Attenuated acid formulations for acid stimulation

Abstract

A method for acid treating a carbonate reservoir with an attenuated acid formulation is provided. The method includes mixing an acidic compound with a hygroscopic chemical to form the attenuated acid formulation, and injecting the attenuated acid formulation into a carbonate formation.

Description

TECHNICAL FIELD

The present disclosure is directed to acid treatment formulations that slow the reactivity of the acid with carbonate formations.

BACKGROUND

The oil and gas industry uses acids to stimulate hydrocarbon reservoirs, for instance, in carbonate formations. The acids are selected based on their reactivity towards the rock matrix or the damage to be removed. In a successful treatment, the dissolution process leads to the formation of a highly conductive channel network, enhancing hydrocarbon production. Acid stimulation of carbonate reservoirs is routinely carried out in matrices, and acid fracturing treatments. Formations including calcite, dolomite, and the like, are typically stimulated using strong mineral acids, such as hydrochloric acid (HCl). This selection is primarily driven by its cost-effectiveness and proven success and efficiency towards dissolving calcium and magnesium-based carbonates. Moreover, the reaction products are readily soluble in water, which helps to prevent formation damage.

However, HCl has significant operational limitations that are detrimental to performance and cost. For example, HCl rapidly reacts with the rock matrix, particularly at elevated temperatures, and as a result, large volumes of acid are required to stimulate the pay zone. The fast reaction of the acid often prevents deeper penetration of live acid into the reservoir. Other drawbacks include undesired acid reactions occurring near the wellbore, causing corrosion and damage to drilling equipment, metal tubulars, and casing. Alternative strategies have been proposed over the years to address some of these limitations.

A number of strategies have been proposed for retarding the reaction rate between acid and the rock matrix. For example, encapsulation or emulsification of the acid can be used to create a temporary barrier between the acid and rock matrix. These techniques can include polymer-type shell or coating, acid-in-diesel emulsion, foaming of the acid, gelled systems, and the like. Changes in temperature, pressure, pH, or shear, among others, can be used to trigger release of the acid in the reservoir. Currently, the most used retarded acid system in the oil and gas industry is emulsification. Here, hydrochloric acid, with a concentration up to 28 wt. % (internal phase), is emulsified in a hydrocarbon (diesel) phase as the continuous phase.

Milder acids have been tested and used to slow the reaction rate of the acid with the rock matrix. These include organic acids, among others.

Acid retardation may also be performed by the introduction of salt solutes. In solutions with the salt solutes, the lack of water molecules increases steric hindrance in the fluid and limits the diffusion of protons. The amount of salt present in the composition can be adjusted to provide a desired acid retardation function.

In some tests, the acid is generated in situ at the point at which reaction is desirable. For example, by generating the downhole acid in a subterranean reservoir. Downhole, as opposed to generating acid on the earth's surface at wellsite and transferring it to a subterranean reservoir downhole.

The acid systems tested do overcome some of the challenges associated with using HCl. However, limitations still exist, particularly with respect to high friction pressures as observed for emulsified acid systems. Organic acids are often plagued by high cost, low dissolving power, and a limited solubility of reaction products.

SUMMARY

An embodiment described in examples herein provides a method for treating a carbonate reservoir with an attenuated acid formulation. The method includes mixing an acidic compound with a hygroscopic chemical to form the attenuated acid formulation and injecting the attenuated acid formulation into a carbonate formation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic drawing of a wellbore illustrating the formation of longer cracks in a carbonate reservoir due to treatment with an attenuated acid.

FIG. 2 is a process flow diagram of a method for treating a reservoir with a formulation that includes a hygroscopic compound to attenuate the reactivity of acid.

FIGS. 3A to 3C are plots of the calculated weight-loss of Indiana limestone core samples for acid formulations containing 15 wt. %, 28 wt. %, and 31 wt. % of hydrochloric acid (HCl).

FIGS. 4A to 4D are plots of the pressure drop across core samples measured during acid injection.

FIG. 5 is a plot of the acid volume to breakthrough for different compositions of attenuated acid solutions.

DETAILED DESCRIPTION

This application provides methods and compositions for attenuating the reactivity of acid within a subterranean formation, by mixing hygroscopic chemicals with acid to form an attenuated acid formulation. Not to be bound by the explanation, the hygroscopic chemicals, such as glycols, will adsorb water from the acid formulation and slow down the dissociation of the acid thus controlling acid (proton) diffusion to the rock surface. This may reduce acid diffusivity, which will result in slower acid attack on the rock. The slower acid attack leads to increased acid penetration and increases the efficiency of the acid treatment. In various embodiments, the attenuated acid formulation composition is delivered to the formation in as a main acid treatment or alternated with other acid treatment systems.

The attenuated acid formulation is a single-phase aqueous-based fluid, with a viscosity close to that of water. For example, the viscosity may be between 1 cP and 49 cP, where 49 cP is the viscosity of triethylene glycol. In some embodiments, the viscosity is between about 1 cP and about 100 cP, or between about 1 cP and 30 cP. In some embodiments, the attenuated acid formulation does includes emulsifiers, foaming agents, or both. As shown in the examples, the acid reactivity profile with a carbonate rock matrix can be decreased through the addition of suitable hygroscopic solvent to the main acid system in volume ratio in the range of 1 to 50% of the total volume of the acid treatment. This is achieved by adding the solvent to the bulk acid, for example, 15 wt. %, 28 wt. %, 31 wt. %, 36 wt. %, HCl and the like, without the need for additional polymers or emulsification procedures.

