[0001] This application claims the benefit of U.S. Provisional Application No. 63/219,970 entitled “Single-Phase Alcohol-Based Retarded Acid,” filed July 9, 2021 , and U.S. Provisional Application No. 63/222,243, filed July 15, 2021 , the disclosures of which are incorporated by reference in their entirety.

FIELD

[0002] This patent application describes methods and apparatus for stimulating hydrocarbon reservoirs. Specifically, methods and materials for acid treating hydrocarbon formations is described.

BACKGROUND

[0003] Almost two-thirds of the world’s remaining oil reserves are contained in carbonate reservoirs. Carbonate formations have a tendency to be highly heterogeneous, with complex porosity and permeability variations, barriers, and irregular flow paths. In order to increase the productivity of wells in a calcareous formation, a range of stimulation techniques can be applied. One of the most common techniques involves the stimulation of the well with acids.

[0004] Acids can be injected into the formation to boost production or increase injectivity in oil and gas fields. Stimulation of carbonate rocks typically involves the reaction between an acid and the minerals calcite (CaCCb) and dolomite [CaMg(C03)2] to enhance the flow properties of the rock. The two main stimulation methods are acid fracturing and matrix acidizing.

[0005]Acid fracturing involves the pumping of a viscous pad at injection rates that result in pressures above the fracture pressure to fracture the rock. Once fractures have been created, an acid stage is pumped to etch and form pathways at and near the fracture surfaces. After the treatment, the fracture closes but the pathways — intricate, high- permeability channels — persist, which allows for an increased conductivity between the formation and the well to remain. [0006] Matrix acidizing, in contrast, is performed at injection rates that result in pressures below the fracture pressure. Acids, solvents, and/or other chemical treatments are pumped into the formation. When acids penetrate into the carbonate formation, some of the minerals in the rock dissolve creating highly conductive channels (wormholes).

[0007] In carbonate reservoirs, hydrochloric acid (HCI) is the most commonly applied stimulation fluid. Organic acids such as formic and acetic acid are used in retarded-acid systems or in high-temperature applications. However, these systems have several limitations. In the case of organic acids, the dissolution of the rock is low when compared to that of HCI, so larger volumes are required to dissolve the same amount of rock. Hydrochloric acid, on the other hand, has a high reaction rate at higher temperatures, meaning that the acid will not penetrate deep into the formation before it is fully spent. To solve these issues, an emulsified acid (acid-in-oil) with a high dissolution capacity and low reaction rates was developed. This fluid system raised other problems such as high friction pressures which limit the pumping rates, health, safety and environmental (HSE) issues due to the use of highly flammable substances (diesel, crude), and quality concerns during the preparation of the emulsion.

[0008] Improved retarded acid systems are needed for stimulation of carbonate reservoirs.

SUMMARY

[0009] Embodiments described herein provide a method of stimulating a hydrocarbon reservoir by preparing an aqueous mixture comprising a strong acid, an alcohol, and a surfactant; and flowing the aqueous mixture into the hydrocarbon reservoir.

[0010] Other embodiments described herein provide a single-phase acid treatment composition, comprising a strong acid, an alcohol, and a betaine-based surfactant.

[0011] Other embodiments described herein provide a method of treating a hydrocarbon reservoir, the method comprising obtaining a single-phase aqueous mixture consisting of water-miscible components, the mixture comprising a strong acid, an alcohol, and a betaine-based surfactant or an alkyl ammonium bromide surfactant; and flowing the aqueous mixture into the hydrocarbon reservoir. DETAILED DESCRIPTION

[0012] Blending a strong acid with an alcohol and a surfactant has been found to yield an acid system with reduced reactivity rate that is useful for acid treating acid-susceptible hydrocarbon reservoirs. The alcohol and the surfactant is added to the acid system and mixed vigorously to homogenize the mixture. The mixture thus formed consists of water- miscible components, and is a single liquid phase. The alcohol is generally an organic molecule with one or more hydroxyl groups. Alcohols with one, two, three, four, five, or six hydroxyl groups, or more, can be used. Molecular weight and hydroxyl number of the alcohol can be selected to provide a target density and/or viscosity. Examples of alcohols that can be used include methanol, isopropanol, ethylene glycol, glycerol, pentaerythritol, and sorbitol. Any alcohol with suitable molecular weight and viscosity for a particular application can be used. Usable alcohols may be aromatic or aliphatic, to any degree. Fused and/or polynuclear aromatic molecules having hydroxyl groups can be used. Linear, branched, cyclic, or multi-cyclic alcohols can be used. Alcohol polymers, for example polyvinyl alcohol, can be used with selected molecular weight, branching, aromaticity, and hydroxyl number. Combinations of alcohols can also be used to tune the acid-retarding features, fluid properties, and/or other features of the mixture. Species such as esters and ethers that can hydrolyze to yield alcohols can be used, alone or in combination with alcohols. For example, an alcohol can be used with the corresponding acetate ester in a retarded acid drilling fluid. Flydrolysis of the ester yields the conjugate alcohol and acetic acid, which can have acid treatment utility. In this way, hydrolyzable species can serve as time-delayed acids and acid-retardant species in some cases.

