster rate.
The scale removal is effected under highly alkaline conditions at pH values in the range of about 10.5 to about 12.0, with optimum values in the range of about 11 to 12, preferably at about 12. Maintaining pFI of the Solvent composition below 10.5 does not result in a required clear solution and if it exceeds above 12 then the rubber parts get deteriorated, deformed and lose out
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its strength. Lithium hydroxide and sodium hydroxide in presence of DTP A and synergist are not soluble at a pH 12. On the other hand, cesium is difficult to obtain, quantity-wise as well as price-wise. Therefore, Potassium Hydroxide (KOH) in the form of potash is the preferred pH adjusting agent for availability as well as for the cost.
The dissolution experiments are carried out at a temperature in the range of 25 to 80°C. The celestite scales dissolve at faster rate at higher temperature, which clearly shows that the dissolution rate is strongly temperature dependent indicating a thermally activated process as shown in figure 3.
In accordance with another aspect of this invention, there is provided a method for removing or dissolving scales from oil wells deposited at various locations comprising following steps:
Chemical composition comprising DTPA and oxalic acid with concentrations in the range of 0.05M to 0.7M of is prepared by admixing diethylene triamine pentaacetic acid (DTPA) (25% to 35% of the mass of the composition) as a chelating agent, oxalic acid as a synergist (in the range of about 10% to 15% of the mass of the composition) and KOH as high alkaline pH maintaining agent (in the range of about 65% to 75% of the mass of the composition). Celestite scales are collected from the tubing of Mumbai High fields and are ground to 70-mesh size powder for providing uniform surface area for dissolution. Celestite powder is dissolved in (100ml) of (0.5M) DTPA and oxalic acid solution and this solution is transferred to labeled plastic bottles (7 in number) with same size (100ml) and shape since the use of glass bottles at high temperature results in leaching of silica from the glass which eventually leads to precipitation of strontium silicate.
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Dissolution experiments are carried out at a temperature in the range of 25 to 80 C for 7 hrs. in an oven. The samples are taken out from each of the labeled bottle at specific time intervals of 5, 10, 30, 60, 120, 240 and 420 min. Only one sample from each bottle at a given time interval is collected. First sample at time interval of 5 min. is taken from the bottle labeled 1, second sample at time interval of 10 min. is taken from the bottle labeled 2. Likewise, seven bottles are collected at 7 different time intervals from the labeled bottles. The dissolution rate at each of the time interval for respective labeled sample bottles is determined by measuring the mass of the residual celestite after drying it completely.
During extensive experimentation, the inventors of the present invention have surprisingly found out that though higher concentrations of the solvent compositions are preferable for dissolving comparatively larger aggregates of scales from production-tubing and well-bore, the most dilute solvent compositions (i.e. 0.05M) have proven to be particularly effective in dissolving trace amount of scales from reservoir and that too at a faster rate of dissolution. Contrary to the conventional solvent compositions (with concentrations > 0.5M), faster and better dissolution rates are observed when dilute solvent compositions (that is 0.05M) are used for dissolution of trace amounts of celestite scales accumulated in parts like reservoir. For example, the DTPA based formulation formulated in accordance with this invention takes only 4 to 5 hours to complete the job.
Figure 4 shows that the most dilute solvent composition (that is 0.05M) has the faster dissolution rate and can be employed for cleaning the whole well bore area. On the other hand, in tubing the Celestite deposition is localized along the
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walls and to clear the tubing faster, more concentrated solvent composition (that is 0.5M) can be used.
The invention will now be described with respect to the following examples which do not limit the invention in any way and only exemplify the invention.
Example 1: only EDTA is used
The solvent composition was prepared by dissolving 20gm of EDTA (0.5M) in
distilled water. The pH of the solution was adjusted to 12 by addition of KOH
(60 gm) to obtain solvent composition.
Strontium sulfate scales were collected from the tubing of Mumbai High fields
and were ground to 70-mesh size powder for providing uniform surface area for
dissolution.
Solvent composition (100ml) prepared above was transferred to each of the
labeled plastic bottles (7 in number) with same size (250ml) and shape. The use
of glass bottles were specifically avoided because glass bottles at high
temperature results in leaching of silica from the glass which eventually leads to
precipitation of strontium silicate. Uniform quantity of celestite powder (1gm)
was introduced in each of the labeled plastic bottle.
Dissolution experiments were carried out at 25 C for 7 hrs. The samples were
taken out from each of the labeled bottle at specific time intervals of 5, 10, 30,