In examples described herein, the hygroscopic solvents include monoethylene glycol, diethylene glycol, and triethylene glycol. The hygroscopic solvents may adsorb a portion of the free water available in the attenuated acid formation resulting in less water for acid dissociation and diffusion toward the rock surface. In turn, this may slow down the reaction between the acid and the rock matrix thereby allowing the live acid to penetrate deeper into the formation prior to neutralization. The deeper penetration will improve the rate of propagation of channels (wormholes) into the reservoir.

FIG. 1 is a schematic drawing 100 of a wellbore 102 illustrating the formation of wormholes 104 in a carbonate reservoir 106 due to treatment with an attenuated acid formulation. The attenuated acid formulation is provided to the carbonate reservoir 106 through the wellbore 102, for example, directly or through a smaller tubular such as a coiled tubing line. Apparatus 108 at the surface 110, such as tanks, pumps, coiled tubing lines, and the like is used to provide the attenuated acid formulation. As described herein, acid treatment with strong mineral acids, such as HCl, may result in faster reaction with the carbonate reservoir 106 leading to less acid penetration. The attenuated acid formation provides further penetration during reaction, increasing the effectiveness of the acid treatment in forming the wormholes 104.

FIG. 2 is a process flow diagram of a method 200 for treating a reservoir with an attenuated acid formulation including an acid and a hygroscopic compound. The method begins at block 202 with the mixing of the acidic compound with a hygroscopic compound to form the attenuated acid formulation. In some embodiments, the attenuated acid formulation additives are to be mixed at the wellsite, for example, in recirculating tanks at 1 atmosphere and temperatures of 25° C. or greater.

The acid can be HCl, or methanesulfonic acid (MSA), among others. In an embodiment, the acid is HCl. Prior to mixing to form the attenuated acid formulation, the HCl may be at a concentration of less than or about 15 wt. %, less than or about 28 wt. % less than or about 31 wt. %, or less than or about 36 wt. %.

Any number of hygroscopic solvents may be used, for example, the hygroscopic solvent may be ethylene glycol, diethylene glycol, propylene glycol (or 1,2 propylene glycol or 1,2-propanediol), triethylene glycol, diglyme (or diethylene glycol dimethyl ether), 2-butoxyethanol (or EGBE or ethylene glycol monobutyl ether), dimethylformamide, propylene carbonate, 1,3 propylene glycol or 1,3-propanediol, diethylene glycol, 2(2-ethoxyethoxy)ethanol (or DEGEE or diethylene glycol monoethyl ether, carbitol, carbitol cellosolve), dipropylene glycol, 1,3 butanediol, dimethylene glycol or 2,3 butanediol, ethylene carbonate, 2(2-methoxyethoxy)ethanol (or diethyleneglycol monomethyl ether), dimethylemalonate, n-methyl-2-pyrrolidone, a mixture of ethylene glycol and water (35/65), a mixture of 60% propylene glycol in water, a mixture of500 ethylene glycol in water and monoethylene glycol (or ethylene glycol). Table 1 is listing of solvents that may be used

TABLE 1
list of hygroscopic solvents that can be used in the attenuated
acid formulation.
Chemical Name	Boiling Point, C.	Melting Point, C.
Ethanol	78.5	−114.1
Methanol	64.4	−98
2-methyl (1,3 propanediol)	125	−91
1-Butanol	117.7	−88.6
2(2-ethoxyethoxy)ethanol-	196	−76
DEGEE
2-Butoxyethanol or EGBE	171	−70
2(2-methoxyethoxy)ethanol	194	−69
Dimethylformamide	153	−60.4
Propylene Glycol	188	−59
EG/35% Water		−52.8
1,3 Butanediol	204	−50
Propylene Carbonate	242	−48.8
60% propylene Glycol in		−48
Water
50% EG in Water		−36.8
1,3 Propylene Glycol	213	−27
Ethylene Glycol	195	−13
Diethylene Glycol	106.2	−10
Triethylene glycol	285	−7
Cesium Formate	>100	−50
Potassium Formate 50%	>100	−50
Cesium and Potassium	>100	−50
Formate (50/50)
Diglyme or Diethylene glycol	162	−68
dimethyl ether
Diglyme or Diethylene glycol	162	−68
dimethyl ether
dimethylemalonate	180	−62
N-Methyl-2-pyrrolidone	202	−24
Dimethyl Succinate	218	−20
Dipropylene Glycol	230	−20
dimethylene glycol	244	−10.5
2,3 Butanediol	177	19
Ethylene Carbonate	243	34
2,2 dimethyl -1,3-Dioxolane -4-	73
methanol
α-Methyl-γ-butyrolactone	80
Urea

In embodiments, the hygroscopic compound may be a hygroscopic solvent, for example, as described in Table 1. The hygroscopic solvent may form about 1 to about 50 vol. % of the total volume of the attenuated acid formulation, or about 10 to about 40 vol. %, or about 15 to about 30 vol. %. In some embodiments, the hygroscopic solvent forms about 20 to about 25 vol. % of the total volume of the attenuated acid formulation.

In some embodiments, the attenuated acid formulation may be combined with suitable inorganic and/or organic acids or acid-producing systems as a means of tailoring the acid reactivity with the rock matrix.