[0013] The types of alcohols that are used are those which are generally unreactive or minimally reactive with strong hydroacids, where in this context a hydroacid is an acidic compound of hydrogen with an anion. Alcohols that accept protons to form relatively stable cations at low pH can react to form stable compounds by eliminating water, thus removing acid from the mixture. Such alcohols generally include those where a hydroxyl group is alpha to a stabilizing structure, such as a tertiary carbon atom (no hydrogen atoms bonded to the carbon atom) or beta secondary carbon atom (one hydrogen atom bonded to the carbon atom). Thus, where an alcohol includes a tertiary carbon or a beta secondary carbon, and the oxygen atom of the hydroxyl group is bonded to the tertiary carbon, the alcohol can be expected to react, under some circumstances, with the acid to eliminate water. These reactions generally have an equilibrium characteristic, so the amount of acid consumed by reaction with the alcohol, and the amount of alkyl halide produced, is defined by the equilibrium. The equilibrium thus affects the amount of acid in the mixture, which can also retard acid function in the formation. Reagents, for example containing or dissociating to common or similar ions, can also be added to affect the equilibrium. For example in one case, a catalytic amount of a more nucleophilic halide, such as bromide or iodide, can be added to return some chloride ions to solution. Where iodide is used, iodide salts may be produced that can also act as corrosion inhibitors.

[0014]Alcohols are believed to retard acid reactions in acid-susceptible formations by reducing the rate at which acid species come into contact with acid-susceptible species in the rock formation. Alcohols that do not react, or react minimally, with acid species can thus be used without limitation. Alcohols that react with acid species under some circumstances can also be used in conditions that tend to reduce the rate of reaction of alcohol with acid species. For example, if an alcohol might be expected to react with HCI at relatively high temperature, that alcohol may be used as an acid retarding reagent if formation temperatures do not approach a reaction temperature of the alcohol with the acid. Species mobility can also be reduced by some alcohols that might react with acid species. For example, where an alcohol increases viscosity and thus reduces mobility of aqueous species, rate of alcohol-acid reactions can be reduced. The acid treatment mixture can also be cooled to reduce any unwanted side-reactions, and to further delay acid activity by adjusting mixture viscosity and/or kinetic characteristics.

[0015]Acids that can be used for the treatment fluid compositions herein strong acid molecules such as hydrogen chloride (HCI, also called hydrochloric acid, hydrogen bromide in water(FIBr, also called hydrobromic acid), hydrogen iodide (HI, also called hydroiodic acid), hydrogen fluoride in water(HF, also called hydrofluoric acid), sulfuric acid (H2SO4), nitric acid (HNO3), phosphoric acid (H3PO4), an alkanesulfonic acid (RSO3H, where R is an alkyl group), an arylsulfonic acid (ArSCb, where Ar is an aromatic or aryl group), or a combination thereof in water solution. The fluids described herein are retarded acid fluids that can be used to penetrate acid-susceptible formations for acid treatment, and provide flowback compositions that are less corrosive that conventional treatment fluids. For example, where conventional treatment fluids generally result in flowback at pH of 0 to 3, the retarded acid fluids described herein generally provide flowback composition of pH from about 3 to about 5.5, resulting in reduced flowback time.

[0016] To the extent some alcohol reacts with acid, perhaps at slow rates, stoichiometrically excess acid can be provided to avoid loss of too much acid while retaining the retarding properties of the alcohol and any byproducts of reaction between alcohol and acid. For example, glycerol reacts relatively slowly with HCI to form small quantities of 3-chloropropane-1 ,2-diol, which is less viscous than glycerol but can still provide a dilution effect to retard acid reactivity. HF, and simple organic acids such as formic acid and acetic acid, which are also commonly used for acid treatment, are generally less reactive with alcohols than HCI, so where such acids are used with HCI, they are generally expected to react much less, or not at all, with any of the alcohols described herein.

[0017] Surfactants are also used in alcohol-acid blends to enhance acid retardation. Surfactants generally occupy sites where acid might react with acid-susceptible species in rock formations. Any surfactant that has affinity for acid-susceptible species in rock formations can be used. Such surfactants may be amphoteric, nonionic, cationic, or anionic. Surfactants that can be used include, but are not limited to, betaine-based materials such as erucic amidopropyl dimethyl betaine and cocam idopropyl betaine; alkyl ammonium bromide materials such as hexadecyltrimethyl ammonium bromide and tetradecyltrimethyl ammonium bromide; and dodecylbenzene sulfonic acid. Combinations of surfactants can be used to tune the effect of acid and alcohol on the acid-susceptible species of the rock formation. The surfactant, or combination of surfactants, is generally added to a mixture of acid and alcohol to complete a single phase treatment mixture.