60, 120, 240 and 420 min. Only one sample from each bottle at a given time
interval was collected. First sample at the time interval of 5 min was taken from
bottle labeled 1; second sample at time interval of 10 min was taken from the
bottle labeled 2. Likewise, seven samples were collected at abovementioned 7
different time intervals from the labeled bottles.
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The dissolution rate at each of the time interval for respective labeled sample bottles was determined by measuring the mass of the residual celestite after drying it completely.
Example 2: (only DTPA is used)
The same procedure was carried out as described in example 1, except that 0.5 M solution of DTPA was prepared by adding (20 gm) of DTPA in 1000ml of distilled water and the pH was adjusted to 12 by adding (60 gm) of KOH.
Example 3: (Preparation of DTPA-Qxalic acid-KQH)
Oxalic acid solution (0.5 M) was prepared by mixing (6 gm) of oxalic acid in
500 ml of distilled water. DTPA solution (0.5 M) was prepared by mixing (20
gm) of DTPA in 500 ml of distilled water. Both the solutions were mixed in
equal proportions to obtain a DTPA and oxalic acid solution (0.5 M).
The pH of the resulting solution was adjusted to 12 by addition of KOH (60
gm) to obtain a solvent composition.
Strontium sulfate scales were collected from the tubing of Mumbai High fields
(oil wells) and were ground to 70-mesh size powder for providing uniform
surface area for dissolution.
Solvent composition (3 00ml) prepared above was transferred to each of the
labeled plastic bottles (7 in number) of same size (250ml) and shape. The use of
glass bottles were specifically avoided because glass bottles at high temperature
results in leaching of silica from the glass which eventually leads to
precipitation of strontium silicate. Uniform quantity of celestite powder (lgm)
so obtained was introduced in each of the labeled plastic bottle.
Dissolution experiments were carried out at 80°C for 7 firs in an oven. The
samples were taken out from each of the labeled bottle at specific time intervals
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of 5, 10, 30, 60, 120, 240 and 420 min. Only one sample from each bottle at a given time interval was collected. First sample at the time interval of 5 min was taken from bottle labeled 1, second sample at time interval of 10 min was taken from the bottle labeled 2. Likewise, seven samples were collected at above mentioned 7 different time intervals from the labeled bottles. The dissolution rate at each of the time interval for respective labeled sample bottles was determined by measuring the mass of the residual celestite after drying it completely.
Example 4: (With DTPA-Qxalic acid)
Similar experiment was carried by following the procedure as described in example 3, except that the solvent composition was prepared by mixing (0.5M) DTPA and (0.5M) oxalic acid in distilled water and the dissolution experiments were carried out 25°C.
Example: 5 (With DTPA- Salicylate)
Similar experiment was carried by following the procedure as described in example 3, except that the solvent composition was prepared by mixing together (0.5M) DTPA with (0.5M) Salicylate in distilled water and the dissolution experiments were carried out 25°C.
Example 6: (With DTPA-oxalic acid-NaOH)
This experiment was carried out as per example 3, wherein the solvent composition was prepared by mixing together DTPA (0.5M) and oxalic acid (0.5M) in distilled water and the pH was adjusted to 12 by adding (60 gm) of solid NaOH and the dissolution experiments were carried out 25 C.
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Example 7; (DTPA-oxalic acid-KOH at 25˚C)
The experiment was carried out as per example 3, wherein the solvent composition was prepared by mixing together DTPA (0.5M) and oxalic acid (0.5M) in distilled water and the pH was adjusted to 12 by adding KOH. The dissolution experiments were carried out at 25°C.
Example 8: (DTPA-oxalic acid-KOH at 80°C)
The experiment was earned out as per example 3, wherein the solvent composition was prepared by mixing together DTPA (0.5M) and oxalic acid (0.5M) in distilled water and the pH was adjusted to 12 by adding KOH.
Example 9: (Preparation of 0.25M solvent composition)
The solvent composition as prepared in example 3 was further diluted to
(0.25M) by adding distilled water and the pH was readjusted to 12 by adding
KOH.
Example 10: (Preparation of 0.125M solvent composition)
The solvent composition as prepared in example 10 was further diluted to
(0.125M) by adding distilled water and the pH was readjusted to 12 by adding
KOH.
Example 11: (Preparation of 0.05M solvent composition)
The solvent composition as prepared in example 3 was further diluted to
(0.05JV1) by adding distilled water and the pH was readjusted to 12 by adding
KOH.
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Field study:
In order to verify the results observed in the laboratory experiments, field
studies were carried out in six wells. Each of the selected wells had drainage
radius of 2.5 meters. Scales from each of these wells were assessed to calculate
exact proportions of the minerals deposited in these wells. Results of the
analysis of scales collected from the selected wells are provided in the table
below.
Scale Analysis:

