WO2000070186A1

WO2000070186A1 - Method for acidizing a subterranean formation

Info

Publication number: WO2000070186A1
Application number: PCT/US2000/012759
Authority: WO
Inventors: Frank Chang; Ronnie L. Thomas; Walter D. Grant, Jr.
Original assignee: Schlumberger Technology Corporation
Priority date: 1999-05-13
Filing date: 2000-05-10
Publication date: 2000-11-23
Also published as: AU4834700A; GB2366584A; NO20015539L; GB0127361D0; NO20015539D0

Abstract

A composition is provided useful in the stimulation of a siliceous clay containing formation to increase production of fluids therefrom wherein the formation contains HCl sensitive minerals and the temperature of the formation ranges from about 80 to about 350 degrees F. The composition has at least an in situ HF-generating source, a boron source capable of reacting with silicates in the formation to form borosilicates therein, and a chelating agent for an ionic aluminum or aluminum containing species in an effective amount to minimize the precipitation of aluminum fluoride and silica gel.

Description

METHOD FOR ACIDIZING A SUBTERRANEAN FORMATION

BACKGROUND OF THE INVENTION

A. Field of the Invention

The invention relates to a method for increasing the permeability of a subterranean formation, wherein the permeability increase is achieved by contacting the formation with an acidic solution to dissolve a portion of the formation. It more particularly relates to an acidizing method for use in formations containing HCl - sensitive materials, e.g., zeolite and chlorites.

B. Description of the Prior Art

Numerous procedures for treating wells with siliceous-material-dissolving acids are known. A good discussion of the known art is found in columns 1 and 2 of Templeton et al., U.S. Pat No. 3,828,854 and in the "Introduction" section of Society of Petroleum Engineers Paper No. 5153, which paper relates to the same invention as the Templeton et al. Patent.

Conventionally, siliceous formations have been acidized by contact with mud acid. The mud acid is usually called "organic mud acid" if the acid in addition to HF is an organic acid, e.g. acetic acid or formic acid. As used herein, "mud acid" refers to an aqueous solution of hydrofluoric acid and at least one of hydrochloric acid, acetic acid, or formic acid; usually, the acid in addition to HF is HCl. As is well understood in the art, the derivation of the HCl and HF is not critical, so that "mud acid" also includes aqueous solutions of chemicals which quickly react to form HCl and HF, i.e., so that by the time the solution reaches the formation, the active ingredients are HF and HCl. The respective concentrations of HCl and HF may vary over wide ranges, with the lower limits being more a matter of practicality rather than operability, and the upper limits being a matter of mutual solubility of the two acids. Thus, any given mud acid solution may have an HCl concentration, by weight, of from about 1 percent or even less up to about 37 percent, and an HF concentration of from about 0.5 percent or even less up to about 25 percent, though as the upper limit is approached for one species, a lesser concentration of the other may be required because of solubility limitations. Most typically, a mud acid is substantially free of other acidic species, and contains from about 3 to about 25 percent HCl and about 1 to about 10 percent HF. A mud acid may also contain one or more functional additives such as inhibitors, diverting agents, and/or surfactants.

Conventional treatments of siliceous clay containing formations with mud acids have generally given excellent results for a short time, but the improvements in production are frequently short lived, with a rapid decline in production being observed thereafter. It has been hypothesized that this phenomenon is observed because the mud acid reacts rapidly with the formation in the first few inches around the borehole, thus spending so rapidly that penetration deep into the formation is not achieved. Subsequently, fines in the surrounding formation migrate into the vicinity of the borehole and replug the acidized portion of the formation.

One approach to this problem is that taught by Templeton et al. in the aforementioned patent and publication. They teach to inject a composition which generates HF slowly, and thus enables the solution to be placed in contact with the formation before any significant amount of the HF is generated. The system there described is a relatively high pH ( >2) aqueous solution of a water soluble fluoride salt and at least one water reactive organic acid ester. From the examples in the patent and paper, it appears that the ester most preferred by Templeton et al. is methyl formate. The method of Templeton et al. suffers from at least two drawbacks. First, many of the organic esters are highly flammable materials which are objectionable from a safety standpoint. Second, as Templeton et al. acknowledge, the fluoride salt-organic ester system actually causes at least temporary formation damage since it causes precipitation of by-products such as ralstonite.

As further background, the use of fluoboric acid in well treating has been previously described. Ayers, Jr., U.S. Pat. No. 2,300,393 teaches treatment with fluoboric acid, optionally with small amounts of HF. Ayers, Jr., warns against using large excesses of HF. Ayers, Jr., also teaches the fluoboric acid may be followed by HCl containing no appreciable amount of hydrofluoric acid, or optionally, by a mixture of HCl and fluoboric acids. Bond et al., U.S. Pat. No. 2,425,415 teaches an acidizing procedure wherein the formation is first contacted with a fluoboric acid solution which contains no free HF, but which contains an excess of boric acid, and thereafter with aqueous fluoboric acid containing excess HF. Kingston el al., U.S. Pat. No. 2,663,689 describes the use of boric acid in aqueous HCl-HF to avoid precipitation of insoluble fluoride salts and fluorosilicic acid.

U.S. 4,151,878 to Thomas is directed to the use of a conventional mud acidizing solution (HCl-HF) followed by fluoboric acid solution. The use of fluoboric acid as an overflush is believed to deter clay migration and thereby significantly reduce or delay production decline which is often otherwise encountered shortly after conventional mud acidizing treatments.

