Description:Field of Invention

The present invention relates, generally, to the field of wellbore fluids used in hydrocarbon bearing subterranean formations. More particularly, the present invention relates to a single-phase retarded acid composition for stimulation of limestone formations during oil and gas exploration and production.

Background of the Invention

Hydrocarbons such as oil and gas are extracted from subterranean geologic formations by drilling a wellbore through a hydrocarbon bearing formation. Once a wellbore is drilled, completion activities are performed to enhance and control productivity. A common method to increase wellbore productivity is by treating the wellbore (also known as wellbore stimulation) with specialized chemicals that aid in stimulation.

Wellbore stimulation is typically performed using two techniques, viz. matrix acidizing or fracturing. In formations with a high limestone content, matrix acidizing is the preferred method to enhance wellbore connectivity. This involves injecting hydrochloric acid into the formation, where it reacts with and dissolves portions of the limestone rock and artificial flow channels known as wormholes are created. These channels facilitate hydrocarbon flow and help remove formation damage, thereby enhancing productivity. Acid Fracturing, on the other hand, involves injecting fluids at high pressure to create fractures in the formation in which subsequent pumping of hydrochloric acid non-uniformly etches the fracture face and imparts conductivity. The choice of matrix acidizing versus acid fracturing is primarily governed by formation permeability wherein the latter is preferred for formations having low permeability.

Conventional plain acid treatments face significant challenges, particularly under high-temperature conditions. One major issue is the limited penetration of the acid into limestone formations. When plain hydrochloric acid reacts with carbonate surfaces at elevated temperatures it leads to rapid facial dissolution of carbonates near the reservoir face, thereby restricting deeper radial penetration and reducing overall effectiveness of matrix acidizing. Furthermore, high reactivity of hydrochloric acid corrodes wellbore equipment, particularly under high-temperature conditions exceeding 200°F. This rapid reaction often limits the applicability of plain acid in matrix acidizing.

Traditionally, to address these challenges, various less-reactive acid formulations have been investigated. Organic acids such as formic and acetic acid, which react slower than hydrochloric acid, have been utilized. Another existing method involves emulsified acid systems, where inorganic acid is suspended in a water-in-oil emulsion with diesel serving as the external phase to slow the reaction rate. While emulsified acid systems enhance radial penetration, they also introduce issues like high viscosity, thermal instability at temperatures exceeding 120°C, environmental concerns related to diesel mixing time and a high demand for surfactants. Moreover, the potential for forming in-situ emulsions after treatment further complicates their practical use.

Efforts have been made to encapsulate inorganic acids in polymer gels, light oils, or similar materials, often in combination with surfactants and chelating agents. While these methods are designed to slow the reaction rate of plain hydrochloric acid with carbonate surfaces, their limitations reduce both their efficiency and feasibility in practical applications.

In light of the aforementioned drawbacks, there is a need for a composition that improves carbonate acidizing. Therefore, there is a need for a composition that provides more controlled acid reaction rates, allowing for deeper and more uniform radial penetration into limestone formations. Furthermore, there is a need for a composition that reduces the risk of premature acid consumption, minimizes damage to wellbore equipment, and improves overall well productivity while addressing the shortcomings of conventional methods.

Summary of the Invention

In various embodiments of the present invention, a single-phase retarded acid composition is provided. The composition comprises a mineral acid, an acid retarder, a biodegradable corrosion inhibitor, an iron control agent, and water. In an embodiment of the present invention, the acid retarder is a combination of surfactant A and surfactant B. In an embodiment of the present invention, the single-phase retarded acid composition comprises mineral acid in a range from 10 to 28 %w/V, acid retarder in a range from 0.5 to 3.0 %w/V, corrosion inhibitor in a range from 0.5 to 2.0 %w/V and iron control agent in a range from 0.1 to 1.0 %w/V, wherein the ratio of mineral acid, acid retarder, corrosion inhibitor, iron control agent, and water is in a range of 12-15:1-2:0.8-1:0.5-0.8:79-86. In an embodiment of the present invention, the single-phase retarded acid composition exhibits enhanced retardation factor as compared to conventional acid formulations.

In an embodiment of the present invention, the mineral acid is selected from a group comprising of hydrochloric acid, methane sulphonic acid, acetic acid, formic acid or their mixture thereof, preferably hydrochloric acid.

