Abstract:
A process for improved acidizing in carbonate formations, where controlled pulse fracturing (CPF) is utilized in combination with a retarded acid and a solidifiable gel. The solidifiable gel forms a solid formation gel in a zone of greater permeability and a gel plug in the wellbore. Said gel is formed from a melamine formaldehyde resin and a cross-linkable polymer. An inhibited acid is placed in an interval of said formation having lessened permeability. The retarded acid comprises hydrochloric, formic, acetic acid or mixtures thereof which is placed in a wellbore adjacent the area to be treated. Said acid can contain retarders and corrosion inhibitors sufficient to make said acid stable in the wellbore. At least one CPF device is placed in said acid near the interval to be treated. Upon detonation of said device, acid is forced into the interval of lessened permeability, thereby enhancing the acidizing treatmemt.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 036,742 which was filed on Apr. 10, 1987 now U.S. Pat. No. 4,787,456. It is also related to Ser. No. 917,324 which was filed on Oct. 9, 1986 now U.S. Pat. No. 4,834,180. 
    
    
     BACKGROUND OF THE INVENTION 
     It is a common practice to acidize subterranean formations in order to increase the permeability thereof. For example, in the petroleum industry, it is conventional to inject an acidizing fluid into a well in order to increase the permeability of a surrounding hydrocarbon-bearing formation and thus facilitate the flow of hydrocarbonaceous fluids into the well from the formation or the injection of fluids, such as gas or water, from the well into the formation. Such acidizing techniques may be carried out as &#34;matrix acidizing&#34; procedures or as &#34;acid-fracturing&#34; procedures. 
     In acid fracturing the acidizing fluid is disposed within the well opposite the formation to be fractured. Thereafter, sufficient pressure is applied to the acidizing fluid to cause the formation to break down with the resultant production of one or more fractures therein. An increase in permeability thus is effected by the fractures formed as well as by the chemcial reaction of the acid within the formation. 
     In matrix acidizing, the acidizing fluid is passed into the formation from the well at a pressure below the breakdown pressure of the formation. In this case, increase in permeability is effected primarily by the chemical reaction of the acid within the formation with little or no permeability increase being due to mechanical disruptions within the formation as in fracturing. 
     In yet another technique involving acidizing, the formation is fractured. Thereafter, an acidizing fluid is injected into the formation at fracturing pressures to extend the created fracture. The acid functions to dissolve formation materials forming the walls of the fracture, thus increasing the width and permeability thereof. 
     In most cases, acidizing procedures are carried out in calcareous formations such as dolomites, limestones, dolomitic sandstones, etc. One difficulty encountered in the acidizing of such a formation is presented by the rapid reaction rate of the acidizing fluid with those portions of the formation with which it first comes into contact. This is particularly serious in matrix acidizing procedures. As the acidizing fluid is forced from the well into the formation, the acid reacts rapidly with the calcareous material immediately adjacent to the well. Thus, the acid becomes spent before it penetrates into the formation a significant distance from the well. For example, in matrix acidizing of a limestone formation, it is common to achieve maximum penetration with a live acid to a depth of only a few inches to a foot from the face of the wellbore. This, of course, severely limits the increase in productivity or injectivity of the well. 
     In order to increase the penetration depth, it has heretofore been proposed to add a reaction inhibitor to the acidizing fluid. For example, in U.S. Pat. No. 3,233,672 issued to N. F. Carpenter, there is disclosed an acidizing process in which inhibitor, such as alkyl-substituted carboximides and alkyl-substituted sulfoxides, is added to the acidizing solution. Another technique for increasing the penetration depth of an acidizing solution is that disclosed by U.S. Pat. No. 3,076,762 issued to W. R. Dill, wherein solid, liquid, or gaseous carbon dioxide is introduced into the formation in conjunction with the acidizing solution. The carbon dioxide acts as a coolant, thus retarding the reaction rate of the acid with the formation carbonates. Also, the carbon dioxide is said to become solubilized in the acidizing solution, thus resulting in the production of carbonic acid which changes the equilibrium point of the acid-carbonate reaction to accomplish a retarding effect. 