However, in formations which contain HCl-sensitive materials, for example, zeolite and chlorite, the use of traditional mud acid is not advisable.

One system which partially addresses this problem is U.S. 4,151,878 to Thomas wherein the permeability of siliceous formation is increased by injecting in sequence a fluoboric acid solution followed by mud acid (HCl-HF) solution. According to this patent, the system may be used in formations which have a tendency to plug initially upon contact with mud acid, or with HCl commonly used as a preflush ahead of mud acid. When contacted initially with fluoboric acid, such formation show little or no plugging effects when subsequently treated with mud acid.

However, the foregoing patent did not specifically address formations containing zeolites and chlorites. It was later discovered that such formations tend to allow the HF to extract aluminum from the formation and deposit hydrated silica thereby causing additional plugging. As noted earlier, typical substitutes for hydrochloric acid include organic acids such as acetic and formic acid. However, these acids were noted as not preventing this extraction of aluminum from the formation and deposition of hydrated silica gel. See R.D. Rogers et al., "Designing a Remedial Acid Treatment for Gulf of Mexico Deepwater Turbidite Sands Containing Zeolite Cement," SPE 39595 presented at the 1998 SPE International Symposium, Lafayette, LA, February 18-19, 1998. Further, severe, damaging precipitation of aluminum fluorides during the HF reactions was discovered with formic-HF and acetic-HF fluid systems. See, C.E. Shuchart, et al., "Improved Success in Acid Stimulations with a New Organic-HF System," SPE 36907 presented at 1996 European Petroleum Conference, Milan, Italy.

Rogers et al. disclosed the use of citric acid as a chelating agent for aluminum to prevent such deposition or formation of hydrated silica gel. The optimum treatment formulation identified therein consisted of 10 percent citric acid and 1.5 percent HF acid, with no additives except corrosion inhibitor. One disadvantage of this particular method is that the use of hydrofluoric acid primarily addresses damage or scaling in the initial few inches of the formation around the wellbore.

Accordingly, there is a need to extend such treatment deeper depths into the formation, for example, up to 3 to 5 feet radius from the wellbore, to avoid a rapid decline in production by stabilizing fines and avoid reprecipitation of acidizing dissolution products near the wellbore.

SUMMARY OF THE INVENTION

The present invention provides an acidizing composition, and a method using the composition as a preflush, main treatment composition or as a postflush in an acidizing treatment. The composition comprises an in situ HF-generating source, a boron source and a chelating agent for aluminum ions or aluminum fluoride species dissolved in an aqueous environment.

The present invention is a method for increasing the permeability of a subterranean formation by injecting an acidizing composition which in a preferred embodiment is prepared by mixing ammonium bifluoride, boric acid and citric acid in water. In the formation, ammonium bifluoride and boric acid hydrolyzes to form fluoboric acid. The fluoboric acid in turn hydrolyzes to form hydrofluoric acid. The citric acid chelates the aluminum in the secondary reactions in the system. This prevents, or at least minimizes, the formation of hydrated silica gel in the formation. Further, the boric acid reacts with the siliceous materials to form borosilicates. The coating of the silica and borosilicates onto the clay particles and fines stabilizes these materials preventing them from causing any damage during flow back.

The method is particularly effective for stimulating formations of the type which exhibit an initial production increase following a conventional mud acidizing treatment but which normally suffer a rapid production decline thereafter. The present treatment permits a prolonged period of increased production from such formations. Although the present invention is not limited by any particular theory, the beneficial results are believed attained because the fluoboric acid stabilizes formation fines deep within the formation by slowly reacting to form borosilicates on the surface of the clays and feldspars, thereby restricting migration of the fines. Further, the chelating agent, e.g., citric acid, chelates aluminum believed to be in the form of free aluminum ions and aluminum fluorides, to help prevent the excessive precipitation of hydrated silica gel and aluminum fluorides by binding silicon with fluorides.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 Sequential acid spending testing to simulate acid penetrating into the formation.

Figure 2 Silicon concentration in spent acid solutions from the sequential acid spending test on minerals containing 58% quartz, 1% plagioclase, 4% calcite, 20% chlorite, and 6% smectite. Test temperature: 170°F. Reaction Time: 1 hour.

Figure 3 Fluoride concentration in spent acid solutions from the sequential acid spending test on minerals containing 58% quartz, 1% plagioclase, 4% calcite, 20% chlorite, and 6% smectite. Test Temperature: 170°F. Reaction Time: 1 hour.

Figure 4 Silicon concentration in spent acid solutions from the sequential acid spending test on minerals containing 90% quartz and 10% zeolite. Test Temperature: 200°F. Reaction Time: 1 hour.

Figure 5 Fluoride concentration in spent acid solutions from the sequential acid spending test on minerals containing 90% quartz and 10% zeolite. Test Temperature: 200°F. Reaction Time: 1 hour. Figure 6 Core flow result showing damage removal by the XE Acid system in formation containing 98% quartz and 2% kaolinite.

Figure 7 Weight study of acid- mineral interaction. 10 ml acid/g mineral.

Mineralogy: 10% Zeolite + 90% Sand. Temperature: 190°F

Figure 8 Weight study of acid- mineral interaction. 5ml acid/g mineral.

Mineralogy: 10% Zeolite + 90% Sand. Temperature: 190°F

Figure 9 Weight study of acid- mineral interaction. 2.5ml acid/g mineral.