In an embodiment of the present invention, surfactant A is selected from a group comprising of lauryl dimethyl amine oxide, myristyl dimethyl amine oxide. In an embodiment of the present invention, the surfactant B is selected from a group comprising of Cocamidopropyl PG–Dimonium Chloride Phosphate, Myristamidopropyl PG–Dimonium Chloride Phosphate and Linoleamidopropyl PG–Dimonium Chloride Phosphate.

In an embodiment of the present invention, surfactant A is in a range from 0.1 to 2.0% w/V and surfactant B is in a range from 0.1 to 1.0% w/V.

In an embodiment of the present invention, the corrosion inhibitor is a combination of propargyl alcohol and cinnamaldehyde. In an embodiment of the present invention, the amount of propargyl alcohol is in a range from 0.1 to 1.0% w/v. In an embodiment of the present invention, the amount of cinnamaldehyde is in a range from 0.1 to 1.0% w/v.

In an embodiment of the present invention, the iron control agent is a combination of sodium erythroborate and disodium EDTA. In an embodiment of the present invention, the amount of sodium erythroborate is in a range from 0.05 to 0.1 % w/v. In an embodiment of the present invention, the amount of disodium EDTA is in a range from 0.1 to 1.0% w/v.

In an embodiment of the present invention, the composition comprises dodecyl benzene sulphonic acid as anti-sludge agent. In an embodiment of the present invention, the amount of the dodecyl benzene sulphonic acid is in a range from 0.1 to 2.0% w/v.

In another embodiment of the present invention, 0.5% w/v XC polymer and nitrogen, in gas to liquid ratio of 40–90%, may be added to the single-phase retarded acid composition to generate stable foam with retarded acid which improves zonal coverage, provides deep stimulation and helps in flow back specially in low pressure reservoirs.

In an embodiment of the present invention, the ratio of the constituents causes the single-phase retarded acid composition to achieve controlled acid consumption and deeper penetration into the reservoir, while reducing rapid facial dissolution. The single-phase retarded acid composition exhibits advantageous properties including thermal stability at elevated temperatures, which ensures consistent performance in high-temperature reservoirs. The single-phase retarded acid composition has low viscosity, which facilitates easy pumping in low-injectivity wells. The single-phase retarded acid composition reduces the risk of corrosion to well components, thereby addressing environmental and operational challenges associated with conventional acidizing methods.

Brief Description of the Drawings

The present invention is described by way of embodiments illustrated in the accompanying drawings herein:

FIG. 1 illustrates various regimes of facial dissolution at the face of limestone reservoir;

FIG. 2(a) depicts an image of L-80 metal coupon tested for corrosion in the single-phase retarded acid composition having 3% cationic corrosion inhibitor, in accordance with an embodiment of the present invention. Fig. 2(b) depicts an image of L-80 metal coupon tested for corrosion in the single-phase retarded acid composition having 1% cinnamaldehyde, in accordance with an embodiment of the present invention;

FIG. 3 depicts a graph illustrating results of core flow tests conducted at 240oF acidized with plain acid, emulsified acid and single-phase retarded acid composition, in accordance with an embodiment of the present invention;

FIG. 4 depicts a graph illustrating results of core flow tests conducted at 275oF, in accordance with an embodiment of the present invention;

FIG. 5 illustrates micro-CT scan images of core plugs acidized with plain acid, emulsified acid and single-phase retarded acid composition, in accordance with an embodiment of the present invention;

FIG. 6 illustrates working principle of the single-phase retarded acid composition, in accordance with an embodiment of the present invention; and

FIG. 7 illustrates adsorption of acid retarder of present invention on to limestone surface, in accordance with an embodiment of the present invention.

Detailed Description of the Invention

The present invention discloses a single-phase retarded acid composition for stimulation of limestone formations. The composition is water-based, which helps slow down the rate of reaction of mineral acid with limestone formations. The composition of the present invention provides much deeper radial penetration of wormholes compared to conventional solutions, thereby increasing the productivity of formations. The single-phase retarded acid composition eliminates the need for emulsifiers and diesel, thereby addressing environmental and operational concerns.

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 people 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. Therefore, the present invention is to be accorded with 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.

In various embodiments of the present invention, the single-phase acid composition is used in matrix acidizing operations to enhance the production of limestone formations. In an advantageous embodiment, the composition slows down the reaction rate of hydrochloric acid without the use of emulsifiers such as diesel. The components and their specific ratio provide the composition dissolving power for limestone similar to that of plain acid, while the rate of reaction with limestone is 15–20 times slower than plain acid at 200°F. Therefore, the single-phase retarded acid composition provides deeper and more effective stimulation than conventional methods, without damaging the formation. The composition also reduces corrosion of wellbore equipment and can be used at bottom hole temperatures up to 275°F. Additionally, it exhibits thermal stability and can be used in high-temperature limestone reservoirs up to 275°F and in low-injectivity wells.