     An additional procedure disclosed in U.S. Pat. No. 2,850,098 issued to Moll et al. involves the removal of contaminants from a water well and the adjacent formation through the injection of gaseous hydrogen chloride. Still another technique for acidizing a calcareous formation is disclosed in U.S. Pat. No. 3,354,957 issued to Every et al. In this process liquid anhydrous hydrogen chloride is forced from a well into the adjacent formations. The liquid hydrogen chloride vaporizes within the formation and the resulting gas dissolves in the formation water to form hydrochloric acid which then attacks the formation. 
     The effectiveness of acidizing in removing wellbore damage and improving productivity in carbonate reservoirs is highly dependent upon acid reactivity and contact with the formation in the vicinity of the damage. If the pay zone is extensive (greater than 20 to 25 feet in thickness), diverting methods, such as ball sealers, benzoic acid flakes or paraffin beads, will be used to inject acid into the formation matrix over the entire interval. Where the total zone thickness is large (greater than about 25 feet), it is very difficult to effectively acidize the entire interval, even when diverting agents are used. 
     Therefore, what is needed is a method whereby an interval in a formation can be isolated, acidized and simultaneously fractured, wherein the acid in its reactive state can penetrate deeply into a formation, thereby increasing its permeability. 
     SUMMARY OF THE INVENTION 
     This invention relates to a method for increasing the permeability of a formation which formation has varying zones of permeability where high energy impulse fracturing is used in combination with an inhibited acid. In the practice of this invention, a pumpable solidifiable gel is directed into a wellbore within the formation&#39;s zone of greater permeability under conditions to selectively close pores in said zone. The gel utilized is formed from a melamine formaldehyde resin and a crosslinkable polymer. Said gel is allowed to solidify in said zone of greater permeability and also in the wellbore where it forms a gel plug sufficient to support a desired volume of acid. Inhibited acid is placed above said gel plug adjacent to a zone of lesser permeability. 
     A canister containing a propellant is suspended into said wellbore substantially near said zone of lesser permeability. Later the propellant is ignited and energy released sufficient to form more than two simultaneous radial fractures in said zone of lesser permeability. Release of this energy causes acid to enter said fractures and react therein. This reaction increases the permeability of said lesser permeability zone. 
     It is therefore an object of this invention to create multiple simultaneous radial fractures in a formation while acidizing said formation. 
     It is another object of this invention to enhance the reactivity of an acid with the formation by contacting said acid with a greater area of the formation when multiple simultaneous radial fractures are created. 
     It is yet another object of this invention to increase the permeability of a formation and stimulate said formation to produce increased volumes of hydrocarbonaceous fluids. 
     It is still yet another object of this invention to increase the permeability of a calcareous formation containing hydrocarbonaceous fluids for production therefrom while minimizing damage to the wellbore. 
     It is a still yet further object of this invention to provide a gelation reaction which can proceed under all pH conditions encountered in a hydrocarbonaceous reservoir. 
     It is even a yet further object of this invention to provide for a substantially stable gel when high temperatures are encountered in a reservoir. 
     It is even a still yet further object of this invention to provide for a gelation reaction which will proceed in a saline hydrocarbonaceous reservoir environment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic representation showing the placement of the canister in the inhibited acid above the solidified gel plug prior to ignition of the propellant. 
     FIG. 2 is a schematic representation depicting the creation of simultaneous fractures in a desired interval above the solidified gel plug. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the practice of this invention, referring to FIG. 1, a solidifiable gel material is directed into wellbore 10. This gel material is placed just below interval 12 of formation 14 which is desired to be treated. Interval 12 contains a zone of lesser permeability. Interval 28 contains a zone of greater permeability. Said solidifiable material is of a size sufficient to plug the more permeable zone 28 and said material is pumped into permeable zone 28 under conditions sufficient to selectively close pores in more permeable zone 28. Said solidifiable material is allowed to set for a time sufficient to form a solid plug 16 sufficient to support an inhibited acid 18 thereabove while becoming a solid and closing pores in interval 28 which has a zone of greater permeability. Thereafter, a canister 20 containing a propellant is placed into wellbore 10 via wire 24 above said solid gel plug 16 into inhibited acid 18 and adjacent to interval 12 containing the zone of lesser permeability. Afterwards, wellbore 10 is closed in a manner sufficient to withstand the ignition of a propellant contained in canister 20. A method for closing the well is disclosed in U.S. Pat. No. 4,039,030, which patent is hereby incorporated by reference. 