Mineralogy: 10% Zeolite + 90% Sand. Temperature: 190°F

Figure 10 Dissolution of glass slides with various HF containing acids at 190°F. Glass Slide Wt. = 4 g, Acid Volume = 250 ml

DESCRIPTION OF THE INVENTION

The present invention provides a method of acidizing HCl sensitive formations utilizing a composition comprising an in situ HF-generating source, a boron source and an aluminum fluoride complexing agent. The in situ HF-generating source allows a deeper penetration of HF into the formation. The complexing agent coordinates or chelates aluminum, possibly in the form of an aluminum ion and/or one or more aluminum fluoride species, to prevent precipitation thereof. The boron source provides boron in an exchange reaction to form borosilicates. Silicates and borosilicates plate out onto clays and fines as a coating thereon and affixes same to sand formation to stabilize the fines against flow back forces and thereby avoid a rapid decline in production caused by mobile fines and clays clogging the pores of the producing area of the formation. Since HCl-sensitive materials remaining in the formation have been coated with silicate and /or borosolicates, the formation may be treated with conventional mud acid (HCl-HF)

Accordingly, the composition of the present invention may be used as the acidizing composition or as a preflush before a conventional mud acid treatment. Further, HF may be used combination therewith to effectively treat the near well bore area without a significant shut-in period to allow for HF generation. In such a situation, a preflush of the aluminum fluoride-complexing agent may be utilized to minimize adverse precipitation of HF dissolution products as a result of the acidizing treatment using HF. In situ HF-Generating Source

The in situ HF-generating source is typically a compound or a composition that reacts to form a compound that hydrolyzes during the acidizing treatment to form HF while the treatment composition is in the formation. Unlike the use of HF itself which is primarily spent within the first few inches from the well bore after injection into the formation, the in situ generating source involves an equilibrium reaction which produces HF at a specific concentration level. As the HF that is generated is spent as it traverses the formation, the equilibrium reaction favors the formation of additional HF to replace that which was spent. This continues until the in situ HF-generating source is exhausted or the conditions do not favor the production of additional HF via the equilibrium reaction.

A preferred in situ HF-generating source is fluoboric acid, which also acts as the boron source required herein. Fluoboric acid hydrolyzes in aqueous solutions according to the following equilibrium reaction:

(1) NH^HFz ± H₂O <→ NH₄OH ± HF

(ammonium (water) (ammonium hydroxide) (hydrofluoric acid) bifluoride)

(2) 4HF ± H₃BO₃ <r-» 3H₂O ± HBF₄

(hydrofluoric (boric (water) (fluoboric acid) acid) acid)

(3) HBF₄ + H₂O «-> HBF₃OH + HF

(fluoboric acid) (water) (hydroxyfluoboric acid) (hydrofluoric acid) Once the injected into the formation and the HF is generated, the HF is spent on clays and other siliceous fines. This reaction in a simplified form just showing reactants and some of the products is as follows:

(4) HF + Al₂Si₄Oι₆ (OH)₂ → H₂SiF₆ + A1F₃ + H₂O

(hydrofluoric acid) (clay) (fluosilicic acid) (aluminum fluoride) (water)

Actually, this is only the initial stage of a complex reaction sequence. Depending on the free fluoride concentration, silicon fluorides can exist as SiF₄, SiF₅ ^1_, and SiF₆ ^2", while the aluminum fluorides are present as Al ³⁺, A1F ²⁺, ... A1F₆.

Because aluminum has a greater affinity for fluorine than silicon, the silicon fluorides and more-fluoride-rich aluminum species react with undissolved clays, extracting aluminum and precipitating hydrated silica. For example, fluosilic acid may react with additional clay to yield a hydrated silica, i.e., silica gel, a soluble aluminum fluoride species and other byproducts as follows:

(5) H₂SiF₆ + Al₂Si₄Oι₆ (OH)₂ ^■* H₂SiO₃ + A1F²⁺ + Si (OH)₄

Reaction 4 is also referred to as the primary reaction and Reaction 5 as the secondary reaction. Silica precipitation occurs according to Reaction 5. The silica precipitation is probably a topochemical reaction occurring only on clay surfaces with aluminum being extracted and the clays coated with a layer of hydrated silica. See, C.W.Crowe, et al., "Precipitation of Hydrated Silica from Spent Hydrofluoric Acid: How Much of a Problem Is It?" J. Petroleum Tech., 1234-1240 (Nov. 1986). According to C.W. Crowe, et al. sand surfaces appeared to be clean with no indication of silica precipitation, which may explain the lack of formation damage in core sections where silica deposition had occurred.

As the HF is spent in Reaction 2, this drives the equilibrium reaction of Reaction 3 to the right in order to generate more HF to replace that which was spent. The effective penetration of HC1/HF acid is rather limited as a result of rapid spending of HF within the matrix of the formation. It is generally believed that HC1/HF acid removes skin damage occurring within the first few inches around the well bore. Since only a limited amount of HF is present at any time, unlike when HF itself is used, the rate of consumption of HF is reduced which allows live HF acid to penetrate deeper into the formation. See, R.L.Thomas et al., "Matrix Treatment Employs New Acid System for Stimulation and Control of Fines Migration in Sandstone Formations," J. Petroleum Tech., pp 1491-1500 (1981), hereby incorporated by reference.

According to Thomas et al. in the foregoing article, the fluoboric acid dissolves clays as effectively as HCl/HF acid but at a slower rate.