In an embodiment of the present invention, the single-phase retarded acid composition comprises a mineral acid, an acid retarder, a corrosion inhibitor, an iron control agent, and water. In an embodiment of the present invention, the mineral acid is selected from a group consisting of hydrochloric acid (HCl), methane sulfonic acid, acetic acid, formic acid or their mixtures. In an exemplary embodiment of the present invention, HCl is used as the mineral acid for the dissolution of limestone formations. In an exemplary embodiment of the present invention, the single-phase retarded acid composition comprises mineral acid in a range from 10-28% w/V, acid retarder in a range from 0.5 to 3.0% w/V, corrosion inhibitor in a range from 0.5 to 2.0% w/V, and iron control agent in a range from 0.1 to 1.0% w/V. In an embodiment, the acid retarder is a combination of surfactant A and surfactant B.

In an embodiment of the present invention, the ratio of a mineral acid, an acid retarder, a corrosion inhibitor, an iron control agent, and water is in the following range 12-15:1-2:0.8-1:0.5-0.8:79–86. The selection and ratio of the constituents work synergistically to achieve deeper and more effective stimulation without damaging the formation.

Traditionally, under desirable temperature conditions, injection rate, pumping of HCl results in the creation of wormholes, which reduces pressure drop, imparts a negative skin factor, and thereby enhances productivity. However, under high-temperature conditions, where the reaction rate of HCl with the calcium carbonate content of the limestone formation is high, the acid quickly reacts near the face of the reservoir, leading to facial dissolution.

As shown in FIG. 1, as the injection rate increases or the acid reactivity decreases, the dissolution regime shifts from facial dissolution to conical wormhole, dominant wormhole, and ramified wormhole. Among these four regimes, the dominant wormhole regime provides the most effective stimulation during acidizing limestone formations. Typically, for formations with temperatures above 100°C, the reactivity of mineral acid such as HCl is appropriately retarded to achieve the dominant wormhole regime under standard pumping conditions. It is highly desirable to retard the rate of reaction of the acid so that worm holing can occur easily under high-temperature conditions.

The single-phase retarded acid composition of the present invention comprises an acid retarder as described herein below which retards the rate of reaction of the mineral acid to achieve worm holing easily under high-temperature conditions.

Acid Retarder

In an embodiment of the present invention, the acid retarder is a combination of surfactant A and surfactant B. In an exemplary embodiment of the present invention, the amount of surfactant A is in a range from 0.1 to 2.0% w/V and the amount of surfactant B is in a range from 0.1 to 1.0% w/V.

In an embodiment, surfactant A is a compound represented by formula I:

where X can be Cl, Br or O, R1, is alkyl/alkenyl chain with carbon length from 8–20, R2 & R3 are alkyl chains with carbon atom from 1–3.

In an exemplary embodiment of the present invention, the surfactant A is selected from a group comprising of lauryl dimethyl amine oxide and myristyl dimethyl amine oxide.

In an embodiment of the present invention, the surfactant B is a compound represented by formula II:

where, x+y = 3, R1 is the alkyl/alkenyl chain having 8–24 carbon atoms, R2, R3 and R4 are alkyl chains having 1–3 carbon atoms.

In an exemplary embodiment of the present invention, the surfactant B is selected from a group comprising of Cocamidopropyl PG–Dimonium Chloride Phosphate, Myristamidopropyl PG–Dimonium Chloride Phosphate and Linoleamidopropyl PG–Dimonium Chloride Phosphate.

Advantageously, the acid retarder of the composition adsorbs onto the surface of the formation, thereby inhibiting the reactivity of the acid by limiting the sites available for bonding between the acid and the formation surface. Additionally, the acid retarder reduces the diffusion of protons to the surface of the rock, thereby facilitating the retardation of the mineral acid's reaction rate, thereby preventing facial dissolution and enabling deep penetration of the wormholes which enhances the formation's permeability.

In an embodiment of the present invention, choice of acid retarders renders the composition to be compatible with sea water, thereby allowing easy preparation of single-phase retarded acid during offshore operations. The acid retarder of the composition also acts as an excellent acid foaming agent generating stable foam with nitrogen or carbon dioxide. The foamed retarded acid while providing deep penetration of wormholes also improves diversion, zonal coverage and aids in flow back post stimulation in low pressure wells.