     Subsequently, the propellant contained in canister 20 is ignited. Upon ignition, as shown in FIG. 2, the inhibited acid 18 is forced into interval 12 containing said zone of lesser permeability 12 via simultaneous multiple radial fractures 22 resultant from energy released from the propellant contained in canister 20 as a consequence of said ignition. Since interval 28 is closed by the solidified gel, this interval is not fractured or acidized. Once the inhibited acid is within the fractures, it becomes active and reacts with the walls of said fractures, thereby increasing the size of said fractures. Said acid also increases the distance the fractures penetrate into the formation. This method is effective in acid treating an interval of a formation, where the total zone thickness is 25 feet or more. Acids which can be utilized include hydrochloric acid, formic acid, acetic acid, gelled acids, and other similar acids known to those skilled in the art. When this method is used, matrix acidizing of a formation is improved, particularly carbonate containing formations. 
     As mentioned above, a pumpable gel mixture is directed into wellbore 10 by pumping the gel mixture into said wellbore by pump means (not shown). After preferably from about 2 hours to about 4 hours, the pumpable gel mixture solidifies. As will be understood by those skilled in the art, the composition of the mixture can be varied to obtain the desired rigidity in the gel. One method of making a suitable pumpable mixture is disclosed in U.S. Pat. No. 4,333,461 issued to Muller on June 8, 1982. This patent is hereby incorporated by reference. The stability and rigidity of the solidified gel depends upon the physical and chemical characteristics of the gel, which characteristics are selected so that the gel plug is of a stability and rigidity adapted to absorb the shock from ignition of the propellant. Generally, these pressures generated upon ignition will vary from about 69,049 kPa to about 551,682 kPa (10,000 psig to 80,000 psig). Instantaneous heat generated upon ignition of the propellant may be greater than about 538° C. (1,000° F.) in the vicinity of the deflagration but is quickly dissipated with propagation. 
     Often, depending upon the kind of propellant used, it will be necessary to increase the density of the pumpable gel to obtain a gel plug having the desired stability and rigidity to absorb the generated energy. To accomplish this, any solid non-reacting material may be added to the pumpable gel mixture. Preferred non-reacting solid materials include solid rock salt, or naturally occurring sodium chloride, calcium carbonate, and suitably crushed mollusk shells, such as oyster shells. 
     Other gel mixtures can be used to obtain a solidified gel having the desired stability and rigidity. A preferred gel mixture which can be used herein is formed from a melamine formaldehyde resin and a crosslinkable polymer. 
     The melamine formaldehyde (&#34;MF&#34;) resin utilized herein is formed as a reaction product of melamine and formaldehyde. Said resin is known as an aminoplast or amino resin which comprises a class of thermo-setting resins made by the reaction of an amine with an aldehyde. The resultant resin is reacted with a crosslinkable polymer in an aqueous medium under all pH conditions and needs no catalyst. Said polymer has at least one functional group selected from a member of the group consisting of an amine, an amide, a hydroxyl, or a thiol group. This reaction can be carried out at ambient conditions, and also under conditions occurring in a subterranean hydrocarbonaceous formation. The gel resultant from said reaction can also be used in the recovery of hydrocarbonaceous fluids from a formation containing same. 
     These gels are novel in that they are unaffected by high saline concentrations often encountered in oil reservoirs. High temperatures encountered in said reservoirs do not adversely affect said gels. Carbonate, bicarbonate, and sulfate anions encountered in oil reservoirs which are known to affect certain metal crosslinked gels do not affect these novel gels. These novel gels can be formed under all pH conditions and are particularly useful in pH conditions of 10 or less. A method for making a kindred gel is discussed in U.S. Pat. No. 4,157,322 which issued to Colegrove on June 5, 1979. Unlike Colegrove, the instant gelation reaction is not catalyzed by a salt which is acid generating upon the application of heat. This patent is hereby incorporated by reference. 