An aqueous fluoboric acid solution may be prepared in any convenient manner. Ayers, U.S. Pat. No. 2,300,393, for example, teaches preparation of fluoboric acid by mixing boric and hydrofluoric acids. Alternatively, boric acid may be added to ammonium fluoride or ammonium bifluoride in the presence of an approximately stoichiometric amount of HCl. Hydrochloric acid reacts with ammonium bifluoride to generate ammonium chloride and hydrofluoric acid (Reaction 6) The hydrofluoric acid then reacts with boric acid to form fluoboric acid (Reaction2).

(6) NH4HF₂ ± HCl <→ NH.C1 ± 2HF

(ammonium (hydrochloric (ammonium chloride) (hydrofluoric acid) bifluoride) acid))

The foregoing would be particularly suited for formations which do not contain HCl- sensitive materials.

For example, an approximately 8 weight percent solution of fluoboric acid may be prepared by admixing the following:

U.S. Metric

Water 340 gal 1.36 m^J

Ammonium bifluoride 5001b. 240 kg 35 wt % HCl 97 gal 0.388 m³ Boric acid 250 lb. 120 kg

Total, approximately 500 gallons 2 m³

Other variations will be readily apparent to those skilled in the art. For example, another suitable fluoboric acid solution may be prepared employing a mixture of HCl and HF as starting materials, e.g., by admixing the following: U.S. Metric

Water 370 gal 1.48 m^J

Ammonium bifluoride 250 lb. 120 kg Aqueous soln. of, by weight, 25% HCl and 20% HF 84 gal 0.366 m³ Boric acid 250 lb. 120 kg

Total, approximately 500 gallons 2 m³

Boron Source

A preferred boron source is fluoboric acid. When it hydrolyzes in Reaction 2, the hydrolysis products thereof are believed to further react with clays, extracting aluminum and replacing same with boron, thereby forming borosilicates. This is based on controlled reactions of fluoboric acid with boron-free slides on which borosilicates are formed. Such is also indicated by R.L.Thomas et al., at J.Petroleum Tech. at 1495.

Other boron sources may be used which are soluble in water and form boric acid in an

acidic aqueous environment, for example, borax [Na₂B₄O₅(OH)₄*8H₂O] and kernite,

[Na₂B₄O₅(OH)₂»2H₂O].

This formation and deposition of borosilicates and partial precipitation of silicic species are believed to be the reason for enhanced stabilization of clay particles and fines. As noted by R.L.Thomas, the clay particles are apparently fused together and to sand particles, which stabilizes the clay to both ionic and mechanical shock. The coating formed on the clays also inhibits their swelling and their ability to attract cations. See R.L. Thomas, J. Petroleum Tech., at 1496-97.

The concentration of fluoboric acid is not sharply critical so long as the concentration and amount employed are effective to achieve an observable improvement in stabilization of the clays and fines in the remote areas of the formation. Such a stabilizing effect can be recognized by improved production over a more prolonged period of time than would have been predicted based on previous experience in that field, or, for example, by laboratory techniques such as core flow tests or by examination of a formation sample using a scanning electron microscope as discussed in Society of Petroleum Engineers Paper No. 6007. Generally, however, treatment compositions once the starting ingredients have been mixed and dissolved in water containing from about 1 weight percent or less up to about 48 weight percent HBF₄ may be employed. More preferably, the treatment composition comprises from about 2 to about 20 weight percent HBF₄ but contains (when injected) less than about 2 percent HCl and less than about 1 percent HF.

The treatment composition may optionally include, functional additives such as a corrosion inhibitor, diverting agent, or the like.

Chelating Agent for Aluminum

Common problems associated with HCl-based acidizing fluids includes sensitivity of clay minerals and zeolites. It has long been known that a select group of minerals will decompose in the presence of HCl or any strong acid. The most common minerals in this group are zeolites and chlorite. Their instability in strong acid is directly related to their layered structure. Alumina layers are attacked by the strong acid, which causes such structures to collapse. The resulting residual gel mass can be severely damaging. For these reasons, organic acids such as formic acid, acetic acid and combinations thereof have been used in place of HCl in acidizing formations containing such minerals. These problems are aggravated by high temperatures. All clays (with their layered structures) have a temperature above which they are no longer stable in HCl. See Figure 1 in SPE 36907, hereby incorporated by reference. HF fluids based on acetic acid and formic acid can successfully overcome these problems.

As noted earlier, SPE 36907 discloses that when acetic-HF and formic-HF fluids are used in acidizing formations, the nearly complete precipitation of aluminum fluoride complexes during HF reactions was observed. To overcome this problem, the acidizing composition of the present invention utilizes an aluminum chelating agent which ties up or coordinates the aluminum ion or aluminum complex. The coordination of the aluminum fluoride complexes alters the aluminum-fluoride equilibrium, which minimizes the amount of A1F₃ in solution and forces the HF reactions to lower F/Al ratios, specifically below the levels at which A1F precipitates. A preferred aluminum chelating agent is a tricarboxylic acid. Such chelating agents do not interfere with the in situ HF generation, HF reactions, nor with the reactions involving the boron source which produce borosilicates. Examples of such tricarboxylic acids include citric acid and N-(2-hydroxyethyl)ethylenediaminetriacetic acid (CAS: 150-39-0). (also referred to herein as H-EDTA).

The use of citric acid as an organic acid in the present system overcomes the precipitation problems and maintains all the advantages of acidic-HF and formic-HF fluids. This is believed in part by the structure of citric acid which allows it to complex aluminum ions. Specifically, citric acid is a tri-carboxylic acid which is soluble in water and in alcohol. Accordingly, compounds which have a similar tri-carboxylic acid configuration and soluble in water would also be useful in present invention.

Citric acid contains three distinct acid hydrogens (-COOH), each one with a different acid strength. The first and the strongest hydrogen of all the citric acid molecules are used up before the second hydrogen is consumed, thus resulting in a weaker acid system.

The citric acid aids in hydrolyzing the ammonium bifluoride to form ammonium citrate and produce HF. It is believed that the active complexing agent is ammonium citrate, with an ionic interaction between the ammonium ion and the citrate ion. Since the affinity of the ionic aluminum species for the citrate is greater than that of the ammonium ion, the aluminum ionic species displaces the ammonium ion. B.A.Rogers, et al., in SPE 39595, favored using concentrated liquid HF with citric acid in their acidizing composition. They taught away from using ammonium fluoride or ammonium bifluoride with citric acid. They cited the different strengths of the three acidic hydrogens thereof and weakness of citric acid, i.e., higher pH, relative to using concentrated liquid HF. This made the formulation of the composition more severe, apparently in view of the fact that much greater amounts of citric acid would be required in their compositions. See SPE 39595 at page 696.

The composition of the present invention incorporates an in situ HF-generating source, a boron source and an aluminum chelating agent. In a specific embodiment, fluoboric acid is generated from ammonium fluoride or ammonium bifluoride and boric acid. The fluoboric acid is both the HF generating source and the boron source. The aluminum chelating agent is citric acid.

Despite the successful application of organic-base HF fluids, a somewhat surprising observation of acetic-HF and formic-HF fluids was the nearly complete precipitation of aluminum fluoride complexes during the HF reactions.

Treatment Sequence:

In a typical treatment, a preflush such as toluene, xylene, or the like may be employed, if desired, to clean the wellbore and surrounding formation of organic deposits such as paraffins or asphaltines. Optionally, the preflush to remove organic deposits may be followed by a preflush of HCl or an acid-organic solvent system to dissolve carbonates in the formation. Where the formation is acid sensitive, i.e., susceptible to an initial decrease in permeability upon contact with HCl, fluoboric acid is beneficially employed as the preflush as taught in U.S. 4,151,878, hereby incorporated by reference.

When fluoboric acid is so employed as a preflush, injection of the acid treatment composition of the present invention may immediately follow injection of the fluoboric acid if desired, but preferably, the well is shut in for at least a brief period to allow the fluoboric acid to react with clays in the formation prior to injecting the acid composition of U.S. 4,151,878, particularly at formation temperatures of about 180° F. (82° C.) and less. Optimum results are achieved when the following minimum shut-in time is used, depending on the bottom hole static temperature (BHST) of the well.

Preferred

BHST Minimum Shut in Time

°F. "C. (calculated from °F.) Minutes

100 38 5 hours 110 43 4 hours 120 49 3 hours 130 54 2 hours 140 60 IV2 hours 150 65 1 hour 160 71 30 minutes 170 77 20 minutes 180 82 10 minutes

When any desired preflushes have been completed, a suitable volume of acid composition is injected in a conventional manner at a matrix rate, i.e., at a rate which does not fracture the formation.

The fluoboric acid is injected following the acid composition, again at a matrix rate. Preferably, an injection rate of about VΛ barrel (42 gallon barrel) per 4 feet of perforations (about 33 liters/meter of perforations) is maintained to assure that migratory fines are not disturbed during the injection. The precise volume employed is not critical. A sufficient volume is preferably employed to obtain penetration of at least about 3 to 4 feet into the formation from the wellbore. Those skilled in the art can determine the approximate volume to use for a given depth of penetration if the porosity is known. Generally, however, about 85-100 gallons per foot (about 1-1.25 m³m/m) of perforations is suitable.

The fluoboric acid may be displaced from the wellbore with a suitable displacement fluid, e.g., an aqueous ammonium chloride solution. Potassium ions are generally to be avoided as they can cause a precipitate to form upon contact with the fluoboric acid. Also, a spacer such as an ammonium chloride or weak organic acid solution is preferably employed between the acid composition and the fluoboric acid overflush. The spacer prevents comingling of the acid composition with the fluoboric acid; comingling can otherwise accelerate the rate of reaction of the fluoboric acid, thereby decreasing the depth of penetration obtainable with the fluoboric acid solution. When a weak organic acid is selected so as to contribute sufficient ionic character to the water to prevent formation shock, yet not appreciably increase the rate at which the fluoboric acid reacts with the formation. Other suitable spacers, e.g., liquid hydrocarbons, alcohols, and the like may also be employed.

Finally, the well is shut in for a period of time sufficient for the fluoboric acid to react with and stabilize the clays. The minimum shut in time depends on the temperature of the formation. While some benefits can be realized with somewhat shorter shut, optimum benefits are realized where the shut in times are at least about as long as the following:

Bottom Hole Static Temperature _Minimum Shut-in Time

°F. °C. (calculated from °F.) Hours

100 38 100

110 43 76

120 49 52

130 54 35

140 60 24

150 65 16

160 71 11

170 77 8

180 82 5

190 88 3

200-225 93-107 2

226-250 108-121 1

251-300 122-149 0.5

Longer shut-in times have not been found to be harmful.

If the treatment composition of the present invention is utilized as the main treatment fluid, a shut in period is preferred. The duration is preferably as that noted in the postflush (over flush) shut-in periods disclosed above, though formation conditions may allow for shorter shut-in periods. Treatment Sequence:

The composition of the present invention is a matrix stimulation fluid containing an in situ HF-generating source, a boron source and an aluminum chelating agent. The composition is particularly suited for acidizing sandstone formations containing high silt and clay content and HCL sensitive minerals, such as zeolites and chlorite. The composition may be used as (1) a preflush for a conventional mud acid treatment, (2) the main acid treatment, or (3) as a postflush to stabilize fines after a conventional mud acid treatment.