Corrosion Inhibitor

Traditionally, corrosion during matrix acidizing is primarily caused by aggressive chemical reactions between the acidizing fluids, such as hydrochloric acid (HCl), and the metallic components of the wellbore equipment. The mineral acid reacts with the metal surfaces of wellbore equipment, including tubing, casing, and pumps, causing the metal to dissolve. In deep reservoirs of the formations, high temperatures increase the reactivity of acids, further accelerating the corrosion process. Corrosion can lead to the thinning and eventual failure of wellbore equipment, posing significant safety concerns.

In an embodiment of the present invention, the corrosion inhibitor is a combination of propargyl alcohol and cinnamaldehyde. In an exemplary embodiment of the present invention, the amount of propargyl alcohol is in the range from 0.1 to 1.0% w/v. In an exemplary embodiment of the present invention, the amount of cinnamaldehyde is in the range from 0.1 to 1.0% w/v.

Advantageously, the single-phase retarded acid composition comprising the corrosion inhibitors reduces the interaction between the mineral acid and the wellbore equipment, thereby slowing the corrosion process. The corrosion inhibitors reduce the interaction of acid and metal by preventing the solubilization of metal ions in the composition. The corrosion inhibitor forms a protective film over the metal surface, thereby preventing corrosion.

Iron Control Agent

Traditionally, during matrix acidizing, mineral acid reacts with iron-bearing minerals in the formation or wellbore equipment, releasing iron ions (primarily ferrous and ferric). The ferric and ferrous ions normally remain dissolved in the retarded acid and can lead to complications during and after the acidizing process. Particularly in sour wells that contain large amounts of hydrogen sulfide, iron sulfide scale tends to form in boreholes, tubulars, and/or formations. The acid used in matrix acidizing can dissolve the iron sulfide, but in the process, hydrogen sulfide is generated. Hydrogen sulfide is toxic and contributes to corrosion. In addition, the dissolved iron tends to precipitate in the form of ferric hydroxide or ferrous sulfide as the acid in the treatment fluid becomes spent. At the end of the matrix acidizing process, ferric hydroxide begins to precipitate and plug the formation. The precipitation of iron is highly undesirable due to the damage it can cause to the permeability of the formation.

In an embodiment of the present invention, the composition comprises an iron control agent to minimize iron precipitation, for example, by sequestering the iron ions in solution or by reducing ferric ions to the more soluble ferrous form of iron. However, the problem with conventional iron sequestering agents is that they are not particularly effective at temperatures above about 125°-150°F.

In an embodiment of the present invention, the iron control agent of the composition is selected from a group comprising of ethylenediaminetetraacetic acid (EDTA), Glutamic acid di-acetate (GLDA), sodium erythroborate, citric acid or a combination thereof. In an exemplary embodiment, the iron control agent is a combination of sodium erythroborate and disodium EDTA. In an exemplary embodiment of the present invention, the amount of sodium erythroborate is in a range from 0.05 to 0.1 % w/v. In an exemplary embodiment, the amount of the disodium EDTA is in a range from 0.1 to 1.0% w/v. Advantageously, the iron control agent prevents precipitation of ferric ions as the pH of spent acid increases beyond 2. Further the iron control agent minimizes the issue of iron catalysed sludge formation.

Anti-Sludge Agent

Elevated temperatures and pressure fluctuations during matrix acidizing operations can destabilize hydrocarbons, such as asphaltenes and resins. The destabilized hydrocarbons may precipitate out of crude oil and aggregate into a thick, tar-like sludge. This is primarily triggered by the interaction of crude oil with mineral acid under favourable conditions. The sludge may precipitate and clog the passageways in formations, thereby reducing the efficacy of the matrix acidizing.

In another embodiment of the present invention, the composition comprises an anti-sludge agent to inhibit the formation of sludge and emulsions that may occur during acidizing operation. In an exemplary embodiment of the present invention, the anti-sludge agent is dodecyl benzene sulphonic acid (DBSA). In an exemplary embodiment of the present invention, the amount of dodecyl benzene sulphonic acid in the single-phase retarded acid composition is in a range from 0.1 to 2.0% w/v.

Advantageously, the single-phase retarded acid composition in accordance with various embodiments of the present invention exhibit slow reaction rate with carbonate surface of the formation while having high dissolution efficiency, low viscosity, thermal stability and ease of preparation. Further, the single-phase retarded acid composition eliminates the limitations associated with the conventional methods viz., plain acid and emulsified acid.