     Polymers having functional groups such as NH 2 , --CONH 2 , --OH, --SH can be gelled with methylated, MF resins. Some acceptable polymers include polyacrylamide, Kelco&#39;s S-130 biopolymer, acrylamide modified polyvinyl alcohol (&#34;AMPVA&#34;), Xanthan biopolymers, poly (acrylamide-co-acryl-amido-2-methyl-propanesulfonate) &#34;AM-AMPS&#34;, &#34;Phillips HE&#34; polymers (a family of acrylamide containing copolymers), and polyvinyl alcohol. More specifically, Phillips HE polymer comprises copolymers of N-vinyl-2-pyrrolidone (VP) and acrylamide (Am) in which the weight ratios of VP:Am preferably range from about 30:70 to about 70:30. Polymers mentioned in U.S. Pat. No. 4,157,322, supra, may be utilized as long as those polymers contain the functional groups above mentioned. Polymer concentration in said gels range from about 0.1 to about 5.0 wt. percent. These polymer concentrations vary depending upon the molecular weight of polymer used. Lower molecular weight polymers require a higher polymer concentration to gel. A polymer concentration of about 0.2-5.0 wt. percent is preferred. This crosslinking/co-gelation method produces high integrity polymer gels able to withstand high temperatures and high salinity conditions often found in subterranean hydrocarbonaceous formations. 
     Methylated MF derived as a reaction product of melamine and formaldehyde has a molar ratio of between 1-6. A ratio of 3-6 is commonly found in commercial resins. The methoyl group, --CH 2  OH and its methylated varieties are reactive to various functional groups such as NH 2 , --CONH 2 , --OH, --SH and can also self-condense to form cured resins. Its preparation is convenient and well documented in preparative polymer manuals. 
     The melamine resin that is utilized in this invention can be a commercial product such as Cyanamid&#39;s Parez® resins. Included among these melamine-formaldehyde (melamine) resins which are useful in this invention are the partially methylated resins and the hexamethoxymethyl resins (i.e. American Cyanamid&#39;s Parez, Cymel™373, Cymel 370, Cymel 303, and Cymel 380). The resin, however, has to be one that is soluble or dispersible in an aqueous medium. Other amino resins can also be used. Non-limiting examples are urea-formaldehyde, ethylene and propylene urea formaldehyde, triazone, uron, and glyoxal resins. The amount of MF resins required for adequate gel formation is in the ratio of 10:1-1:10 polymer to amino resins. Preferred polymer concentrations are from about 0.2 to about 5.0 wt. percent. Amino resins are preferred crosslinkers because they (1) are economical to use; (2) can be applied to a wide variety of polymers; (3) from thermally stable, brine tolerant gels; and (4) do not need an acid or base catalyst. 
     The gelation rate of the composition depends on the amount of each of the components and the temperature at which the reaction is conducted. Thus, one can tailor the gel rate and gel strength of the composition by adjusting the amount of the polymer, the resin amount and the temperature. The higher the temperature at given concentrations of resin and polymer will result in a faster gelation time. If a thicker gelled composition is desired, the polymer and resin concentrations may be increased for a given temperature. 
     Gels resultant from the gelation reaction were formed in about a 15 to 30 wt.% brine solution containing at least about 1500 ppm Ca(II) and at least about 500 ppm Mg(II). Said formed gels were stable as determined by sustained gel integrity and low gel shrinkage for at least about 195° F. for at least three months. Examples of preferred gel compositions are set forth below. 