The composition removes formation damage caused by clay and other aluminosilicate minerals. It also minimizes hydrated silica precipitation, as a result of the secondary reaction (Reaction 3). The aluminum chelating agent minimizes or prevents the precipitation of aluminum fluoride. Undissolved fines are also prevented from migrating as a result of the deposition of a coating of borosilicates and silicates, which fuse the fines to each other and to the sand matrix of the formation. This coating is also useful during a preflush application to desensitize the HCl sensitive minerals by coating them and thereby protecting them from the adverse effects of HCl.

In a situation wherein the composition is used for the main acidizing treatment, a preferred treatment sequence is as follows:

1. Circulate and establish injectivity with ammonium chloride brine. 2. Inject an aqueous solution containing 5 percent by weight ammonium chloride and 10 percent by weight ethylene glycol monobutylether (a mutual solvent).

3. Preflush with an aqueous solution containing 10 percent by weight glacial acetic acid.

4. Inject composition of the present invention, with an optional shut-in period.

5. Overflush or postflush with an aqueous solution containing 5 percent by weight ammonium chloride or 10 percent by weight glacial acetic acid.

6. Flowback.

As earlier noted, the composition may also contain HF to treat the near well bore area with the in situ HF generating source providing a deeper treatment. Alternately, an HF/citric acid treatment fluid may be used before or after the injection of the composition of the present invention, preferably after so that further increase the flow capacity near the wellbore, where the highest concentration of damaging clay materials are residing, can be achieved.

EXAMPLES

In the following examples, compositions according to the present invention are referred to as XE Acid and XE Acid II. The composition of the present invention is prepared by mixing the starting ingredients into fresh water. The citric acid or HEDTA and the ammonium bifluoride were mixed in the fresh water first until totally dissolved. Thereafter, the boric acid was then slowly added until it was totally dissolved. The boric acid will not easily dissolve in water unless the other two components are already in the system. Therefore, the boric acid is preferably added last. These compositions may be used at temperatures ranging from about 80 to about 350°F.

XE Acid was prepared by mixing about 13.4 percent by weight citric acid, about 9.8 percent by weight ammonium bifluoride and about 4.9 percent by weight boric acid into fresh water. The ammonium bifluoride and the boric acid react to form fluoboric acid, which in turn generates HF in situ. The boric acid provides the boron source for the fluoboric acid. The citric acid, believed to be in the form of an ammonium citrate, chelates aluminum when the aluminosilicate is dissolved by HF acid. By chelating aluminum, the mixture of fluoboric acid and citric acid continues to extract aluminum form the clays and dissolution proceeds.

XE Acid II was prepared by mixing about 13.4 percent by weight H-EDTA, about 9.8 percent by weight ammonium bifluoride and about 4.9 percent by weight boric acid into fresh water. The ammonium bifluoride and the boric acid react to form fluoboric acid, which in turn generates HF in situ. The boric acid provides the boron source for the fluoboric acid. The HEDTA is the chelating agent in this composition. Example 1:

A sandstone formation in South America contains 58% quartz, 12% plagioclase, 4% calcite, 20% chlorite, and 6% smectite. The formation minerals were first treated 10% citric acid as preflush. The minerals were then reacted with the 12/3 HCl HF, 9/1 HCl/HF, 3/1 HCl/HF, fluoboric acids, as well as the XE Acid. The starting ingredients of the acid compositions are described in Table 1.

For each acid, a sequential spending test was performed to simulate the process of acid penetrating into formation and reacting with the formation rock. The testing procedure is depicted in Figure 1. Before reacting with the HF containing acid, the minerals were allowed to react with 10% citric acid for 1 hour. A 50 ml fresh acid solution was loaded into a plastic bottle and heated to 170°F. Five grams of the 10% citric acid treated mineral sample was added into the bottle to react with the acid for 1 hour. The spent acid in this first batch was filtered out, and a 5 ml filtrate sample was extracted for ion analysis. Second batch of the 10% citric acid treated mineral with weight of 4.5 grams was added to the remaining 45 ml of filtrate. This is designated as batch #2 to simulates the spent acid penetrating into formation and contacting un-reacted minerals. Fifty ml of fresh acid was added into the minerals from batch #1 which has previously been reacted with the HF containing acid. This simulates near well bore where minerals contact fresh acid all the time during pumping. After one hour, the spent acid in batch #2 was filtered out, and a 5 ml filtrate sample was extracted for ion analysis. A third batch of the 10% citric acid treated mineral with weight of 4.0 grams was added to the remaining 40 ml of filtrate. This was designated as batch #3. The filtrate from batch #1 was added into mineral batch #2 after extracting 5 ml for analysis. Fifty ml of fresh acid was then added into mineral batch #1 which contained the reacted minerals. The sequence was repeated for overall four batches.

Table 1

The compositions of every 100 ml of HF containing acids

Figure 2 shows that as the mud acids penetrated into the formation, reacted acid (spent acid) came into contact with fresh formation minerals, silicon ions were gradually depleted from the solution of HCl/HF acid systems. The fluoboric acid's capability of slowly generating HF to continue dissolve aluminosilicate minerals rendered a flat profile as dissolution and precipitation processes reach equilibrium. The XE Acid system dramatically improved the fluoboric acid by preventing precipitation while the dissolution process continued. Therefore, the silicon concentration continued to increase.