In an embodiment of the present invention, the ratio of 12-15:1-2:0.8-1:0.5-0.8:79–86 is advantageous to the composition in exhibiting synergy. The amount of the components is specifically formulated to achieve good dissolution of reservoir rock and facilitates propagation of wormholes while achieving retardation factor of more than 15 at 200 0F. The amount of corrosion inhibitor is also specifically formulated to effectively control corrosion without interfering with the acid retarder. The amount of iron control agent in the composition is also optimum for application in limestone reservoir.

By way of the composition of the present invention, a surprising effect is observed in terms of retardation in the rate of reaction between the acid and limestone formations. The composition exhibits high dissolution efficiency and low viscosity similar to that of plain acid, along with a retardation rate comparable to that of an emulsified acid. Being water-based, the composition of the present invention eliminates stability issues of emulsions, and high viscosity observed with conventional emulsified acids.

The composition of the present invention, in accordance with various embodiment of the present invention, functions by using water-soluble acid retarders that adsorb onto the surface of the limestone reservoir, forming a film that slows down the diffusion of H+ ions toward the surface of limestone formations, thereby resulting in a reduced overall rate of reaction between the limestone and mineral acid, as shown in FIG. 6. In an embodiment of the present invention, the amount of acid retarder and the relative dosage of surfactant A and B are optimized to provide the desired rate of retardation.

The mechanism of the single-phase retarded acid composition of the present invention is illustrated in FIG. 7, in accordance with an embodiment of the present invention. As shown in FIG. 7, since the point of zero charge (PZC) of calcite is in the range of 8–9.5, limestone minerals are positively charged under matrix acidizing conditions. As a result, the reaction rate of HCl is retarded by adding suitable zwitterionic surfactants. As shown in FIG. 6, the negatively charged ends of the single-phase retarded acid bind to the limestone surface while the positively charged ends of the single-phase retarded acid slow down the attack by H+ ions, in accordance with an embodiment of the present invention. As shown in FIG. 7, specialty surfactants, where the negatively charged ends adsorb onto the limestone surface, and the additional positive charge in the surfactant molecule combined with steric hindrance further reduces the molecular diffusion of H+ ions toward the CaCO3 mineral at higher temperatures.

The properties of the single-phase acid composition are provided in table 1, in accordance with an embodiment of the present invention.
Table 1
Composition Viscosity 25 0C Thermal Stability Retardation factor @ 2000F
Mineral acid (10-28 %w/V) 1.25 cp >3 days 20
Acid retarder (0.5-3.0 %w/V)
Corrosion inhibitor (0.5-2.0 %w/V)
Iron control agent (0.1-1.0 %w/V)
Water (Balance Amount)

Advantageously, the reaction rate of the composition with limestone is significantly retarded, thereby promoting deep wormhole penetration under high-temperature/low-injectivity conditions. Surprisingly the retardation factor of the acid formulation is more than 15 at 200oF. The composition is thermally stable and effective for stimulating high-temperature limestone and dolomite reservoirs, including horizontal wells. In an embodiment of the present invention, the acid retarders are biodegradable and environmentally friendly, thereby making them easy to handle. The single-phase retarded acid composition is less viscous, allowing it to be easily pumped into tight limestone reservoirs. The minimal use of acid retarder also reduces the cost of matrix acidizing operations. Also, the single-phase acid composition eliminates the use of diesel, which is typically required in substantial quantities in emulsified acids. As a result, the composition reduces the carbon footprint, cost, and operational safety risks. Furthermore, being single-phase composition, it can be prepared more easily and, in less time, thereby increasing productivity.

In an embodiment of the present invention, a method of preparing the single-phase retarded acid composition is provided. The method comprises adding suitable amounts of corrosion inhibitor to water. Subsequently, HCl is added and mixed for 15–20 minutes allowing complete activation of corrosion inhibitor. Further, iron control agent is added and mixed to form a homogeneous mixture. Acid retarder is finally added in the suitable proportions and mixed for 1–10 minutes to form a homogeneously mixed single-phase retarded acid.

The prepared single phase acid formulation is then injected into the subterranean limestone formation for simulation. In an embodiment of the present invention, during simulation, the single-phase retarded acid composition is maintained at a lower pressure than the minimum in situ fracture pressure. In another embodiment of the present invention, during simulation in tight reservoirs, the single-phase retarded acid composition is maintained at higher pressure than fracture pressure.