Further, friction losses resulting from the high viscosity of many acid systems, such as gelled and emulsified acids, during the delivery may increase costs and lower efficiency due to pressure losses. The attenuated acid formulations provided herein are generally low-viscosity systems, which facilitates pumping the formulations to a desired location with minimal pressure losses due to friction. In some embodiments, the attenuated acid formulation is mixed at the wellhead, no need for batch mixing and transportation and storage facilities, saving money and time.

While the attenuated acid formulation can be used as the acidizing system without further changes, other wettability alteration chemicals can be added easily to the solvent phase. For example, relatively low concentrations, for example, less than 10.0 vol. %, of a retarding agent, such as a fluorinated surfactant, can be added. The retarding agent can be added to the attenuated acid formulation at the wellsite in an on-the-fly fashion, e.g., as the attenuated acid formulation is mixed and pumped into the wellbore. The agent concentration can be tailored to deliver a targeted wormhole penetration depth, for example, a higher concentration of the agent will yield a higher retardation effect.

Other additives may be used. For example, in some embodiments, the attenuated acid formulation includes surfactants that help to provide surface repellency to acid. The surfactants can be anionic, cationic, or neutral in nature. For example, a fluorinated surfactant can be functionalized to include a hydrophilic head-group with chemical affinity and retention on the rock surface and functionalized hydrophobic tail, that contains long chain fluorinated alkyl groups, serving the purpose of repelling water or aqueous solutions, such as acids, as well as oleic phases, such as condensate or crude oil, from the rock surface thus creating a temporary barrier to delay acid reactivity.

Suitable fluorinated surfactant molecules can include but are not limited to fluoro-surfactants, fluoro-polymers, fluoro-polymeric surfactants. For example the fluorinated surfactant molecules can include non-ionic acrylic fluorinated copolymer, nonionic fluoro-surfactant partially fluorinated acrylic copolymer, nonionic fluorinated methacrylate polymer, anionic phosphate fluoro-surfactant. Any of these structures can be linear, branched, or cross-linked chemical species. Other fluorinated surfactants include NW100 Fluorinated Surfactant in water with concentration of greater than about 5.0 wt. % and less than about 20.0 wt. %, available from VerdeChem Technologies of Alberta, CA. WS-1200 available from 3M of St. Paul, MN. Zonyl® UR, a mixture of anionic phosphate fluoro-surfactants, a phosphate hydrophilic head and a fluorocarbon hydrophobic tail, available from Chemours of Wilmington, DE. Another material from Chemours that can be used is Zonyl TBS. Zonyl TBS is an anionic sulfonate fluorinated surfactant, whose structure is given by R^FCH₂CH₂SO₃Y where R^F is F(CF₂CF₂)_1-9 and Y is a mixture of hydrogen and ammonium.

Another fluorinated surfactant that can be used has the structure R^f—SO₂N—R¹·M⁺ wherein R^f is a C₁ to C₁₂ perfluoroalkyl group, R¹ is H, an alkyl group, a hydroxyalkyl group, an alkylamine oxide, an alkylcarboxylate group, or an aminoalkyl group, and M⁺ is a cation. The acid may include hydrogen fluoride or an ammonium fluoride complex.

Another fluorinated surfactant that can be used has the formula R^f-Q-R1-SO₃-M⁺ wherein R^f is a C₁ to C₁₂ perfluoroalkyl group, R¹ is an alkylene of the formula—C_nH₂n(CHOH)_oC_mH_2m—, wherein n and m are independently 1 to 6, and o is 0 or 1, and is optionally substituted by a catenary oxygen or nitrogen group; M⁺ is a cation; and Q is —O—, or —SO₂NR²—, wherein R² is an H—, alkyl, aryl, hydroxyalkyl, aminoalkyl, or sulfonatoalkyl group, optionally containing one or more catenary oxygen or nitrogen heteroatoms. The fluoro-containing pendant group can be bonded to a primary, secondary or tertiary carbon

These fluoro-polymers, fluoro-surfactants, or fluorinated polymeric surfactants can be linear, branched chain, star-shaped, or brush-like molecules. Further, surface modified nanoparticles may be used.

Notably, the functionality of the surfactant molecule can be tailored to increase adsorption and retention to the rock surface in addition to degree of repellency to best meet the application needs. Where used, surfactants are present in the fluids in an amount sufficient to prevent incompatibility with formation fluids and/or well bore fluids.

The surfactants may be used in a liquid or powder form. If included, a liquid surfactant may be added in an amount of from about 0.01 vol. %, to about 10 vol. %, or about 0.5 vol. % to about 5 vol. %, or about 1 vol. % to about 3 vol. %. If included, a powdered surfactants may be present in an amount in the range of from about 0.001 wt. % to 10 wt. %, from about 0.5 wt. % to about 5 wt. %, or about 1 wt. % to about 3 wt. %.

Examples of suitable surfactants are non-emulsifiers that are commercially available from Halliburton Energy Services, Inc., of Duncan, Oklohama, under the tradenames “LOSURF-259™” nonionic non-emulsifier, “LOSURF-300™” nonionic surfactant, “LOSURF-357™” nonionic surfactant, and “LOSURF-400™” surfactant. Another example of a suitable surfactant is a non-emulsifier commercially available from Halliburton Energy Services, Inc., of Duncan, Oklahoma, under the tradename “NEA-96M™” Surfactant. In some embodiments, it is beneficial to add a surfactant to a viscosified fluid of a composition as described in this application as that fluid is being pumped down hole to help eliminate the possibility of foaming if so desired.

