Acidizing subterranean well formations containing deposits of metal compounds
In acidizing procedures for increasing the permeability of subterranean well formations, aqueous acid solutions are often used. For example, aqueous solutions of hydrochloric acid, hydrofluoric acid, formic acid, acetic acid and various mixtures of such acids are commonly utilized, with aqueous hydrochloric acid being the most commonly used acid solution. When the formation being acidized contains deposits of metal compounds such as iron compounds or clays containing aluminum compounds, the acid solution dissolves such deposits as well as other reactive substances contained in the formation, but upon becoming spent, iron and/or aluminum contained in the solution precipitate as hydroxides which can reduce the permeability of the treated formation. 
     In order to prevent the precipitation of metal compounds from spent acidizing solutions, sequestering additives have heretofore been utilized in the acidizing solutions. For example, acetic acid has been commonly used as a sequestering agent for iron and the like in spent aqueous hydrochloric acid solutions. U.S. Pat. No. 3,142,335 describes iron sequestering additives for use with hydrochloric acid solutions which are comprised of a mixture of citric acid, or a salt thereof, and a low molecular weight organic acid or salt thereof, such as acetic acid or formic acid. 
     By the present invention sequestering additives, acidizing compositions containing the additives and methods of acidizing subterranean well formations are provided which are more effective in preventing the precipitation of metal compounds in formations than the heretofore used additives, compositions and methods. 
     The sequestering additives of the present invention are comprised of a mixture of a first ingredient selected from the group consisting of levulinic acid, a salt of levulinic acid and mixtures thereof, and a second ingredient selected from the group consisting of citric acid, a salt of citric acid and mixtures thereof. The relative proportions of the ingredients can vary, but generally fall within a levulinic acid and/or salt thereof to citric acid and/or salt thereof weight ratio in the range of from about 5:1 to 1:5. The specific amount of each ingredient used in the additive depends on the metal compound content of the formation to be acidized and other factors, but the relative proportions of the ingredients are adjusted so that they are sufficient to prevent precipitation of metal compounds from a spent aqueous acid solution when added to the live solution in a given amount for a longer period of time than obtainable with like amounts of either ingredient alone. 
     The acidizing compositions of this invention for treating subterranean well formations containing metal compound deposits are comprised of an aqueous solution of one or more of the acids mentioned above and the above-described additive for sequestering metals. The total amount of the sequestering additive in the acidizing composition required to prevent precipitation of metal compounds from the composition after becoming spent in a subterranean formation varies with the concentration of metal in the spent solution. However, in most applications an additive concentration in the range of from about 10 pounds to about 400 pounds per 1,000 gallons of aqueous acid solution is sufficient. Preferably, the citric acid or salt thereof ingredient is combined with the acid solution in a solid state in an amount in the range of from about 10 to about 300 pounds per 1,000 gallons of the solution. The levulinic acid ingredient is preferably combined with the acid solution in an amount in the range of from about 0.2% to about 6% by volume of the solution. An equivalent amount of a salt of levulinic acid, preferably alkali metal or ammonium, can be utilized. Such amounts and proportions of ingredients are sufficient in most applications to prevent precipitation of iron and/or aluminum compounds from the acid composition after becoming spent in a subterranean well formation for a period of time sufficient to recover the spent composition from the formation. 
     The most preferred acidizing composition of this invention is an aqueous solution of hydrochloric acid and a levulinic acid-citric acid metal sequestering additive, the hydrochloric acid being present in the composition in an amount of about 15% by weight of the composition, the citric acid being present in the composition in an amount of about 50 pounds per 1,000 gallons of the composition and the levulinic acid being present in the composition in an amount of about 1% by volume of the composition. 
     As will be understood by those skilled in the art, other conventional well formation treating additives can be combined with the acidizing composition such as non-emulsifying agents, corrosion inhibitors, anti-sludging agents, fluid loss additives, etc. 
     In preparing the acidizing compositions, a corrosion inhibitor, if utilized, is first added to water in a mixing tank. The acid or mixture of acids used is next combined with the water in an amount sufficient to obtain a solution of desired acid concentration and mixed thoroughly. The levulinic acid or salt thereof is next combined with the acid solution followed by combining the citric acid or salt thereof with the solution. The resulting composition is mixed or agitated for a period of time sufficient to completely dissolve the sequestering additive ingredients followed by the addition of other conventional formation treating additives, if used. 
     Once the acidizing composition of this invention has been prepared as described above, it is introduced into a subterranean well formation to be acidized. The acidizing composition dissolves deposits of metal compounds as well as other reactive substances in the formation and maintains the metals in solution after becoming spent for a period of time sufficient to recover the spent composition, i.e., the spent composition is recovered from the subterranean formation by producing the formation, by driving the spent composition through the formation to a recovery well, or by driving the spent composition over such a wide area that any precipitate that forms cannot have a detrimental effect. 
    