In an embodiment of the present invention, the single-phase retarded acid composition can also be used in acid fracturing to improve the penetration of live acid in the fracture and improve the etched frac length. The low viscosity of the solution further helps in improving frac length by process of “viscous fingering” when used in conjunction with frac gel and/or gelled acid.

The disclosure herein provides for examples illustrating the process for preparing the single-phase retarded acid 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 further enable those of skill in the art to practice the embodiments. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein.

Working Examples
Preparation of the Single-phase Retarded Acid Composition

A measured amount of water was poured into a beaker to which 0.5% w/v of cinnamaldehyde,0.3% w/v propargyl alcohol and 15% w/v of HCl was added and mixed with magnetic stirrer for 10 – 15 min. 0.5% w/v of Na2EDTA and 0.3% (w/v) of DBSA was added to the mixture and stirred further for 5–10 minutes for complete dissolution. 1.0% w/v of Lauryl Amine Oxide and 0.5% w/v of Cocamidopropyl PG–Dimonium Chloride Phosphate was added and mixed to form a homogeneous single-phase retarded acid.

The single-phase retarded acid composition prepared in accordance with Example 1 was evaluated for (a) retardation factor (b) corrosion inhibition (c) thermal stability (d) effectiveness for limestone acidizing by conducting core flooding at different conditions of injection rates and temperature. Further, the wormhole propagation was evaluated by conducting micro-CT scan of the acidized core plugs.

Determination of Retardation Factor

The retardation factor for the single-phase retarded acid composition, with varying concentrations of mineral acid, acid retarder, corrosion inhibitor, and iron control agent, was tested by maintaining a 50 ml solution of each composition at a temperature of 200°F on a hot plate magnetic stirrer. 3.5g of calcium carbonate chips were added to the composition, and a timer was started to measure the reaction time. The reaction time was recorded from the moment of carbonate chips addition to the point where the carbon dioxide effervescence stopped, indicating the disappearance of the calcium carbonate chips. The retardation factor at 200°F was calculated from the ratio of the reaction time with the acid retarder to the reaction time with plain acid, without any acid retarder. The reaction time and retardation factor for conventional acid formulation are provided in table 2(a) below. The reaction time and retardation factor for single-phase retarded acid composition in accordance with various embodiments of the present invention are provided in table 2 below.
Table 2
S. No. Conventional Acid Formulation
Reaction Time (min) Retardation Factor
1 15% HCl + 0.5% Na2EDTA + 1% conventional corrosion inhibitor 0.5 1
2 15% HCl + 0.5% Na2EDTA + 1% conventional corrosion inhibitor + 19% MgCl2 1.0 2
3 15% HCl + 0.5% Na2EDTA + 1% conventional corrosion inhibitor + 19% MgCl2 + 2% Nonionic Surfactant 2.5 5

Table 2(a)
S. No. Single Phase Retarded Acid Composition
Reaction Time (min) Retardation Factor
1 15% HCl + 0.5% Na2EDTA + 0.5% Cinnamaldehyde + 2% Linoleamidopropyl PG Dimonium Chloride Phosphate 6.5 13
2 15% HCl + 0.5% Na2EDTA + 0.2% Propargyl Alcohol + 0.3% Cinamaldehyde + 2% Cocamidopropyl PG Dimonium Chloride Phosphate 7.0 14
3 15% HCl + 0.5% Na2EDTA + 0.5% Cinnamaldehyde + 2% Lauryl Dimethyl Amine Oxide 6.5 13
4 15% HCl + 0.5% Na2EDTA + 0.5% Cinnamaldehyde + 1% Lauryl Dimethyl Amine Oxide + 0.5% % Linoleamidopropyl PG Dimonium Chloride Phosphate
8.5
17
5 15% HCl + 0.5% Na2EDTA + 0.5% Cinnamaldehyde + 1% Lauryl Dimethyl Amine Oxide + 0.5% Cocamidopropyl PG Dimonium Chloride Phosphate 9.5 19

From the results shown in table 2, the retarded acid composition comprising mixture of lauryl dimethyl amine oxide and Cocamidopropyl PG Dimonium Chloride Phosphate or Linoleamidopropyl PG Dimonium Chloride Phosphate, resulted in a substantial retardation of the reaction rate of HCl with calcium carbonate, without the addition of large amounts of conventional acid retarder such as magnesium chloride. Further, the retardation factor observed at 200°F was upto 19, which is significantly higher and better suited for high-temperature and low-injectivity conditions.