In some embodiments, a mutual solvents is used to help keep other additives in solution, such as, surfactants as described above. Suitable mutual solvents include, but are not limited to, Halliburton's MUSOL® Mutual Solvent, MUSOL® A Mutual Solvent, MUSOL® E Mutual Solvent, ethyleneglycolmonobutylether, propyleneglycolmonobutylether, water, methanol, isopropyl alcohol, alcohol ethers, aromatic solvents, other hydrocarbons, mineral oils, paraffins, and derivatives and combinations thereof, among others. If used, the mutual solvent may be included in an amount of from about 1 vol. % to about 20 vol. % or from about 5 vol. % to about 10 vol. %.

The solvent system and the added retarding agent and surfactants, if used, are not expected to lead to formation damage nor pose compatibility concerns with other oilfield chemistry additives. The use of the hygroscopic solvents and the control of acid diffusivity in both the well bore and the formation may provide additional benefits such as corrosion control in tubulars.

In addition to the retarding agent and surfactants, other additives such as friction reducers, may be added to the attenuated acid formulation to reduce friction pressure during pumping. This may allow the injection of attenuated acid formulation to be performed at a higher injection rate, further propagation of the acid inside the reservoir.

In some embodiments, the attenuated acid formulation is in the form of an emulsion. The emulsion is formed by the use on an emulsifying surfactant, such as nonionic surfactants including sorbitan esters, AF-61™ Emulsifier, and AF-70™ emulsifier, among others. In some embodiments, a surfactant is an oil external surfactant, for instance, AF-61™ Emulsifier and AF-70™ Emulsifier. For water external emulsions, surfactants that may be used include SEM-7™ Emulsifier, WS-36™ Dispersant, and WS-44™ Emulsifier. If a surfactant is used, generally an amount between 0.1 vol. % to 3 vol. % is sufficient to form the emulsion. In some embodiments, the emulsion can be mixed and then pumped. In some embodiments, the components can be pumped and then mixed down hole. In some embodiments, a surfactant is a viscoelastic surfactant.

The attenuated acid formulation may include one or more viscosifying agents. In some embodiments, the viscosifying agent is a polymer system, such as a cross-linked polymer system, where the crosslinker comprises zirconium or ferric metal clusters.

The attenuated acid formulation may include a gelling agent. Any gelling agent suitable for use in subterranean applications may be used in these fluids, including, but not limited to, natural biopolymers, synthetic polymers, cross-linked gelling agents, viscoelastic surfactants, and the like. Guar and xanthan are examples of suitable gelling agents. A variety of gelling agents may be used, including hydratable polymers that contain one or more functional groups such as hydroxyl, carboxyl, sulfate, sulfonate, amino or amide groups. Suitable gelling agents include polysaccharides, biopolymers, synthetic polymers, or a combination thereof. Examples of suitable polymers include, but are not limited to, guar gum and derivatives thereof, such as hydroxypropyl guar and carboxymethylhydroxypropyl guar, cellulose derivatives, such as hydroxyethyl cellulose, locust bean gum, tara, konjak, tamarind, starch, cellulose, karaya, diutan, scleroglucan, wellan, gellan, xanthan, tragacanth, and carrageenan, and derivatives and combinations of one or more of such polymers.

Further, synthetic polymers and copolymers may be used, alone or in combination with polysaccharides, biopolymers, or other polymers. Examples of such synthetic polymers include, but are not limited to, polyacrylate, polymethacrylate, polyacrylamide, polyvinyl alcohol, and polyvinylpyrrolidone. Commonly used synthetic polymer acid-gelling agents are polymers and/or copolymers consisting of various ratios of acrylic, acrylamide, acrylamidomethylpropane sulfonic acid, quaternized dimethylaminoethylacrylate, quaternized dimethylaminoethylmethacrylate, mixtures thereof, and the like. In other exemplary embodiments, the gelling agent molecule may be depolymerized. The term “depolymerized,” as used herein, generally refers to a decrease in the molecular weight of the gelling agent molecule. If used, a gelling agent may be present in the attenuated acid formulations in an amount from about 0.01 wt. % to about 5 wt. %, or about 0.5 wt. % to 4 wt. %, or about 2 wt. % to about 4 wt. % of the base fluid.

As some polymer gelling agents may be problematic under certain conditions in the formation, surfactants may be used as gelling agents. For example, when mixed with a fluid in a concentration above the critical micelle concentration, the molecules (or ions) of surfactants may associate to form micelles. The micelles may function, among other purposes, to stabilize emulsions, break emulsions, stabilize foam, change the wettability of a surface, solubilize certain materials, or reduce surface tension, among other functions. When used as a gelling agent, the molecules (or ions) of the surfactants associate to form micelles of a certain micellar structure, such as rod-like, wormlike, vesicles, and the like (which may be referred to as “viscosifying micelles”). Under certain conditions, for example, concentration, ionic strength of the fluid, and the like, the viscosifying micelles are capable of increasing the viscosity to a particular fluid or forming a gel. In some examples, viscosifying micelles may increase the viscosity of a fluid sufficiently that the fluid exhibits viscoelastic behavior, such as shear thinning properties. Further, because the viscosifying micelles may be sensitive to pH and hydrocarbons, the viscosity of these viscoelastic surfactant fluids may be reduced after introduction into the subterranean formation by the conditions on the formation. An example of a surfactant that may be useful for forming viscosifying micelles is Armovis EHS, Armovis EHS-W, Aromox APA-T, Aromox APA-TW, and Ethoquad O/12-PG from Nouryon. Chemically, these viscoelastic surfactants are oleyl methyl ammonium chloride ethoxlate, tallow amidoamine oxide, and zwitterionic surfactant such as erucamidopropyi hy droxypropylsultaine. The decrease in viscosity in the formation allows a substantial portion of the viscoelastic surfactant fluids to be produced back from the formation without the need for expensive remedial treatments. If used, the surfactants may be used in an amount of up to 10 wt. % of the attenuated acid formulation.