[0018] Other components can be added to the single-phase acid mixtures described above for use in acid treatment of hydrocarbon formations. Such components include corrosion inhibitors, friction reducers, scale inhibitors, biocides, surfactants, H2S scavengers, iron control reagents, diversion agents, viscosifiers, chelating reagents, solvents, clay stabilizers, and calcium inhibitors. These reagents can be added to the mixture neat or dissolved in water or another compatible solvent. For example, such reagents can be added to an alcohol to form a premix, and the premix can then be added to an acid solution to form a single-phase treatment mixture.

[0019] The single-phase aqueous mixtures described above can be used as acid treatment compositions with no further additional components, and can be used in a single-step acid treatment process, wherein the single-phase aqueous mixture consisting of water-miscible components and comprising a strong acid, an alcohol, and a surfactant is pumped into a well to acidify the interior of a hydrocarbon formation adjacent to the well. Additional components can be added to the single-phase aqueous mixture to enhance the properties and performance thereof. Adding these components may results in a multi-phase mixture in some cases, or the mixture may remain single-phase after the additional components are added. Acid treatment mixtures described herein may also be used in multi-step processes that might include pre-treatment operations to flush the formation with flush compositions that may be liquid, gas, or a mixture thereof, and may be aqueous, oleaginous, or a mixture thereof. In some instances a dilute acid flush may be used prior to acid treatment to remove any unwanted components from the formation prior to acid treatment.

EXAMPLES

[0020] To evaluate the retardation effects provided to the reaction between HCI and calcite, rotating disk experiments (RDEs), which serve to study the interplay between transport and reaction under defined conditions, were conducted at room temperature. Three fluids were tested: unmodified 15% HCI, 15% HCI with 20% glycerol, and 15% HCI with 1 % surfactant (erucic amidopropyl dimethyl betaine). Fig. 1 is a graph showing the results of the dynamic tests. As shown in Fig. 1 , the surfactant on its own does not significantly slow down the rate of dissolution of the rock by HCI. Glycerol, on the other hand, reduces dissolution of the rock by half under the same test conditions.

[0021] Mass loss experiments were conducted to test the performance of various surfactants with glycerol. The control fluid (labeled HCI) only contains 15% HCI whereas the rest contain 15% HCI and 20% glycerol as a retarding agent. The retardation package is then completed with the addition of a surfactant. The surfactants tested are erucic amidopropyl dimethyl betaine (EADB), cocam idopropyl betaine (CB), hexadecyltrimethyl ammonium bromide (CTAB), and dodecylbenzene sulfonic acid (DBS). EADB and CB were both added at a concentration of 10% whereas CTAB and DBS were added at a concentration of 0.4g/100 ml_ fluid. The materials dissolved were marble cores 1” in diameter x 0.5” in thickness.

[0022] Fig. 2 is a graph showing the results of the tests. As seen in Fig. 2, glycerol on its own retards the reaction of HCI with the marble. Flowever, the addition of the other component of the retardation package, the surfactant, greatly improves the rate of retardation. The core in the control fluid dissolved fully after close to 12 minutes, whereas that in glycerol took almost twice as long to fully dissolve. The cores in fluids with glycerol and a surfactant reacted much slower, with EADB interfering with the reaction the most and DBS the least.

[0023] The dynamic test results of Fig. 1 and the static test results of Fig. 2 both show a clear performance advantage of a single-phase aqueous mixture consisting of water- miscible components and comprising a strong acid, an alcohol, and a surfactant over use of acid alone or acid and surfactant alone. Interaction of the acid with acid-susceptible species in the rock is slowed, allowing time for the mixture to flow deeper into the formation to improve permeability deep into the formation.

[0024] The treatment fluids described herein can be used as a foamed material. A foaming agent, which can also be a surfactant or a co-surfactant, can be included and a gas introduced to form a foam. The foam can be pumped into the subterreanean formation to apply retarded acid compositions to the formation. Foaming reduces the amount of acid, retardant, and other agents that must be used to treat the formation.

[0025] The methods described herein can be used to treat subterranean formations having acid susceptible structures, such as carbonate formations. Subterranean formations such as hydrocarbon reservoirs and injection reservoirs having acid- susceptible formations can be treated using the compositions described herein. The retarded acid treatment fluids described herein use an alcohol and a surfactant to slow reaction of a strong acid with the acid-susceptible structures, allowing the treatment fluid to retain its activity longer as the fluid is flowed into the formation to reach a target location. [0026] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the present disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.