Corrosion Testing

Corrosion tests with various acid corrosion inhibitors were conducted under the conditions specified in table 3 below. Metal coupons were immersed in the single-phase retarded acid composition of the present invention, which contained 15% HCl, 0.5% Na2EDTA, 1% Lauryl Dimethyl Amine Oxide, 0.5% Cocamidopropyl PG Dimonium Chloride Phosphate + 0.3% DBSA as base fluid at the desired temperature and duration. Corrosion rates were measured as weight loss per unit of total surface area after adding corrosion inhibitors of varying type and dosage to the base fluid. The metal coupons were visually examined for pitting after the test. The results of the corrosion testing are provided in table 4 herein below.
Table 3
S. No Parameter Value
1 Material of Metal Coupons L – 80
3 Temperature 100 0C & 120 0C
4 Time 6 hrs.
5 Acceptable corrosion limit =0.024 g/cm2

Table 4
Temperature / Time Corrosion Inhibitor Corrosion Rate (g/cm2)
100 0C / 6 hrs. 3% Cationic Corrosion Inhibitor 0.260
1% Propargyl Alcohol 0.0097
1% Cinnamaldehyde 0.0136
0.5% Propargyl Alcohol + 0.5% Cinamaldehyde 0.0089
1% Propargyl Alcohol + 1% Cinnamaldehyde 0.0024
120 0C / 6 hrs. 3% Cationic Corrosion Inhibitor 0.545
2% Propargyl Alcohol 0.0162
2% Cinnamaldehyde 0.022
0.5% propargyl Alcohol + 0.5% Cinnamaldehyde 0.020
1% Propargyl Alcohol + 1% Cinnamaldehyde 0.0081

Results in table 4 indicate that selected organic corrosion inhibitors (cinnamaldehyde and propargyl alcohol) are compatible with the acid retarders and significantly reduced the corrosion of downhole tubulars. Photographs of metal coupon after corrosion testing with 3% cationic corrosion inhibitors and 1% cinnamaldehyde are shown in FIG. 2.

Core Flow Studies

Core flow studies were conducted using various compositions as provided in table 5 herein below. The tests for core flow study were carried out at 1000 psi back pressure and 2500 – 3500 psi of confining pressure.
Table 5
Acid Type Formulation
Brine 2% KCl
Plain Acid 15% HCl + 0.5% EDTA + 3% ACI + 2% Non-ionic Surfactant
Emulsified Acid (EA) 15% HCl (in aqueous phase) + 30% Diesel + 2% ACI + 1.8% Span – 80 + 0.2% Tween – 80 + 0.5% EDTA
Present invention – Aqueous Retarded Acid (ARA) 15% HCl + 1% Lauryl Dimethyl Amine Oxide + 0.5% Cocamidopropyl PG Dimonium Chloride Phosphate + 0.5% Cinnamaldehyde + 0.5% Propargyl Alcohol + 0.5% Na2EDTA + 0.3% DBSA

The summary of core flow tests at 240oF is provided in FIG. 3, in accordance with an embodiment of the present invention. Results in FIG. 3 show that at 240oF, the single-phase retarded acid composition requires much fewer pore volumes to breakthrough (PVbt) than plain acid over the entire range of injection rate, which is a highly desirable feature for acidizing limestone reservoirs. The PVbt of composition of the present invention is also marginally lower than emulsified acid over the same range which shows that the composition of present invention is a good alternative to emulsified acid as it eliminates the use of diesel and makes operation simple without compromising on the effectiveness of stimulation job. The pore volume to breakthrough observed at 240°F and 275°F was 0.21 and 0.47, respectively, indicating superior stimulation performance compared to existing solutions. The composition provides much deeper penetration of acid/wormholes compared to conventional solutions, thereby leading to increased production gains after stimulation in oil and gas wells within limestone reservoirs.

The summary of the core flow tests at 275°F is provided in FIG. 4, in accordance with an embodiment of the present invention. No breakthrough was observed at 275°F during core flooding with plain acid, even after pumping approximately 3.5 pore volumes. Significant facial dissolution was observed in the core sample through a micro-CT scan, as shown in FIG. 5. The results in FIG. 4 and FIG. 5 indicated that plain acid was not suitable for stimulating limestone reservoirs under such conditions. The remaining tests at 275°F were carried out with emulsified acid and the single-phase retarded acid composition of the present invention.