If a cross-linkable gelling agent is used, a cross linking agent can be included in the attenuated acid formulation, for example, to further increase viscosity. Cross-linking agents typically include a metal ion that is capable of cross-linking the molecules of the gelling agent. The crosslinking agents can be selected based on the functional groups on the gelling agent. Examples of suitable cross linking agents include, but are not limited to, zirconium compounds, such as zirconium lactate, zirconium lactate triethanolamine, zirconium carbonate, zirconium acetylacetonate, zirconium maleate, zirconium citrate, zirconium oxychloride, and zirconium diisopropylamine lactate; titanium compounds, such as titanium lactate, titanium maleate, titanium citrate, titanium ammonium lactate, titanium triethanolamine, and titanium acetylacetonate; aluminum compounds, such as aluminum lactate or aluminum citrate; and colemanite; antimony compounds; chromium compounds; iron compounds; copper compounds; zinc compounds; or a combination thereof.

An example of a suitable commercially available zirconium-based cross-linking agent is “CL-24™” cross linker from Halliburton Energy Services, Inc., Duncan, Oklahoma. An example of a suitable commercially available titanium-based crosslinking agent is “CL-39™” cross linker from Halliburton Energy Services, Inc., Duncan Oklahoma. An example of a suitable borate-based crosslinking agent is commercially available as “CL-22™” delayed borate cross linker from Halliburton Energy Services, Inc., Duncan, Oklahoma.

Divalent ions, such as calcium chloride and magnesium oxide, may be used as cross-linking compounds for some gelling agents. An example of a suitable divalent ion cross-linking agent is commercially available as “CL-30™” from Halliburton Energy Services, Inc., Duncan, Oklahoma. Another example of a suitable crosslinking agent is “CL-15,” from Halliburton Energy Services, Inc., Duncan Okla. When included, the crosslinking agent is present in an amount sufficient to provide the desired degree of cross-linking. In some embodiments, the crosslinking agent is present in the attenuated acid formulation in an amount in the range of from about 0.01 wt. % to about 5 wt. %, or 0.05 wt. % to about 4 wt. %, or about 0.4 wt. % to about 2 wt. %.

In some embodiments, if a gelling agent, such as a cross-linked gelling agent, is used, a suitable breaker may be used to ultimately reduce the viscosity of the attenuated acid formulation, depending on the gelling agent and its interaction with the acid-generating compound, the generated acid, and the well bore conditions. Any breaker suitable for the subterranean formation and the gelling agent may be used. The amount of a breaker will depend on the amount of gelling agent present in the attenuated acid formulation

The attenuated acid formulation may include bactericides to protect the subterranean formation, the fluid, or both from attack by bacteria. Such attacks may be problematic because they may lower the viscosity of the fluid, for example, resulting in poorer performance. Bacteria also can cause plugging by bacterial slime production and generate H₂S from sulfur containing compounds, turning the formation sour. Any bactericides known in the art are suitable. Where used, the bactericides are present in an amount sufficient to destroy at least a substantial portion of the bacteria that may be present. Examples of bactericides that may be used include, but are not limited to, a 2,2-dibromo-3-nitrilopropionamide, commercially available under the tradename “BE-3 S™” biocide from Halliburton Energy Services, Inc., of Duncan, Oklahoma, and a 2-bromo-2-nitro-1,3-propanediol commercially available under the tradename “BE-6™” biocide from Halliburton Energy Services, Inc., of Duncan, Oklahoma. In one embodiment, the bactericides are present in a provided composition in an amount in the range of from about 0.001 wt. % to about 0.003 wt. % by weight of the attenuated acid formulation. In certain embodiments, such bactericides may be present in a composition in an amount in the range of from about 0.01% to about 0.1% by volume.

The attenuated acid formulation may include other additives as desired. Examples of such additional additives include, but are not limited to, oxidizing agents, lost circulation materials, scale inhibitors, surfactants, clay stabilizers, corrosion inhibitors, paraffin inhibitors, asphaltene inhibitors, penetrating agents, clay control additives, iron control additives, reducers, oxygen scavengers, sulfide scavengers, emulsifiers, foaming agents, gases, derivatives thereof and combinations thereof, and the like. These may be selected to be compatible with the chemistry of the attenuated acid formulation.