     
         ______________________________________Gelation of Melamine-Formaldehyde Crosslinker                  30%     Deionized                                  ParezExample  Polymer         Brine.sup.8                          Water   613.sup.1______________________________________  10% AMPVA.sup.2#1     5 g             5 g     0         0.4 g#2     2.5 g           5 g     2.5  g    0.4 g  AMPS-AMPVA.sup.3 10%#3     2.5 g           5 g     2.5  g    0.4 g#4     5 g             5 g     0         0.4 g  PVA.sup.4 5%#5     5 g             2.5 g   2.5  g    0.4 g  AMPS-PVA.sup.5 10%#6     5 g             2.5 g   2.5  g    0.4 g  Magnifloc.sup.6 1%#7     5 g             5 g     0         0.4 g#8     5 g             2.5 g   2.5  g    0.4 g  AM-AMPS.sup.7 1%#9     5 g             5 g     0         0.4 g #10   2.5 g           5 g     2.5  g    0.4 g______________________________________Gelation with trimethylolmelamine (TM)                  30%     DeionizedExample  Polymer         Brine.sup.8                          Water   TM______________________________________  S-130 1%.sup.9#11    5 g             5 g     --        0.4 g#12    5 g             5 g               0.2 g  HE B 2%.sup.10#13    2.5 g           5 g     2.5  g    0.4 g#14    2.5 g           5 g     2.5  g    0.4 g  HE E 2%#15    2.5 g           5 g     2.5  g    0.4 g#16    2.5 g           5 g     2.5  g    0.2 g  Xanthan.sup.11 2%#17    2.5 g           5 g     2.5  g    0.4 g#18    2.5 g           5 g     2.5  g    0.2 g______________________________________ .sup.1 A commercial 80% active amino resin obtainable from American Cyanamid .sup.2 Acrylamide modified polyvinyl alcohol .sup.3 Acrylamido2-methyl-propanesulfonate/acrylamide modified polyvinyl alcohol .sup.4 Polyvinyl alcohol .sup.5 Acrylamido2-methyl propanesulfonate/polyvinyl alcohol .sup.6 Polyacryamide obtained from American Cyanamid .sup.7 Poly (acrylamideco-acrylamido-2-methyl-propanesulfonate) .sup.8 30% NaCl, 2000 ppm Ca, 1000 ppm Mg .sup.9 Kelco &#34;S130&#34; biopolymer .sup.10 Phillips HE .sup.11 Pfizer Flocon biopolymer 
    
     The gel plug as described herein is especially advantageous, since it will not &#34;melt&#34; or break down because the intense heat generated by the detonation of the high energy impulse device is localized. Once the acidizing method is completed, hydrochloric acid can be used to chemically break down any remaining gel plug or solid gel within the more permeable zone should it be desired to produce hydrocarbonaceous fluids from the acidized zone and the more permeable zones(s). A concentration of about 15% by weight HCl in water is especially useful. This acid with necessary corrosion inhibitors, etc., is available from the service companies. The HCl solution is circulated down the wellbore using coiled tubing and nitrogen. It is often used with this operation to minimize the amount of fluid in contact with the zone of interest following the high energy impulse treatment. 
     When using propellants to generate the desired fracturing pressure, it is often desirable to produce a gel plug which will withstand temperatures from about 149° C. to about 232° C. (300° F. to 450° F.) for from about 0.5 of a day to about 4 days. A thermally stable solid gel plug is obtained by mixing into the pumpable gel mixture an oxygen scavenger chemical composition, for example, sodium thiosulfate or a short-chain alcohol or carbinol (such as methanol, ethanol, and isopropanol). However, sodium thiosulfate is preferred. The concentration of the oxygen scavenger, of course, will depend upon the thermal stability desired to be obtained for the gel plug and the solidified gel within said fractures. Moreover, the preferred concentration of the oxygen scavenger chemical composition in the pumpable gel mixture is from about 0.10 percent by weight to about 0.75 percent by weight, most preferably 0.50 percent by weight, based on the total weight of the mixture. 
     Several different ways are provided for easy removal of any remaining solid gel plug. One variation, which can be utilized to facilitate removal of the gel plug from the wellbore, is to form a solid gel containing a gel breaker. This gel breaker, included in the pumpable gel mixture, is selected from a group of chemical compounds which can break down the solid gel at temperatures of less than about 16° C. to about 121° C. (60° F. to 250° F.). Generaly, this breakdown of the gel stem will occur within from about 2 hours to about 24 hours after solidification of the gel mixture in the wellbore, depending upon type and concentration of the gel breaker added. Chemical compositions satisfactory for use as gel breakers, and which may be incorporated into the pumpable gel mixture, include certain enzymes and certain oxidizing agents (such as sodium persulfate) which are suitable for breaking down the solid gel. Other gel breakers are disclosed in U.S. Pat. No. 4,265,311 issued to Ely on May 5, 1981. This patent is hereby incorporated by reference. Enzyme breakers may be obtained commercially from oil field service companies. 
     The concentration of the gel breaker incorporated into the pumpable gel mixture may vary from about 0.01 weight percent to about 0.10 weight percent, preferably about 0.05 weight percent of the gel mixture. Although the temperature upon ignition in the wellbore may generally exceed about 66° C. (150° F.), the gel plug will remain intact during the generation and dissipation of energy after ignition of the propellant. Upon cooling to a temperature of from about 16° C. to 66° C. (60° F. to about 150° F.), a suitable gel breaker will break down the solid gel plug stem in the wellbore, causing the plug to liquify. 