SR. NO COMPOSITION ANALYSIS
BARC-
1 BARC-2 UNI-CHEM PETRO-LITE GEO CHEM-1 GEO CHEM-
2
1 SrS04 93% 93.2% 93% 93% 94.4% 94%
2 CaC03 - - -
3 BaS04 2% 5% 3%
4 CaS04 - 2% 4%
The results clearly showed that proportion of strontium sulfate was the highest (in the range of about 93% to 95%) in all the scale samples. Assuming the porosity of the wells to be 20%, the solvent requirement per meter of perforated interval was calculated by extrapolating the experimental quantity and was found out to be 3 m3(3900 liters). Considering 5 meters of perforated interval for each well, the total solvent requirement for each well was calculated (3900 x 5=19,500 liters, ~ 20 m3). Accordingly, this total solvent composition for each of the wells was prepared using required/calculated
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quantities of the respective active ingredients. The escalated quantities of active ingredients of the solvent composition for wells are provided in the table herein below.

Sr.
No. Chemical Name Quantity for 1 m3
(kg) Quantity for 20 m3 (kg)
1. DTPA (di-ethylene triamine penta-acetic acid) 19.80 396.00
2. Oxalic acid 6.35 127.00
3. Potassium hydroxide 60.00 1200.00
Six offshore wells were selected and identified for the evaluation based on the well history, pressure and production history, produced water chemistry and data to ascertain obstruction due to the scaling. Based on the well data, detailed job designs were prepared. These six offshore wells were treated with the developed solvent composition for removal of celestite scales from tubing as well as from surface bore area. Out of six offshore wells Well No. A was treated with 0.25M solvent composition, Well No. B was treated with 0.125M solvent composition, well no. C was treated with 0.05M composition and remaining three well namely D, E and F were treated with 0.5M solvent composition.
From the detailed analysis, it was observed that scale dissolution using said solvent composition was found to be technically & economically successful in all wells (pay back period 14.66 days only). It was observed that 0.05M solvent composition dissolves the celestite scales in formation at faster rate and covered the whole well bore area. However, in tubing celestite deposition was localized
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along the walls and 0.5M solvent composition was found to be more suitable to remove the deposited scales at a faster rate.
The solvent composition of the present invention was tested on pilot basis. Net gain of 955 bbl per day of liquid was reported during the testing while the corresponding net gain in extra oil was reported to be 273 bbJ per day. Respective liquid gain from each of the wells is provided in the following table.
Field results

Well No. Liquid gain bbl/day
A 3
B 20
C 94
F 572
D 103
E 163
Net 955
While considerable emphasis has been placed herein on the specific structure of the preferred embodiment, it will be appreciated that many alterations can be made and that many modifications can be made in the preferred embodiment without departing from the principles of the invention. These and other changes in the preferred embodiment as well as other embodiments of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.
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Claim:
1. A chemical composition for dissolving Strontium Sulphate scales
comprising an intimate mixture of:
a. diethylene Triamine Penta-acetic Acid (DTPA) as a chelating
agent in the range of about 25% to 35% of the mass of the
composition;
b. oxalic acid as a synergist in the range of about 10% to 15% of the
mass of the composition; and
c. high alkaline pH maintaining agent in the range of about 65 to
75% of the mass of the composition.
2. A chemical composition as claimed in claim 1, wherein the chelating agent is in the range of 0.3 to 0.6 M.
3. A chemical composition as claimed in claim 1, wherein the oxalic acid is in the range of 0.1 to 0.7M.
4. A chemical composition as claimed in claim 1, wherein the pH maintaining agent is one selected from a group consisting of sodium hydroxide, potassium hydroxide, cesium hydroxide and lithium hydroxide.
5. A chemical composition as claimed in claim 1, wherein the pH maintaining agent is potassium hydroxide.
6. A chemical composition as claimed in claim 1, wherein the chemical


composition is adapted to operatively maintain a pH in the range of 10.5 to 12 at the time of dissolution of the scales.
7. A method for dissolving Strontium Sulphate scales as claimed in claim 1 from oil wells deposited at various locations comprising following steps:
• mixing a predetermined quantity of Diethylene Triamine Penta-acetic Acid (DTPA) with an Oxalic acid;
• adding Potassium Hydroxide as a high alkaline pH maintaining agent; and
• introducing the composition into oil wells having Strontium Sulphate scales at a temperature in the range of 25 to 80 C to dissolve the various scales.
Dated this 19th day of August 2008.

Mohan Dewan OfR. K. Dewan&Co Applicants' Patent Attorneys

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