In some embodiments, the attenuated acid formulation includes a foaming agent. Examples of foaming agents include but are not limited to: surfactants, such as water-soluble, nonionic, anionic, cationic, or amphoteric surfactants; carbohydrates, such as polysaccharides, cellulosic derivatives, guar or guar derivatives, Xanthan, carrageenan, starch polymers, gums, polyacrylamides, polyacrylates, betaine-based surfactants, viscoelastic surfactants, natural or synthetic clays; polymeric surfactants, such as partially hydrolyzed polyvinyl acetate, partially hydrolyzed modified polyvinyl acetate, block or copolymers of polyethylene, polypropylene, polybutylene or polypentene, proteins, and partially hydrolyzed polyvinyl acetate, polyacrylate and derivatives of polyacrylates, polyvinyl pyrrolidone and derivatives; N2; CO; CO2; air; and natural gas.

At block 204, the attenuated acid formulation is delivered to the formation, for example by a bull-heading or coiled tubing process. Upon injection into the wellbore, the attenuated acid formulation makes contact with the formation, allowing the hygroscopic solvent to adsorb part of the free water available. This may reduce the diffusion rate of protons to the rock surface, and calcium and magnesium ions from the rock surface into the liquid system. This mechanism may provide the lower activity of the attenuated acid formulation use to improve acid and wormhole penetration rate. The hygroscopic solvent can be injected as a part of the attenuated acid formulation. The treatment may be allowed to soak in the reservoir for up to 5 hours, or up to 10 hours, or up to 15 hours, or longer, to insure that all the acid injected reacted with formation and the acid is totally spent, for example, as determined by a pH increase from about 0 to about 4.

In some embodiments in which a fluorinated surfactant is used, an aqueous solution of the fluorinated surfactant and the attenuated acid formulation are introduced into the formation in separate stages. This may be done through the same or different tubing. The aqueous fluids to mix within the formation. In some embodiments, the aqueous solution of the fluorinated surfactant is introduced into the formation first.

EXAMPLES

The examples below show that the attenuated acid formulation described herein dissolved less limestone in the same exposure time, under static and ambient conditions, than the bulk acid as shown in Table 2. Furthermore, a lower volume of the attenuated acid formulation was required to form a wormhole through a core sample. This demonstrates that larger quantities of live acid can reach pay zones located deeper into the reservoir thereby increasing hydrocarbon production.

In the following examples, the abbreviation “g” stands for grams, the abbreviation “mL” stands for milliliters, and the abbreviation “ppm” stands for parts per million and can also be expressed as milligrams per liter.

Example 1. Core-plug Dissolution Experiments

The objective of this study was to determine the dissolution profile for a series of different attenuated acid formulations under analogous testing conditions. The parameters included ambient pressure and temperature, fluid volume (100 mL) and exposure time to acid (5 minutes). The acid formulations were prepared by mixing different glycols and hydrochloric acids (HCl) in ratios from 10/90 all the way up to 50/50, as shown in Tables 2-4.

In a typical experiment, a homogenous Indiana limestone core sample having a permeability between 4-8 mD was cut to have a diameter and length of 1.5″ D x 0.5″ L, respectively. One core sample was used for each individual test. The cores were dried in an oven at 248° F. overnight. Each of the dried cores was then saturated in DI-H₂O under vacuum for 12-24 h. The dry and saturated weight for the pre-treated core was recorded and porosity was calculated.

For each experiment, a saturated core was transferred to a 300 mL beaker containing 100 mL of the attenuated acid formulation (Listed in Tables 2, 3, and 4). For each experiment, the core sample was placed standing-up in the solution to ensure consistency across the series.

The weight of the saturated acidized core sample was measured for both the dry and saturated sample. From this, the percentage weight loss for each core was calculated and compared.

TABLE 2
Calculated weight loss of Indiana limestone core samples, post-acidizing, for the acid formulations
containing the benchmark acid, i.e. 15 wt. % HCl, and 15 wt. % HCl in the presence of the
noted additive-Rocks are exposed to acids for 5 minutes.
	HCl		Additive Concentration	Calcite Dissolved
Formulation	(wt. %)	Additive Name	(gpt)	(%)
1	15	N/A	0	45.7
2	15	Diethylene Glycol	10	29
3	15	Diethylene Glycol	25	19.5
4	15	Diethylene Glycol	50	5.5
5	15	Mono ethylene Glycol	10	30.4
6	15	Mono ethylene Glycol	25	20.4
7	15	Mono ethylene Glycol	50	7.0
8	15	Triethylene Glycol	10	27.4
9	15	Triethylene Glycol	25	18.3
10	15	Triethylene Glycol	50	7.6

TABLE 3
Calculated weight loss of Indiana limestone core samples, post-acidizing, for the acid formulations
containing the benchmark acid 28 wt. % HCl, and 28 wt. % HCl in the presence of the
noted additive-Rocks are exposed to acids for 5 minutes.
	HCl		Additive Concentration	Calcite Dissolved
Formulation	(wt. %)	Additive Name	(gpt)	(%)
1	28	N/A	0	57
2	28	Diethylene Glycol	10	34.7
3	28	Diethylene Glycol	25	28.4
4	28	Diethylene Glycol	50	4.6
5	28	Mono ethylene Glycol	10	40.1
6	28	Mono ethylene Glycol	25	30.4
7	28	Mono ethylene Glycol	50	6.3
8	28	Triethylene Glycol	10	35.8
9	28	Triethylene Glycol	25	33.8
10	28	Triethylene Glycol	50	12.3