Based on the results in FIGs. 3 and 4, a marked difference was observed between the performance of emulsified acid and single-phase retarded acid at injection rates below 1.5 cc/min. It was noted that under low injectivity conditions, the single-phase retarded acid performed significantly better than emulsified acid. Additionally, under high injectivity conditions, the performance of single-phase retarded acid closely matched that of emulsified acid, suggesting that single-phase retarded acid was a better choice for high-temperature applications. Surprisingly, the single-phase retarded acid of the present invention displayed low PVbt across a wide range of injection rates, which was a highly desirable feature. This characteristic not only allowed for deeper penetration of live acid but also enabled more uniform acid penetration in long pay zones or horizontal intervals. These studies demonstrated that the single-phase retarded acid composition of the present invention was highly effective in stimulating high-temperature, low-injectivity limestone reservoirs while being compatible with wellbore equipment metallurgy.

Micro CT scan of Acidized Core Plugs

FIG. 5 is micro-CT scan image of core plugs acidized with plain acid, emulsified acid and single-phase retarded acid composition, in accordance with an embodiment of the present invention. Micro CT scan results shown in FIG. 5 indicate severe facial dissolution with plain acid at 275oF and very poor penetration of plain acid in the core plug. Therefore, the result indicates that plain acid may not yield desirable stimulation results under such high temperature conditions. For emulsified acid formulation under same temperature conditions, CT scan images suggested conical wormhole regime at 0.5 cc/min. Surprisingly, dominant wormhole regime was observed with single-phase retarded acid composition at same conditions. This indicates that the single-phase retarded acid composition of the present invention would be highly effective for stimulation of tight and high temperature wells compared to conventional alternatives.

[0070] 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.
, Claims:We Claim:

1) A single-phase retarded acid composition, comprising:
a mineral acid in a range from 10 to 28 %w/V;
an acid retarder in a range from 0.5 to 3.0 %w/V;
a corrosion inhibitor in a range from 0.5 to 2.0 %w/V;
an iron control agent in a range from 0.1 to 1.0 %w/V; and water,
wherein ratio of the mineral acid, acid retarder, corrosion inhibitor and iron control agent and water is 12-15:1-2:0.8-1:0.5-0.8:79-86, wherein the single phase retarded acid achieves enhanced retardation factor as compared to conventional acid formulations.

2) The composition as claimed in claim 1, wherein the mineral acid is selected from a group comprising of hydrochloric acid, methane sulphonic acid, formic acid, acetic acid or their mixture thereof, preferably hydrochloric acid.

3) The composition as claimed in claim 1, wherein the acid retarder is a combination of a surfactant A and surfactant B.

4) The composition as claimed in claim 3, wherein the surfactant A is in a range from 0.1 to 2.0% w/v.

5) The composition as claimed in claim 3, wherein the surfactant B is in a range from 0.1 to 1.0% w/v

6) The composition as claimed in claim 3, wherein the surfactant A is selected from a group comprising of lauryl dimethyl amine oxide, myristyl dimethyl amine oxide.
7) The composition as claimed in claim 3, wherein the surfactant B is selected from a group comprising of Cocamidopropyl PG–Dimonium Chloride Phosphate, Myristamidopropyl PG–Dimonium Chloride Phosphate and Linoleamidopropyl PG–Dimonium Chloride Phosphate.

8) The composition as claimed in claim 1, wherein the corrosion inhibitor is a combination of propargyl alcohol and cinnamaldehyde.

9) The composition as claimed in claim 8, wherein amount of the propargyl alcohol is in a range from 0.1 to 1.0% w/v.

10) The composition as claimed in claim 8, wherein amount of the cinnamaldehyde is in a range from 0.1 to 1.0% w/v.

11) The composition as claimed in claim 1, wherein the iron control agent is a combination of sodium erythroborate and disodium EDTA.

12) The composition as claimed in claim 11, wherein amount of the sodium erythroborate is in a range from 0.05 to 0.1 % w/v.

13) The composition as claimed in claim 11, wherein amount of the disodium EDTA is in a range from 0.1 to 1.0% w/v.

14) The composition as claimed in claim 1, wherein the composition comprises an anti-sludge agent is dodecyl benzene sulphonic acid.

15) The composition as claimed in claim 14, wherein amount of the dodecyl benzene sulphonic acid is in a range from 0.1 to 2.0% w/v.

16) The composition as claimed in claim 1, wherein the retardation factor of the acid formulation is more than 15 at 2000F.

Dated this 10th day of February, 2025

Oil and Natural Gas Corporation Limited
(Jogeshwar Mishra)
IN/PA – 2578
of Shardul Amarchand Mangaldas & Co.
Attorneys for the Applicant