Figure 3 shows the fluoride concentrations in the spent acid solutions of the 12/3 HCl/HF, 9/1 HCl/HF, 3/1 HCl/HF, fluoboric acids, and the new acid. The mud acids generated fluoride rapidly so the fluoride concentration reached a plateau. The XE Acid generated fluoride slowly as it was generated by the reaction between the acid and the minerals. This provides deep fresh acid penetration during acidizing. The silicon and fluoride concentration data are tabulated in Table 2.

Table 2. Silicon and fluoride concentrations in the filtrate samples as it sequentially reacted with minerals in

Example 1

Example 2: This example involves acidizing a sandstone formation with typical lithology that is seen in the Gulf of Mexico. The formations usually contain HCl sensitive minerals, namely zeolite, in the range of 3 to 20 percent. A synthetic blend of 90% silica sand with 10% zeolite was used in this example to simulate such a formation. The minerals were reacted with 9/1 HCl/HF, 3/1 HCl/HF, fluoboric acids, and the XE acid. The testing procedure was the same as that in Example 1. Figure 4 shows mud acids were not capable of keeping dissolved silicon in spent acid solution. However the XE Acid continued to dissolve minerals and maintains the silicon ions in solution; and, therefore the silicon concentration in solution accumulated from samples extracted from Batch #1 through Batch #4.

Figure 5 shows that fluoride was continuously generated as the XE Acid reacted with the minerals. Therefore, the fluoride concentration rose as the acid traveled into the formation. The fluoride concentrations reached equilibrium rapidly in the mud acids. Accordingly, the mud acids were unable to continuously dissolve minerals deep into the formation. Table 3 shows the silicon and fluoride concentrations numerically.

Table 3. Silicon and fluoride concentrations in the filtrate samples as it sequentially reacted with minerals in

Example 2

Example 3:

In this example, a sandstone core containing 2% kaolinite clay was used. The core was sensitized injecting by 6% NaCl solution, then damaged by fresh water (Figure 6). The core was then acidized by XE Acid. First a 3% NH C1 solution was injected as preflush, followed by 15% HCl. The core was then treated with XE Acid. No apparent increase of permeability was observed during the acid injection. The core was shut in for 16 hours at 200°F. The permeability was then measured again using 6% NaCl. A significant recovery of permeability was achieved. The 6% NaCl solution was also to again sensitize the core. However, though fresh water was injected following the 6% NaCl in an attempt to again damage the core, no damage occurred showing that XE Acid successfully stabilized undissolved clay to prevent post acidizing damage.

Example 4

Long term spending tests were performed to investigate whether the reaction byproducts which cause precipitants will potentially plug the sandstone matrix. Batches of acid/minerals were placed in water bath set at 190°F temperature for 24 hours, and weight loss was measured as a function of time. If a batch showed weight gain during reaction, it was an indication that precipitation had occurred.

The weight loss measurements were conducted also to investigate the effect of acid volume to mineral mass ratio on reactions. Figure 7 through 9 show the results of reacting 25 ml of acid with 2.5, 5, and 10 grams of solids. The solid composition was 10% zeolite and 90% 100 mesh silica sand. The acids used included XE Acid, fluoboric acid, 9/1 HCl/HF mud acid, and 3/1 HCl/HF mud acid. Mud acids (9/1 and 3/1 HCl/HF) were rapidly spent and became non-reactive shortly after acid-mineral contact. Potential weight gain due to precipitation was also observed. Fluoboric acid did slowly generate HF to sustain mineral dissolution. However, it caused precipitation by gaining weight during the reaction process. This again demonstrated that a chelating agent, citric acid, in the XE Acid system was essential in preventing precipitation. It was an improvement over the fluoboric acid. The chelating agent strongly bound aluminum so that fluoride ions in the solution can bind silicon to prevent precipitation. The XE Acid continuously dissolved minerals, even at a low acid/mineral ratio (2.5 ml acid/g mineral (Figure 9), no weight gain was seen, the precipitation potential was therefore minimal. Table 4 through 6 tabulate the data for this series of tests.

Table 4 Weight Loss % from Initial 2.5 grams of minerals (90% Silica sand+10% zeolite) reacting with 25 ml of acid

Table 5 Weight Loss % from Initial 5 grams of minerals (90% Silica sand+10% zeolite) reacting with 25 ml of acid

Table 6 Weight Loss % from Initial 10 grams of minerals (90% Silica sand+10% zeolite) reacting with 25 ml of acid

Example 5 The long term weight loss of glass slides were measured by reacting the various acid formulations with silica glass to determine the dissolving power of the acids. The tests were conducted at 190°F. The acids used in this series of tests include XE Acid, fluoboric acid,

12/3, 9/1, 3/1 mud acids, and XE-II Acid. The composition of XE-LI Acid was 13.4% N-(2- hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), which is also a chelant agent for aluminum, 9.8% ammonium bifluoride (NH HF₂), and 4.9% boric acid (H BO₃). Two hundred fifty milliliters of each acid was allowed react with a 4.5 gram silica glass slide for

48 hours. The slide was dried and weighed during the time period. Figure 8 shows that mud acids rapidly reacted with glass and lost the strength needed to continue the reaction.