TABLE 4
Calculated weight loss of Indiana limestone core samples, post-acidizing, for the acid formulations
containing the benchmark acid 31 wt. % HCl, and 31 wt. % HCl in the presence of the
noted additive-Rocks are exposed to acids for 5 minutes.
	HCl		Additive Concentration	Calcite Dissolved
Formulation	(wt. %)	Additive Name	(gpt)	(%)
1	31	N/A	0	67.2
2	31	Diethylene Glycol	10	40.4
3	31	Diethylene Glycol	25	29.7
4	31	Diethylene Glycol	30	28.0
5	31	Mono ethylene Glycol	35	22.7
6	31	Mono ethylene Glycol	40	18.6
7	28	Mono ethylene Glycol	45	12.6

FIGS. 3A to 3C are plots of the calculated weight-loss of Indiana limestone core samples for various acid formulations containing 15 wt. %, 28 wt. %, and 31 wt. %. The calculated weight loss of Indiana limestone core samples, post-acidizing, for the acid formulations containing 15 and 28 wt. % HCl in the presence of the noted additive shows that the dissolution of rock in acid was also observed at both low and high concentration of the fluorinated surfactants.

Example 2. High Temperature/High Pressure Coreflow Experiment

Coreflow experiments are commonly performed in the oil and gas industry to evaluate and benchmark the performance of an assortment of oilfield reservoir stimulation fluids, including acid systems. Accordingly, a linear coreflow experiment was performed to validate that the retardation behavior observed under static, ambient temperature and pressure applied to reservoir conditions, for example, 300° F. and 3000 psi. Attenuated acid formulations containing 28 wt. % HCl and different volume fractions for diethylene glycol (or DEG) (ratios are 10, 25 and 50 vol. % of the total formulation) were selected to test the breakthrough.

For acidizing applications, the volume of acid required to dissolve a path in the core plug, for example, from the inlet to the outlet of the core sample, is one indication of success at the lab-scale. This value is commonly referred to as pore volume to breakthrough, PV_BT. Acid systems having higher acid-rock reactivity will be associated with higher PV_BT values under analogous testing conditions and vice versa. Thus, lower PV_BT values are desired because the expectation is that at the field-scale the result will correlate with increased stimulation of the treated zone. This results from the tendency of live acid to penetrate deeper into the reservoir, thereby increasing the relative permeability for oil and gas to be produced.

Based on the attenuation results, it was anticipated that attenuated acid formulations tested would require less acid to achieve breakthrough as compared to 28 and 36 wt. % HCl in the absence of DEG. As shown in Table 5 below, the acid volume needed to achieve breakthrough was reduced by 75% by adding the DEG to the 28 wt. % HCl. Even when compared to 26 wt. % HCl emulsified acid, the acid system used here showed around 50% reduction in the volume of acid need to achieve breakthrough. In other words, there was a 100% improvement in the acid penetration rate when using the HCl/DEG formulation.

TABLE 3
Summary of coreflood data collected for 12-inch outcrop Indiana limestone core samples treated
with different acid systems at temperature, pressure and flow rate of 300° F., 3000 psi and 2
and 5 mL/min injection rate, respectively.
		Inj. Rate,	Core Length	Core Diameter
Fluid ID	PVBT	mL/min	(inch)	(inch)
28 wt. % HCl	1.1	5	12	1.5
26 wt. % Emulsified Acid	0.55	5	12	1.5
28 wt. % HCl + DEG (75/25)	0.23	5	12	1.5
28 wt. % HCl + DEG (75/25)		2	12	1.5
36 wt. % HCl + DEG (50/50)	0.23	2	12	1.5
36 wt. % HCl + DEG (75/25)	0.2	2	12	1.5
36 wt. % HCl + DEG (90/10)	0.21	2	12	1.5

Core Preparation Procedures for Coreflood Studies.

Core samples having a porosity ranging from 14.3 to 16.3% were selected for this study. The absolute permeability for each DI-H₂O saturated core sample was measured in a horizontal fashion using a high temperature, high-pressure (HT/HP) coreflood apparatus equipped with a 12″ coreholder. The permeability was calculated by flowing DI-H₂O through the core sample at various flow rates (e.g. ranging from 0.5 to 4 cm³/min) until the flow stabilized. For each rate, the average differential pressure across the core (DP) was recorded and applied to Darcy's equation to determine the initial permeability.

FIGS. 4A to 4D are plots of the pressure drop across core samples measured during acid injection. The pressure drop across the core sample measured during acid injection indicated acid breakthrough, from which the acid volume was measures. FIG. 5 is a plot of the acid volume to breakthrough for different compositions of attenuated acid solutions.

Embodiments

An embodiment described in examples herein provides a method for treating a carbonate reservoir with an attenuated acid formulation. The method includes mixing an acidic compound with a hygroscopic chemical to form the attenuated acid formulation, and injecting the attenuated acid formulation into a carbonate formation.

In an aspect, the acidic compound includes a mineral acid. In an aspect, the mineral acid includes hydrochloric acid, or methane sulfonic acid, or combinations thereof.

In an aspect, the acidic compound includes a compound that forms an acid in an aqueous solution. In an aspect, the acidic compound includes an alkylsulfonic acid. In an aspect, the alkylsulfonic acid is methanesulfonic acid (MSA).

In an aspect, the method includes mixing the acidic compound with the hygroscopic chemical to form the attenuated acid formulation on-the-fly as materials are injected into the formation.