     Another method for breaking the gel according to the invention comprises contacting the solidified gel stem with a mineral acid after ignition of the propellant and lapse of a suitable time interval. In those instances where it is undesirable to have a gel breaker incorporated into the gel mixture prior to ignition to remove the solid gel plug, it is preferred to use hydrochloric acid of a strength sufficient to solubilize the solid gel. Hydrocloric acid, and acids similar thereto, can be used to break down the solid gel on contact. Hydrochloric acid of a concentration of about 5.0 percent to about 28  percent, preferably about 15 percent by volume of solution, will generally be sufficient for this purpose. Although hydrochloric acid has been mentioned, other similar mineral acids or strong organic acids may be used. 
     After the gel is solidified, a solution of inhibited acid is injected into the wellbore over the formed gel plug. Said inhibited acid does not dissolve the gel plug. The solution of acid employed may be any of the aqueous solutions of acid commonly employed for acidizing subterranean calcareous formations. For example, the solution of acid may be an aqueous solution of hydrochloric acid. Commonly, the aqueous solutions of hydrochloric acid employed for acidizing subterranean calcareous formations contain between 5 and 28 percent by weight of hydrogen chloride. An aqueous solution of acetic acid may be also employed. Additionally, an aqueous solution of formic acid may be employed. As is known, when the acid solution becomes spent as the result of reacting with the material of the formation, the solubility of calcium sulfate, i.e., anhydrite or gypsum, dissolved in the acid decreases. Thus, any calcium sulfate dissolved from the formation or derived from the water employed in preparing the solution of acid can precipitate with a consequent decrease in the permeability of the formation. 
     Accordingly, it is preferred that the solution of acid that is employed contain an agent to inhibit the precipitation of calcium sulfate. Thus, where hydrogen chloride is employed, the solution thereof may contain up to 40 percent by weight of calcium chloride. Additionally, the solution of acid may also contain any of the commonly employed inhibitors for preventing corrosion of metal equipment, such as casing, liner, or tubing in the well. 
     The amount of solution of acid to be employed will vary according to the radial distance from the well to which the formation is to be acidized and, as stated, this distance may vary up to 15 feet but will not, in most cases, exceed about 10 feet from the well. The amount of solution of acid to be employed will also vary according to the extent to which the material of the formation is to be dissolved. Preferably, the amount of acid should be one hydrocarbon pore volume of the portion of the formation to be acidized. However, lesser amounts may be employed. Generally, the amount employed will be that ordinarily employed to conventional, commercial acidizing operations. 
     Also, as disclosed in U.S. Pat. No. 3,233,672 issued to Carpenter, inhibitors, such as alkyl-substituted carboximides and alkyl-substituted sulfoxides, can be added to the acidizing solution. This patent is hereby incorporated by reference. 
     After the inhibited acid has been placed into the wellbore to the desired formation interval sought to be treated, a high energy impulse device or canister containing propellant therein is located within the wellbore. Propellant is contained in the canister or high energy device sufficient to create simultaneous multiple radial fractures within the formation where the acid in its activated state reacts within said fractures. Said fractures are enlarged and lengthened. Upon the creation of these fractures, the reactive acid is forced into said fractures. Once the acid has entered the formation via said fractures, the acid reacts with the formation, thereby increasing the permeability within said formation. This increase in permeability allows for increased volumes of hydrocarbonaceous fluids to be produced from a formation containing same. 
     Said propellant can belong to the modified nitrocellulose or the modified or unmodified nitroamine propellant class. Another suitable propellant is a composite propellant which contains ammonium perchlorate in a rubberized binder. Other suitable propellants are discussed in U.S. Pat. No. 4,590,997 which issued to Stowe on May 27, 1986. This patent is hereby incorporated by reference. 
     After the steps of this method are completed, instead of removing the solid gel plug, additional inhibited acid and another propellant device can be placed into the wellbore and the method repeated. Also, by directing additional solidifiable gel material into the wellbore, the level of the gel plug in the wellbore can be raised, and other desired intervals can be treated where a multi-interval formation is encountered. If needed, the method can be repeated until the formation has been acidized to the extent desired. 
     Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be restored to without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims.