STABILIZED MICROEMULSION CARRIER FLUID

BACKGROUN D

[0001 ] The present disclosure generally relates to acidizing a subterranean formation, and, more specifically, to acidizing fluids that may minimize the severity of precipitation during an acidizing process.

[0002] Treatment flu ids can be used in a variety of su bterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimu lation operations, production operations, sand control treatments, and the like. As used herein, the terms "treat," "treatment," "treating," and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, u nless otherwise specified herein. Illustrative treatment operations can include, for example, drilling operations, fractu ring operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.

[0003] Acidizing operations can be used to stimulate a su bterranean formation to increase production therefrom. Du ring an acidizing operation, an acid-soluble material in the su bterranean formation can be dissolved by one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, or to remove acid- solu ble precipitation damage in the subterranean formation . The acid-solu ble material being dissolved by the acid(s) can be part of the native formation matrix or can have been deliberately introduced into the subterranean formation in conjunction with a stimulation or like treatment operation (e.g. , proppant or gravel particulates) . Illustrative su bstances within the native formation matrix that may be dissolved by an acid include, but are not limited to, carbonates, silicates and alu minosilicates. Other substances can also be dissolved during the cou rse of performing an acidizing operation, and the foregoing su bstances should not be considered to limit the scope of substances that may undergo acidization. As further discussed below, certain components dissolved during an acidizing operation can be problematic and possibly detrimental for future production from the subterranean formation.

[0004] Carbonate formations can contain minerals that comprise a carbonate anion (e.g., calcite and dolomite). When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to solubilize the carbonate material. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g. , acetic and formic acids) can be used to treat a carbonate formation, often with similar degrees of success.

[0005] Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term "siliceous" refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e. , silica), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g. , aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite). Acidizing a siliceous formation or a formation containing a siliceous material is thought to be considerably different than acidizing a carbonate formation. Specifically, the mineral and organic acids that can be effective for acidizing a carbonate formation may have little effect on a siliceous formation, since these acids do not effectively react with siliceous materials to affect their dissolution. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution. Oftentimes, a mineral acid or an organic acid can be used in conjunction with hydrofluoric acid to maintain a low pH state as the hydrofluoric acid becomes spent during dissolution of a siliceous material. The low pH state may promote initial silicon or aluminum dissolution and aid in maintaining these substances in a dissolved state.

[0006] Despite the advantages that can be realized by acidizing a siliceous formation, silicon and aluminum can sometimes further react to produce damaging precipitates after their dissolution that can often be more detrimental for production than if the acidizing operation had not been performed in the first place. The equilibrium solubility levels of silicon and aluminum in a fluid usually depend upon one another. That is, by maintaining high levels of dissolved aluminum during an acidizing operation, silicon dissolution can also be promoted. In this regard, dissolved aluminum can be maintained in a fluid by coordination with fluoride ions, but such aluminum coordination can leave insufficient fluoride ions remaining for effective silicon solubilization to take place, thereby leading to damaging silicon precipitation. Chelating agents can be used to increase the degree of silicon solubilization by maintaining aluminum in a dissolved state. By chelating aluminum to form a soluble aluminum complex, increased levels of dissolved silicon may be realized, since more free fluoride ions are left available to affect its solubilization.

[0007] Even when chelating agents are used during an acidizing operation, precipitation can still be problematic. If the amount of aluminum or another metal needing sequestration exceeds the amount of chelating agent that is available, precipitation can still occur. An even more significant issue is that of precipitation of insoluble fluorosilicates and aluminosilicates that can occur in the presence of Group 1 metal ions (i.e. , alkali metal ions). The terms "Group 1 metal ions" and "alkali metal ions" will be used synonymously herein. Under low pH conditions (e.g., below a pH of about 3), dissolved silicon can react with Group 1 metal ions (e.g., Na ⁺ and K ⁺ ) to produce insoluble alkali metal fluorosilicates and aluminosilicates. Other metal ions, including Group 2 metal ions (e.g. , Ca ²⁺ and Mg ²⁺ ), may also be problematic in this regard. In many instances, pre-flush fluids may be introduced to a subterranean formation prior to performing an acidizing operation in order to decrease the quantity of available alkali metal ions. In some instances, such pre-flush fluids can contain ammonium ions (NH ₄ ⁺ ) that can displace alkali metal ions in the subterranean formation and leave it desirably conditioned for an acidizating operation. In contrast to alkali metal ions, ammonium ions are not believed to promote the formation of insoluble fluorosilicates and aluminosilicates. The use of pre-flush fluids, particularly those containing ammonium ions, can considerably add to the time and expense needed to perform an acidizing operation. In addition to problematic alkali metal ions in the subterranean formation itself, the precipitation of alkali metal fluorosilicates and fluoroaluminates can considerably limit the sourcing of carrier fluids that may be used when acidizing a subterranean formation.

[0008] Other techniques can also be applied for mitigating precipitation during an acidizing operation. Without limitation, these additional techniques can include adding agents to a subterranean formation that directly complex silicon or that increase the fluid solubility of highly insoluble substances. Despite the various approaches that can be used for mitigating precipitation during an acidizing operation, precipitation represents a complex problem that can arise from a number of different sources. As a result, precipitation can be a near- inevitable problem that must be addressed in some manner during the course of conducting a downhole operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The following figures are included to illustrate certain aspects of the present invention, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, as will occur to one having ordinary skill in the art and the benefit of this disclosure.

[0010] FIGURE 1 shows a plot of differential pressure and permeability of the core sample as a function of time.

[0011] FIGURE 2 shows a plot of concentration versus the sample fraction for the ionic constituents obtained during treatment of the core sample.

DETAILED DESCRIPTION

[0012] The present disclosure generally relates to acidizing a subterranean formation, and, more specifically, to acidizing fluids that may minimize the severity of precipitation during an acidizing process.

[0013] Despite the various precautions that may be undertaken to reduce the incidence or severity of precipitation during an acidizing operation, at least some precipitation inevitably occurs. If the quantity and/or size of the precipitates can be kept sufficiently small, however, they may not be overly damaging to the subterranean formation. Small quantities of precipitates may not appreciably impact the permeability of a subterranean formation by blocking flow pathways therein. Precipitates that are small in size may remain fluidly mobile such that they can be conveyed by a fluid and produced from the subterranean formation. By formulating an acidizing fluid to better transport precipitates that do form during an acidizing operation, the acidizing operation may be made to be more tolerant toward precipitation. By making an acidizing fluid more tolerant toward precipitate occurrence, there may be a reduced need to formulate the acidizing fluid to contain additional measures that minimize the occurrence of precipitates, thereby potentially lowering the cost of conducting an acidizing operation.

[0014] In order to increase the tolerance of acidizing fluids toward precipitation, the acidizing fluids described herein comprise a stabilized microemulsion carrier fluid. As used herein, the term "stabilized microemulsion" refers to a composition containing a continuous fluid phase and a discontinuous phase of emulsion particulates in the continuous fluid phase, where the emulsion particulates are about 10 microns or smaller in size. The term "stabilized" refers to the feature that the microemulsion does not break into two or more immiscible continuous fluid phases on its own. Stabilized microemulsions also encompass stabilized nanoemulsions, which have emulsion particulates that are about 100 nm or smaller in size. As used herein, the terms "stabilized microemulsion" and "stabilized nanoemulsion" may be used interchangeably with the terms "complex microfluid" or "complex nanofluid." It is to be recognized that any embodiment described herein in reference to a stabilized microemulsion may be practiced in a like manner with a stabilized nanoemulsion.

[0015] Stabilized microemulsions may be particularly advantageous for mitigating the effects of precipitation during an acidizing operation. As a carrier fluid, stabilized microemulsions may possess good fluidity and therefore a ready ability to flow through the porosity of a subterranean formation. The good flow characteristics of stabilized microemulsions contrast those of emulsions having larger particulate sizes, which can exhibit significant viscosity and/or gelation and sometimes only flow with difficulty. By maintaining good flow characteristics, acidizing fluids formulated with a stabilized microemulsion carrier fluid may readily migrate through the flow pathways in low permeability subterranean formations, including shale formations, thereby wetting the surface of the subterranean formation. The enhanced wetting of the subterranean formation may improve the effectiveness of the acidizing fluid upon the surface of the subterranean formation. Moreover, the stabilized microemulsion can provide good wetting to the surface of any precipitates that may form during the course of an acidizing operation. Surface wetting of the precipitates may be particularly desirable to facilitate their suspension in the stabilized microemulsion carrier fluid. By facilitating suspension of the precipitates in the stabilized microemulsion carrier fluid, the precipitates may be produced from the subterranean formation as the stabilized microemulsion carrier fluid is flowed back to the earth's surface.

[0016] Not only do stabilized microemulsion carrier fluids promote the production of precipitates from a subterranean formation, but also the microemulsion structure may allow larger precipitates to be supported and transported in the microemulsion carrier fluid than would otherwise be possible without the emulsion particulates being present. Moreover, as a further advantage, the presence of a stabilized microemulsion during precipitate formation may promote the formation of smaller precipitates than would otherwise be formed in its absence. Still further, stabilized microemulsion may also reduce the rate of precipitation in a subterranean formation once precipitation begins.

[0017] As discussed above, chelating agents can be used in conjunction with an acidizing operation to lessen the occurrence of damaging precipitates in a subterranean formation. Chelating agents and other substances used to address downhole precipitation can readily be employed in conjunction with a stabilized microemulsion carrier fluid, thereby targeting the downhole precipitation from competing mechanistic pathways. By using a microemulsion carrier fluid in combination with one or more other agents that suppress precipitation, the amount of precipitation can be kept sufficiently low for suspension of the precipitates in the microemulsion carrier fluid and production from the subterranean formation. By addressing precipitation via competing mechanistic pathways, it may be possible to acidize a subterranean surface more vigorously with less fear of formation damage taking place, thereby reducing the time and expense needed to conduct the acidizing operation. Moreover, by employing a stabilized microemulsion carrier fluid, there may be an increased tolerance for alkali metal ions in the subterranean formation, which may also decrease the time and expense of performing the acidizing operation by reducing the need for pre-flush treatment cycles.

[0018] As an additional advantage of the acidizing fluids described herein, some stabilized microemulsions can be formed from naturally occurring materials, including those that are biodegradable. Biodegradability represents a significant advantage for the downhole use of stabilized microemulsions and related materials. For example, in some embodiments, a biodegradable chelating agent can be used in conjunction with a biodegradable stabilized microemulsion in order to provide an acidizing fluid with a highly desirable environmental profile.

[0019] As used herein, the term "acid source" will refer to an acid or an acid-generating compound.

[0020] As used herein, the term "spent fluid" will refer to an acidizing fluid in which an acid source therein has been at least partially consumed.

[0021] As used herein, the term "siliceous material" refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates.

[0022] As used herein, the term "precipitate" refers to a solid substance formed in the course of reacting a substance in a subterranean formation with an acidizing fluid.

[0023] In some embodiments, methods described herein can comprise: providing an acidizing fluid comprising a stabilized microemulsion carrier fluid, a chelating agent, and an acid source; introducing the acidizing fluid into a subterranean formation; at least partially dissolving a surface in the subterranean formation with the acidizing fluid, thereby forming a spent fluid; and producing the spent fluid from the subterranean formation. In some embodiments, a precipitate may be formed in the course of at least partially dissolving a surface in the subterranean formation. In some embodiments, the methods may further comprise producing the precipitate from the subterranean formation with the spent fluid.

[0024] The identity of the stabilized microemulsion carrier fluid used in the acidizing fluids described herein is not believed to be particularly limited. Illustrative examples of stabilized microemulsions may be found in United States Patent Nos. 7,380,606 and 8,357,639 and United States Patent Application Publication 2008/0287324 (now abandoned). Generally, the stabilized microemulsions contain one or more surfactants, an alcohol, and a continuous phase comprising one or more solvents. The continuous phase may be an aqueous phase or an organic phase, and the stabilized microemulsions may be water-in-oil microemulsions or oil-in-water microemulsions. For acidizing operations conducted with aqueous acidizing fluids, the latter may be preferred. One particularly suitable stabilized microemulsion may be GASPERM 1000® fluid additive, which is available from Halliburton Energy Services. Other suitable stabilized microemulsions may be available from Flotek. Another suitable stabilized microemu lsion is LOSURF 360 su rfactant, which is available from Hallibu rton Energy Services, Inc. The stabilized microemulsions may be added to a carrier fluid, such as an aqueous carrier flu id, that is of a non-emulsified nature.

[0025] Illustrative solvents that may be present in the continuous phase of the microemulsions include, for example, terpenes (including monoterpenes and diterpenes) ; alkyi, cyclic or aryl acid esters of short chain alcohols (e.g. , ethyl lactate), water, brine, unsatu rated salt solutions, diesel, kerosene, esters, linear a-olefins, poly a-olefins, internal olefins, linear alkyi benzenes, and the like. One example of a terpene that may be particularly desirable is d-limonene, which has good dissolution properties and is biodegradable. Ethyl lactate also possesses good biodegradation properties. Other su itable solvents can be envisioned by one having ordinary skill in the art.

[0026] For oil-in-water microemu lsions, suitable surfactants may include, for exam ple, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan monooleate, linear alcohol alkoxylates, alkyi ether su lfates, dodecylbenzenesu lfonic acid, linear nonyl- phenols, dioxane, ethoxylated castor oil, dipalmitoylphosphatidylcholine, polyoxyethylene nonyl-phenol ether, sodium bis-2-ethylhexylsu lfosuccinate, tetraethyleneglycoldodecylether, sodium octylbenzenesu lfonate, alkyi propoxy- ethoxysu lfonates, alkyi propoxy-ethoxysu lfates, alkylarylpropoxy- ethoxysu lfonates, benzenesulfonates, and derivatives thereof. Other surfactants suitable for use in oil-in-water microemulsions can be envisioned by one having ordinary skill in the art.

[0027] For water-in-oil microemu lsions, suitable surfactants may include, for example, sorbitan monopalmitate, sorbitan monostearate, and sorbitan monooleate. Mixtu res of the foregoing with the surfactants used in oil- in-water emu lsions may also be used . Other surfactants suitable for use in water-in-oil microemulsions can be envisioned by one having ordinary skill in the art.

[0028] Other suitable surfactants that may be present in the microemulsions may include, for example, alkyi sulfates, alkyi ether sulfonates, alkyi sulfonates, alkylaryl su lfonates, linear and branched al kyi ether su lfates and sulfonates, polypropoxylated su lfates and su lfonates, polyethoxylated sulfates and sulfonates, alkyi disu lfonates, alkylaryl disulfonates, alkyi disulfates, alkyl sulfosuccinates, alkyl ether sulfates, carboxylates, fatty acid carboxylates, phosphate esters, amides, diamides, polyglycol esters, alkyl polyglycosides, sorbitan esters, methyl glucoside esters, alcohol ethoxylates, arginine methyl esters, alkanolamines, and alkylenediamines.

[0029] Without being bound by any theory of mechanism, it is believed that the alcohol component of the stabilized microemulsions may serve as a coupling agent between the solvent and the surfactant and thereby stabilize the microemulsion. Illustrative alcohols that may be present in the stabilized microemulsion can include Ci-C ₂ o alcohols such as, for example, methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol, t-butanol, n-pentanol, n- hexanol, and the like. Other suitable alcohols can include glycols such as ethylene glycol, polyethyleneglycol, propylene glycol, and triethylene glycol. It is to be recognized that in some embodiments, a stabilized microemulsion may be formed without an alcohol being present.

[0030] The type of subterranean formation being treated with the acidizing fluids described herein is not believed to be particularly limited. In some embodiments, the subterranean formation may comprise a carbonate formation, such as a limestone or dolomite formation, for example. Particularly in embodiments where the acidizing fluid comprises hydrofluoric acid or a hydrofluoric acid-generating compound, the subterranean formation may comprise a siliceous formation or have had a siliceous material introduced thereto. In some embodiments, the subterranean formation may comprise a sandstone formation or a clay-containing formation. In some embodiments, the subterranean formation may comprise a subterranean formation having a low permeability, such as a shale formation, for example. In some or other embodiments, the subterranean formation may comprise native minerals such as, for example, authigenic or detrital minerals, particularly layered aluminosilicates, feldspathic minerals, or purely siliceous minerals.

[0031] In addition to subterranean formations that natively contain the above minerals, in some embodiments, the subterranean formation may have had any of the above minerals introduced thereto. For example, in some embodiments, proppant or gravel particulates that comprise a potentially problematic mineral can be introduced to a subterranean formation and subsequently be treated therein. That is, in some embodiments, the acidizing flu ids described herein may be used for treating a proppant pack or a gravel pack in a subterranean formation .

[0032] In more particular embodiments, the subterranean formation may comprise an aluminosilicate material, which can be natively present in the subterranean formation or have been introduced thereto . Alu minosilicate materials that may be present in a siliceous su bterranean formation include clays (including mixed layer and swelling clays), zeolites, kaolinite, illite, chlorite, bentonite, and feldspars, for example. It is to be recognized that, in some embodiments, a siliceous subterranean formation may comprise other materials that are non-siliceous in nature. For example, in some embodiments, a siliceous subterranean formation or a siliceous material in another type of su bterranean formation may comprise about 1% to about 35% of a carbonate material by mass. In some embodiments, the subterranean formation can comprise a matrix that is substantially non-siliceous in nature but contains a siliceous material therein (e.g. , introduced proppant or gravel particulates or siliceous particulates within a carbonate formation matrix) .

[0033] In some embodiments, acidizing fluids described herein can have a pH ranging between about 0 and about 7, or between about 0 and about 2, or between about 1 and about 2, or between about 1 and about 3, or between about 1 and about 4, or between about 1 and about 5, or between about 1 and about 6, or between about 1 and about 7. Such pH values may be especially advantageous for promoting dissolution of silicates and alu minosilicates at a desirable rate, as wel l as maintaining silicon in a dissolved state once dissolution occurs. The low pH values can discourage the formation of precipitates in the subterranean formation.

[0034] In some embodiments, the acidizing fluids described herein may include an acid or an acid-generating compound . The acid or acid-generating compound may be a mineral acid or an organic acid . Su itable mineral acids may include, for example, hydrochloric acid, hydrofluoric acid and the like. Suitable organic acids may include, for example, formic acid, acetic acid, chloroacetic acid, dichloroacetic acid, trichloroacetic acid, methanesu lfonic acid and the like. In some embodiments, hydrofluoric acid may be present in the acidizing flu ids described herein in combination with an acid-generating compound or another acid, such as hydrochloric acid or an organic acid, for example. Use of hydrofluoric acid in combination with another acid or an acid-generating compound may help maintain the pH of the acidizing fluid in a desired range as the hydrofluoric acid of the acidizing fluid becomes spent. For example, the acid or acid-generating compound may be present in the acidizing fluid in a quantity sufficient to maintain the pH of the acidizing fluid within the ranges set forth above. In other embodiments, however, hydrofluoric acid may be the only acid present, other than an acid comprising the chelating agent.

[0035] Examples of suitable acid-generating compounds may include, for example, esters, aliphatic polyesters, orthoesters, poly(orthoesters), poly(lactides), poly(glycolides), poly(E-caprolactones), poly(hydroxybutyrates), poly(anhydrides), ethylene glycol monoformate, ethylene glycol diformate, diethylene glycol diformate, glyceryl monoformate, glyceryl diformate, glyceryl triformate, triethylene glycol diformate, and formate esters of pentaerythritol.

[0036] In some embodiments, the acidizing fluids described herein may comprise hydrofluoric acid or a hydrofluoric acid generating compound. Examples of suitable hydrofluoric acid-generating compounds may include, for example, fluoroboric acid, fluorosulfuric acid, hexafluorophosphoric acid, hexafluoroantimonic acid, difluorophosphoric acid, hexafluorosilicic acid, potassium hydrogen difluoride, sodium hydrogen difluoride, boron trifluoride acetonitrile complex, boron trifluoride acetic acid complex, boron trifluoride dimethyl ether complex, boron trifluoride diethyl ether complex, boron trifluoride dipropyl ether complex, boron trifluoride dibutyl ether complex, boron trifluoride t-butyl methyl ether complex, boron trifluoride phosphoric acid complex, boron trifluoride dihydrate, boron trifluoride methanol complex, boron trifluoride ethanol complex, boron trifluoride propanol complex, boron trifluoride isopropanol complex, boron trifluoride phenol complex, boron trifluoride propionic acid complex, boron trifluoride tetrahydrofuran complex, boron trifluoride piperidine complex, boron trifluoride ethylamine complex, boron trifluoride methylamine complex, boron trifluoride triethanolamine complex, polyvinylammonium fluoride, polyvinylpyridinium fluoride, pyridinium fluoride, imidazolium fluoride, ammonium fluoride, ammonium bifluoride, tetrafluoroborate salts, hexafluoroantimonate salts, hexafluorophosphate salts, bifluoride salts, and any combination thereof.

[0037] When used, a hydrofluoric acid-generating compound can be present in the acidizing fluids described herein in an amount ranging between about 0.1% to about 20% of the acidizing fluid by weight. In other embodiments, an amount of the hydrofluoric acid-generating compound can range between about 0.5% to about 10% of the acidizing fluid by weight or between about 0.5% to about 8% of the acidizing fluid by weight. Hydrofluoric acid may be used in the acidizing fluids in similar concentration ranges.

[0038] In various embodiments, the acidizing fluids described herein may comprise a chelating agent, and the methods described herein may further comprise complexing a metal ion in the subterranean formation with the chelating agent. The metal ion may be produced by the reaction of a surface in the subterranean formation with an acid, or the chelating agent may react directly with the surface to affect dissolution. As described above, inclusion of a chelating agent in the acidizing fluid and complexation of a metal ion therewith may be desirable to discourage precipitation in a subterranean formation such that the stabilized microemulsion carrier fluid can better protect the subterranean formation from precipitates that do form.

[0039] In some embodiments, traditional chelating agents such as, for example, ethylenediaminetetraacetic acid (EDTA), propylenediaminetetraacetic acid (PDTA), nitrilotriacetic acid (NTA), N-(2- hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA), hydroxyethyliminodiacetic acid (HEIDA), cyclohexylenediaminetetraacetic acid (CDTA), diphenylaminesulfonic acid (DPAS), ethylenediaminedi(o-hydroxyphenylacetic) acid (EDDHA), glucoheptonic acid, gluconic acid, citric acid, any salt thereof, any derivative thereof, or the like may be included in the acidizing fluids described herein.

[0040] In some or other embodiments, a biodegradable chelating agent may be included in the acidizing fluids described herein. As used herein, the term "biodegradable" refers to a substance that can be broken down by exposure to environmental conditions including native or non-native microbes, sunlight, air, heat, and the like. Use of the term "biodegradable" does not imply a particular degree of biodegradability, mechanism of biodegradability, or a specified biodegradation half-life. Suitable biodegradable chelating agents may include, for example, glutamic acid diacetic acid (GLDA), methylglycine diacetic acid (MGDA), β-alanine diacetic acid (β-ADA), ethylenediaminedisuccinic acid, S,S-ethylenediaminedisuccinic acid (EDDS), iminodisuccinic acid (IDS), hydroxyiminodisuccinic acid (HIDS), polyamino disuccinic acids, N-bis[2-(l,2- dicarboxyethoxy)ethyl]glycine (BCA6), N-bis[2-( l,2- dicarboxyethoxy)ethyl]aspartic acid (BCA5), N-bis[2-( l,2- dicarboxyethoxy)ethyl]methylglycine (MCBA5), N-tris[( l,2- dicarboxyethoxy)ethyl]amine (TCA6), N-methyliminodiacetic acid (MIDA), iminodiacetic acid (IDA), N-(2-acetamido)iminodiacetic acid (ADA), hydroxymethyl-iminodiacetic acid, 2-(2-carboxyethylamino) succinic acid (CEAA), 2-(2-carboxymethylamino) succinic acid (CMAA), diethylenetriamine- N,N"-disuccinic acid, tnethylenetetramine-N,N"'-disuccinic acid, 1,6- hexamethylenediamine-N,N'-disuccinic acid, tetraethylenepentamine-N,N""- disuccinic acid, 2-hydroxypropylene- l,3-diamine-N,N'-disuccinic acid, 1,2- propylenediamine-N,N'-disuccinic acid, l,3-propylenediamine-N,N'-disuccinic acid, cis-cyclohexanediamine-N,N'-disuccinic acid, trans-cyclohexanediamine- Ν,Ν'-disuccinic acid, ethylenebis(oxyethylenenitrilo)-N,N'-disuccinic acid, glucoheptanoic acid, cysteic acid-N,N-diacetic acid, cysteic acid-N-monoacetic acid, alanine-N-monoacetic acid, N-(3-hydroxysuccinyl) aspartic acid, N-[2-(3- hydroxysuccinyl)]-L-serine, aspartic acid-N,N-diacetic acid, aspartic acid-N- monoacetic acid, any salt thereof, any derivative thereof, or any combination thereof. In some embodiments, a biodegradable chelating agent may be used in combination with a biodegradable stabilized microemulsion carrier fluid, thereby improving the overall environmental profile of the acidizing fluids.

[0041] In some embodiments, the concentration of the chelating agent in the acidizing fluid may range between about 0.1% to about 50% of the acidizing fluid by weight, or between about 0.1% to about 5% of the acidizing fluid by weight, or between about 1% to about 10% of the acidizing fluid by weight, or between about 5% to about 15% of the acidizing fluid by weight, or between about 0.5% to about 5% of the acidizing fluid by weight, or between about 10% to about 25% of the acidizing fluid by weight, or between about 5% to about 50% of the acidizing fluid by weight.

[0042] In some embodiments, methods described herein may comprise:

providing an acidizing fluid comprising a stabilized microemulsion carrier fluid, a chelating agent, and a hydrofluoric acid source; introducing the acidizing fluid into a subterranean formation comprising a siliceous material; and at least partially dissolving the siliceous material in the subterranean formation with the acidizing fluid, thereby forming a spent fluid. In some embodiments, the siliceous material may comprise a silicate, an aluminosilicate, or any combination thereof.

[0043] In some embodiments, the methods may further comprise producing the spent fluid from the subterranean formation. In some embodiments, precipitates may be formed in the process of interacting the acidizing fluid with the subterranean formation. In some embodiments, methods described herein may further comprise forming a precipitate in the subterranean formation while at least partially dissolving the siliceous material, and producing the precipitate from the subterranean formation with the spent fluid.

[0044] In some embodiments, methods described herein may comprise: introducing a stabilized microemulsion carrier fluid, a chelating agent, and a hydrofluoric acid source to a subterranean formation comprising a siliceous material; at least partially dissolving a siliceous material in the subterranean formation with the hydrofluoric acid source; and complexing a metal ion in the subterranean formation with the chelating agent.

[0045] As described above, in some embodiments, the stabilized microemulsion carrier fluid, the chelating agent, and the hydrofluoric acid source may be combined in an acidizing fluid that is introduced to the subterranean formation. Although it may be desirable to include the stabilized microemulsion carrier fluid with the acidizing fluid in order to minimize the number of treatment stages, it is not necessary to do so. In some embodiments, the stabilized microemulsion carrier fluid may be introduced to the subterranean formation before the acidizing operation is carried out, that is, before the hydrofluoric acid source, the chelating agent, or both. Introduction of the stabilized microemulsion to the subterranean formation before acidization may be desirable to achieve better wetting of a surface within the subterranean formation. In other embodiments, the stabilized microemulsion carrier fluid may be introduced to the subterranean formation after the acidizing operation is carried out, that is, after the hydrofluoric acid source, the chelating agent, or both. Introduction of a stabilized microemulsion carrier fluid after acidization may be desirable to facilitate better wetting of precipitates that do form during the acidizing operation. Moreover, if the location of precipitates in the subterranean formation is rather localized, addition of the stabilized microemulsion carrier fluid as a spot treatment may allow the precipitates to be more efficiently produced from the subterranean formation with smaller volumes of the stabilized microemu lsion carrier fluid than wou ld otherwise be needed .

[0046] In some embodiments, the acidizing fluids described herein may further comprise a compound having two or more quaternized amine groups, as described in commonly owned United States Patent Application 13/588, 158, entitled "Treatment Fluids Comprising a Stabilizing Compound Having Quaternized Amine Grou ps and Methods for Use Thereof," filed on August 17, 2012. As described therein, such compounds may desirably decrease the incidence of precipitation during an acidizing process. In more particular embodiments, the compound having two or more quaternized amine groups may comprise a bis-quaternary ammoniu m compound . Illustrative bis-quaternary ammonium compounds that may be used in the treatment fluids described herein are fu rther set forth in United States Patent Application Publications 2004/0235677 (now abandoned) and 2006/0013798, and United States Patent No. 7,947,633. One exam ple of a bis-quaternary ammonium compou nd composition that can be suitable for use in the treatment flu ids described herein is product BQA-6170 (produced by Corsitech, (Houston, Texas) and commercially available through Hallibu rton Energy Services, Inc. ), which is a mixture of bis-quaternary ammonium compounds.

[0047] In some embodiments, the acidizing fluids described herein may further comprise a silica scale control additive. As used herein, the term "silica scale control additive" will refer to any substance capable of su ppressing silica scale build-up by increasing the solu bility of dissolved silicon, inhibiting polymer chain propagation of dissolved silicon to produce precipitates, and/or decreasing the size and/or quantity of precipitates formed from dissolved silicon. Utilization of a silica scale control additive in combination with metal ion complexation provided by a chelating agent may beneficially permit a greater level of dissolved silicon to be realized than is possible through metal ion complexation alone. Suitable silica scale control additives may include, for example, phosphonates, aminocarboxylic acids, polyaminocarboxylic acids, polyalkyleneimines (e.g. , polyethyleneimine), polyvinylamines, polyallylamines, polyallyldimethylammonium chloride, polyaminoamide dendrimers, any derivative thereof, and any combination thereof. Illustrative commercially available silica scale control additives include, for example, ACUMER 5000 (Rohm and Hass), and CLA-STA® XP and CLA-STA® FS (Halliburton Energy Services, Inc.)- Other suitable silica scale control additives may include ortho- dihydroxybenzene compounds, such as tannic acid, for example, as described in commonly owned United States Patent Application Publication 2012/0145401.

[0048] In some embodiments, the acidizing fluids described herein may further comprise a silicate complexing agent, such as a functionalized pyridine compound, as described in commonly owned United States Patent Application 13/444,883, titled "Treatment Fluids Comprising a Silicate Complexing Agent and Methods for Use Thereof," filed on April 12, 2012. Again, use of a silicate complexing agent may address one of the complex processes through which precipitation occurs during an acidizing operation.

[0049] In some embodiments, acidizing fluids described herein may further comprise an alkali metal complexing agent, such as a crown ether, azacrown ether, or pseudocrown ether, as described in commonly owned United States Patent Application 13/444,897, published as US2013/0269944 and titled "Treatment Fluids Comprising an Alkali Metal Complexing Agent and Methods for Use Thereof," filed on April 12, 2012. Use of such compounds may address precipitation effects that occur in the presence of alkali metal ions.

[0050] In further embodiments, the acidizing fluids described herein may further comprise any number of additives that are commonly used in treatment fluids including, for example, surfactants, gel stabilizers, anti- oxidants, polymer degradation prevention additives, relative permeability modifiers, scale inhibitors, corrosion inhibitors, foaming agents, defoaming agents, antifoaming agents, emulsifying agents, de-emulsifying agents, iron control agents, proppants or other particulates, particulate diverters, salts, acids, fluid loss control additives, gas, catalysts, clay control agents, dispersants, flocculants, scavengers {e.g., H ₂ S scavengers, C0 ₂ scavengers or 0 ₂ scavengers), gelling agents, lubricants, breakers, friction reducers, bridging agents, viscosifiers, weighting agents, solubilizers, pH control agents (e.g., buffers), hydrate inhibitors, consolidating agents, bactericides, catalysts, clay stabilizers, breakers, delayed release breakers, and the like. Combinations of these additives can be used as well. Given the benefit of the present disclosure, one of ordinary skill in the art will be able to formulate a acidizing fluid of the present disclosure such that the acidizing fluid has properties suitable for a given application. [0051] To facilitate a better understanding of the present invention, the following examples of preferred or representative embodiments are given. In no way should the following examples be read to limit, or to define, the scope of the invention.

EXAMPLES

[0052] Example 1: Flow Test in a Sandstone Core from the

Bandera Outcrop. A treatment fluid having a pH of 2.5 and containing 0.6 M methylglycine diacetic acid (TRILON M commercially available from BASF), 1% HF by weight, 1% of a bis-quaternary ammonium compound by weight, and 0.5% of a stabilized microemulsion by weight was prepared and injected onto a sandstone core from the Bandera outcrop. The stabilized microemulsion was LOSURF 360 (commercially available from Halliburton Energy Services, Inc. of Houston, Texas). The core composition is set forth in Table 1. The core sample's initial porosity was 14%.

[0053] The treatment fluid was injected in a continuous fashion during the entire duration of the treatment phase, and 3% KCI brine was used for pre- and post-flushes. The temperature was held at 225°F during the treatment phase, and 10 pore volumes of the treatment fluid were injected onto the core. The effluent pH ranged between 5.2 and 6.3. The effluent had ionic analyses within the following ranges: Al ³⁺ (3,500-4,500 mg/L), Ca ²⁺ (13,900-15,500 mg/L), Mg ²⁺ (4,600 mg/L) and Fe ³⁺ (9,300- 10,900 mg/L). Further chemical analyses of the effluent are illustrated in FIGURE 2.

Table 1

Percent

Core Component Composition (%)

Quartz 62

Sodium feldspar 14

Calcite 1

Dolomite 14

Kaolinite 4

Illite/Mica 5

[0054] FIGURE 1 shows a plot of differential pressure and permeability of the core sample as a function of time. As shown in FIGURE 1, the permeability of the core sample increased considerably after the treatment stage was completed. The permeability increase was about 2.5-fold over the pretreatment permeability, and the differential pressure decreased to about one- third of its original value. FIGURE 2 shows a plot of concentration versus the sample fraction for the ionic constituents obtained during treatment of the core sample. As shown in FIGURE 2, high levels of aluminum and sodium were maintained during the treatment stage, indicating that these substances did not precipitate to any appreciable degree.

[0055] Embodiments disclosed herein include:

[0056] A. Methods of acidizing a subterranean formation. The methods involve providing an acidizing fluid comprising a stabilized microemulsion carrier fluid, a chelating agent, and an acid source; introducing the acidizing fluid into a subterranean formation; at least partially dissolving a surface in the subterranean formation with the acidizing fluid, thereby forming a spent fluid; and producing the spent fluid from the subterranean formation.

[0057] B. Methods of acidizing a subterranean formation. The methods involve providing an acidizing fluid comprising a stabilized microemulsion carrier fluid, a chelating agent, and a hydrofluoric acid source; introducing the acidizing fluid into a subterranean formation containing a siliceous material; and at least partially dissolving the siliceous material in the subterranean formation with the acidizing fluid, thereby forming a spent fluid.

[0058] C. Methods of complexing a metal ion in a subterranean formation. The methods involve introducing a stabilized microemulsion carrier fluid, a chelating agent, and a hydrofluoric acid source to a subterranean formation comprising a siliceous material; at least partially dissolving the siliceous material in the subterranean formation with the hydrofluoric acid source; and complexing a metal ion in the subterranean formation with the chelating agent.

[0059] Each of embodiments A, B and C may have one or more of the following additional elements in any combination :

[0060] Element 1 : complexing a metal ion in the subterranean formation with the chelating agent.

[0061] Element 2 : forming a precipitate in the subterranean formation while at least partially dissolving the surface in the subterranean formation, and producing the precipitate from the subterranean formation with the spent fluid. [0062] Element 3 : wherein the acid source comprises hydrofluoric acid or a hydrofluoric acid-generating compound.

[0063] Element 4: wherein the chelating agent, the stabilized microemulsion carrier fluid, or both are biodegradable.

[0064] Element 5 : wherein the chelating agent comprises glutamic acid diacetic acid, methylglycine diacetic acid, or any combination thereof.

[0065] Element 6: wherein the acidizing fluid further comprises a compound having two or more quaternized amine groups.

[0066] Element 7 : wherein the siliceous material comprises a silicate, an aluminosilicate, or any combination thereof.

[0067] Element 8: wherein the siliceous material further comprises about 1% to about 35% of a carbonate material by mass.

[0068] Element 9 : producing the spent fluid from the subterranean formation.

[0069] Element 10 : wherein the subterranean formation comprises a shale formation.

[0070] Element 11 : wherein the stabilized microemulsion carrier fluid, the chelating agent, and the hydrofluoric acid source are combined in an acidizing fluid that is introduced to the subterranean formation.

[0071] Element 12: wherein the stabilized microemulsion carrier fluid is introduced to the subterranean formation before the chelating agent, the hydrofluoric acid source, or both.

[0072] Element 13 : wherein the stabilized microemulsion carrier fluid is introduced to the subterranean formation after the chelating agent, the hydrofluoric acid source, or both.

[0073] Element 14: forming a precipitate in the subterranean formation while at least partially dissolving the siliceous material, and producing the precipitate from the subterranean formation with the spent fluid.

[0074] By way of non-limiting example, exemplary combinations applicable to A, B and C include:

[0075] Combination 1 : The method of A, B or C in combination with

Elements 2 and 3.

[0076] Combination 2: The method of A, B or C in combination with Elements 3 and 5. [0077] Combination 3 : The method of A, B or C in combination with

Elements 1 and 7.

[0078] Combination 4: The method of A, B or C in combination with Elements 1 and 10.

[0079] Combination 5 : The method of A, B or C in combination with Elements 3 and 11.

[0080] Combination 6: The method of A, B or C in combination with Elements 3 and 12.

[0081] Combination 7: The method of A, B or C in combination with Elements 3 and 13.

[0082] Combination 8: The method of A, B or C in combination with

Elements 3 and 14.

[0083] Combination 9 : The method of A, B or C in combination with Elements 3, 5 and 6.

[0084] Combination 10 : The method of A, B or C in combination with Elements 3, 5 and 7.

[0085] Combination 11 : The method of A, B or C in combination with Elements 3, 5 and 10.

[0086] Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present invention. The invention illustratively disclosed herein suitably may be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein. While compositions and methods are described in terms of "comprising," "containing," or "including" various components or steps, the compositions and methods can also "consist essentially of" or "consist of" the various components and steps. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, "from about a to about b," or, equivalently, "from approximately a to b," or, equivalently, "from approximately a-b") disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles "a" or "an," as used in the claims, are defined herein to mean one or more than one of the element that it introduces.