In an aspect, the hygroscopic chemical is at least one of ethylene glycol, diethylene glycol, propylene glycol (or 1,2 propylene glycol or 1,2-propanediol), triethylene glycol, diglyme (or diethylene glycol dimethyl ether), 2-butoxyethanol (or EGBE or ethylene glycol monobutyl ether), dimethylformamide, propylene carbonate, 1,3 propylene glycol or 1,3-propanediol, diethylene glycol 2(2-ethoxyethoxy)ethanol (or DEGEE or diethylene glycol monoethyl ether, carbitol, carbitol cellosolve), dipropylene glycol, 1,3 butanediol, dimethylene glycol or 2,3 butanediol, ethylene carbonate, 2(2-methoxyethoxy)ethanol (or diethyleneglycol monomethyl ether), dimethylemalonate, n-methyl-2-pyrrolidone, a mixture of ethylene glycol and water (35/65), a mixture of 60% propylene glycol in water, a mixture of 50% ethylene glycol in water and monoethylene glycol (or ethylene glycol).

In an aspect, the acidic compound is a 15 wt. % solution of hydrochloric acid. In an aspect, the acidic compound is a 28 wt. % solution of hydrochloric acid. In an aspect, the acidic compound is a 31 wt. % solution of hydrochloric acid.

In an aspect, the hygroscopic chemical is diethylene glycol (DEG). In an aspect, the DEG makes up between 5 and 40 vol. % of the attenuated acid formulation. In an aspect, the DEG makes up between 20 and 25 vol. % of the attenuated acid formulation.

In an aspect, the hygroscopic chemical is monoethylene glycol (MEG). In an aspect, the MEG makes up between 5 and 40 vol. % of the attenuated acid formulation. In an aspect, the MEG makes up between 20 and 25 vol. % of the attenuated acid formulation.

In an aspect, the attenuated acid formulation dissolves between about 15 wt. % calcite and 30 wt. % calcite from a limestone core sample in about 5 minutes. In an aspect, the attenuated acid formulation dissolves between about 20 wt. % calcite and 25 wt. % calcite from a limestone core sample in about 5 minutes. In an aspect, the attenuated acid formulation achieves a pore volume to breakthrough (PV_BT) of less than 0.3.

In an aspect, the attenuated acid formulation includes a surfactant. In an aspect, the surfactant includes a fluorinated surfactant.

In an aspect, the attenuated acid formulation is an emulsion. In an aspect, the attenuated acid formulation is a foam.

In an aspect, the method includes viscosifying the attenuated acid formulation by a gelling agent.

In an aspect, the attenuated acid formulation includes a viscoelastic surfactant.

In an aspect, the attenuated acid formulation includes a crosslinkable gelling agent.

In an aspect, the method includes acidizing a carbonate matrix with the attenuated acid formulation, acid fracturing with the attenuated acid formulation, acidizing an open hole with the attenuated acid formulation, cleaning a wellbore with the attenuated acid formulation, cleaning scale with the attenuated acid formulation, increasing the permeability of injection wells with the attenuated acid formulation, cleaning up filter cake, or cleaning up drilling mud.

Other implementations are also within the scope of the following claims.

Claims

1. A method for treating a carbonate reservoir with an attenuated acid formulation, comprising: mixing an acidic compound with a hygroscopic chemical to form the attenuated acid formulation, wherein the acidic compound comprises a hydrochloric acid solution and the hydroscopic chemical comprises diethylene glycol, wherein the DEG comprises between 5 and 40 vol. % of the attenuated acid formulation; andinjecting the attenuated acid formulation into a carbonate formation, wherein the pore volume for breakthrough (PVBT) is less than 0.23 for an attenuated acid formulation comprising either a 28 wt. % or a 36 wt. % solution of HCl.

2. The method of claim 1, wherein the acidic composition further comprises methane sulfonic acid.

3. The method of claim 1, wherein the acidic compound comprises a compound that forms an acid in an aqueous solution.

4. The method of claim 1, wherein the acidic compound comprises an alkylsulfonic acid.

5. The method of claim 4, wherein the alkylsulfonic acid is methanesulfonic acid (MSA).

6. The method of claim 1, comprising mixing the acidic compound with the hygroscopic chemical to form the attenuated acid formulation on-the-fly as materials are injected into the formation.

7. The method of claim 1, wherein the DEG comprises between 20 and 25 vol. % of the attenuated acid formulation.

8. The method of claim 1, wherein the attenuated acid formulation comprises a surfactant.

9. The method of claim 8, wherein the surfactant comprises a fluorinated surfactant.

10. The method of claim 1, wherein the attenuated acid formulation is an emulsion.

11. The method of claim 1, wherein the attenuated acid formulation is a foam.

12. The method of claim 1, comprising viscosifying the attenuated acid formulation by a gelling agent.

13. The method of claim 1, wherein the attenuated acid formulation comprises a viscoelastic surfactant.

14. The method of claim 1, wherein the attenuated acid formulation comprises a crosslinkable gelling agent.

15. The method of claim 1, comprising: acidizing a carbonate matrix with the attenuated acid formulation;acid fracturing with the attenuated acid formulation;acidizing an open hole with the attenuated acid formulation;cleaning a wellbore with the attenuated acid formulation;cleaning scale with the attenuated acid formulation;increasing the permeability of injection wells with the attenuated acid formulation;cleaning up filter cake; orcleaning up drilling mud.