Therefore, the weight loss curves leveled off. The fluoboric acid continued to dissolve the silica glass over the 48 hour time period. Sustained dissolving power was also seen by the

XE Acid, as well as XE-LI Acid, in the glass slide studies. The HEDTA in XE-JJ Acid being a higher molecular weight (M.W. 278) material, equivalent weight percent generated less molar of HF than the citric acid (M.W. 192) based XE Acid. Therefore, the dissolution capability was much less. The 13.4% citric acid generated about 2.2% HF, which is about

1.1 molar, upon initial mixing of the starting ingredients with a final generated amount of about 3.5% HF. The 13.4% HEDTA, generated only about 0.75 molar of HF upon mixing of initial ingredients with a final generated amount of HF of about 2.5%.

Table 7 Long dissolution of silica glass slides in acids at 190°F. 250 ml of acid reacting with 4.5 g of glass slide

A formation with permeability from 60 to 500 md, containing 60% quartz, 9% dolomite, 10% zeolite, 5% smectite, 1% chlorite, 10% feldspar, and 5% albite is to be acidized to remove damaged caused due to clay swelling and dispersion. XE Acid can be applied to restore the permeability without further potential damage to the formation. The preflush fluid will be 5% by wt. NH4CI for a volume of 40 gallon per ft of the formation penetrated by the well bore, followed by 5% by wt. acetic acid for a volume of 50 gallons per ft of the formation penetrated by the well bore. The preflush is followed by 100 gallon per ft of the formation the main treating fluid XE Acid prepared by mixing 13% by wt. citric acid or HEDTA, 10% by wt. NH₄HF₂, and 2 to 5% by wt. H₃BO₃ water. The main treating acid is followed by 5% acetic acid for a volume of 50 gallon per ft of the formation penetrated by the well bore, and finally displaced by 5% NH₄CI for a volume of 50 gallon per ft of the formation penetrated by the well bore. The common additives can be used in the acetic acid and main acid stages. These common additives include corrosion inhibitors, non-emulsifying agents, anti-sludging agents, and water wetting agents.

Example 7: (Prophetic) Used as a Preflush

A formation contains 80% quartz, 10% illite, 2% calcite, and 3% feldspar, and 5% kaolinite. The formation can be acidized by first preflush with 3% NH4C1 for a volume of 50 gallon per ft of formation penetrated by the well bore. The formation will then be preflushed with the XE Acid (prepared by mixing 10 to 13% by wt. citric acid or HEDTA, 10% by wt. NH₄HF₂, and 2 to 5% by wt. H₃BO₃ in water) for a volume of 50 gallons per ft of the formation penetrated by the well bore. The formation will then be treated with a 9/1 HCl/HF mud acid for a volume of 100 gallons per ft of the formation penetrated by the well bore, followed by 15% HCl for a volume of 50 gallons per ft of the formation penetrated by the well bore. Finally the formation will be displaced by 3% NH_tCl for a volume of 50 gallons per ft of the formation penetrated by the well bore.

Claims

What is Claimed Is:

1. A method for increasing the permeability of a subterranean formation comprising into said formation a composition having at least an in situ HF- generating source, a boron source capable of reacting with silicates in the formation to form borosilicates therein, and a chelating agent for an ionic aluminum or aluminum containing species in an effective amount to minimize the precipitation of aluminum fluoride and silica gel.

2. The method of Claim 1 wherein the in situ HF generating source and the boron source is fluoboric acid.

3. The method of Claim 1 , wherein the chelating agent is a tricarboxylic acid.

4. The method of Claim 1, wherein the composition is prepared by mixing an ammonium fluoride compound, boric acid and a tricarboxylic acid in water.

5. The method of Claim 4, wherein the ammonium fluoride compound is ammonium bifluoride.

6. The method of Claim 5, wherein the tricarboxylic acid is citric acid.

7. A method for stimulating a siliceous clay containing formation to increase production of fluids therefrom wherein the formation contains HCl sensitive minerals and the temperature of the formation ranges from about 80 to about 350 degrees F, the method comprising injecting a composition having at least an in situ HF-generating source, a boron source capable of reacting with silicates in the formation to form borosilicates therein, and a chelating agent for an ionic aluminum or aluminum containing species in an effective amount to minimize the precipitation of aluminum fluoride and silica gel.

8. The method of Claim 7, wherein the in situ HF generating source and the boron source is fluoboric acid.

9. The method of Claim 7, wherein the chelating agent is a tricarboxylic acid.

10. The method of Claim 7, wherein the composition is prepared by mixing an ammonium fluoride compound, boric acid and a tricarboxylic acid in water.

11. The method of Claim 10, wherein the ammonium fluoride compound is ammonium bifluoride.

12. The method of Claim 11, wherein the tricarboxylic acid is citric acid.

13. A composition useful in the stimulation a siliceous clay containing formation to increase production of fluids therefrom wherein the formation contains HCl sensitive minerals and the temperature of the formation ranges from about 80 to about 350 degrees F, the composition comprising an in situ HF-generating source, a boron source capable of reacting with silicates in the formation to form borosilicates therein, and a chelating agent for an ionic aluminum or aluminum containing species in an effective amount to minimize the precipitation of aluminum fluoride and silica gel.

14. The composition of Claim 13, wherein the in situ HF generating source and the boron source is fluoboric acid.

15. The composition of Claim 13, wherein the chelating agent is a tricarboxylic acid.

16. The composition of Claim 13, wherein the composition is prepared by mixing an ammonium fluoride compound, boric acid and a tricarboxylic acid in water.

17. The composition of Claim 16, wherein the ammonium fluoride compound is ammonium bifluoride.

18. The composition of Claim 17, wherein the tricarboxylic acid is citric acid.