    
     The following examples are given to further illustrate the present invention. 
     EXAMPLE 1 
     Laboratory tests are conducted to compare the iron sequestering ability of various sequestering additives of this invention. Samples of 15% by weight aqueous hydrochloric acid solutions containing various concentrations of iron (Fe+++) in solution are prepared. Levulinic acid-citric acid sequestering additives having proportions of ingredients given in Table I below are combined with the aqueous acid solutions, and the resultant compositions are reacted with calcite in containers until spent. The spent compositions in the containers are covered and allowed to stand in contact with excess calcite at atmospheric pressure and room temperature until precipitation of iron hydroxide occurs. The times required for the precipitation of iron hydroxide from the spent compositions are shown in Table I below. 
     
                                           TABLE I                                 
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COMPARISON OF PRECIPITATION TIMES OF SPENT AQUEOUS HYDROCHLORIC ACID      
CONTAINING VARIOUS CONCENTRATIONS OF IRON (Fe+++) AND VARIOUS             
LEVULINIC ACID-CITRIC ACID SEQUESTERING ADDITIVES                         
                         PRECIPITATION TIME VS IRON CONCENTRATION         
    Levulinic Acid                                                        
              Citric Acid                                                 
    Concentration,                                                        
              Concentration,                                              
                         2500    5000    7500    10,000 12,500            
    Pounds per 1,000                                                      
              Pounds per 1,000                                            
                         Milligrams                                       
                                 Milligrams                               
                                         Milligrams                       
                                                 Milligrams               
                                                        Milligrams        
Solu-                                                                     
    Gallons of                                                            
              Gallons of Per Liter                                        
                                 Per Liter                                
                                         Per Liter                        
                                                 Per Liter                
                                                        Per Liter         
tion                                                                      
    Solution  Solution   (Fe+++) (Fe+++) (Fe+++) (Fe+++)                  
                                                        (Fe+++)           
__________________________________________________________________________
1   150       0           12 HRS 0.5 HRS 0.5 HRS 0.5 HRS                  
                                                        0.5 HRS           
2   125       25         170 HRS 170 HRS 170 HRS 170 HRS                  
                                                        0.5 HRS           
3   100       50         170 HRS 170 HRS 170 HRS 170 HRS                  
                                                        0.5 HRS           
4   75        75         170 HRS 170 HRS 170 HRS 170 HRS                  
                                                        0.5 HRS           
5   50        100        170 HRS 170 HRS 170 HRS 0.5 HRS                  
                                                        0.5 HRS           
6   25        125        170 HRS 170 HRS 170 HRS 0.5 HRS                  
                                                        0.5 HRS           
7   0         150        170 HRS 170 HRS 0.5 HRS 0.5 HRS                  
                                                        0.5               
__________________________________________________________________________
                                                        HRS               
 
    
     From Table 1 it can be seen that the levulinic acid-citric acid sequestering additives of the present invention can prevent, for at least 170 hours, the precipitation of iron compounds from spent aqueous hydrochloric acid solutions containing up to 10,000 milligrams of iron per liter of solution and that for spent acid solutions containing more than about 5000 milligrams of iron per liter, the additives of this invention are more effective in the prevention of precipitation than like amounts of levulinic acid or citric acid alone. 
     EXAMPLE 2 
     The laboratory procedure set forth in Example 1 is repeated using levulinic acid alone, acetic acid alone, mixtures of levulinic and citric acids and mixtures of acetic and citric acids as sequestering additives. The results of these tests are set forth in Table II below. 
     
         TABLE II
  COMPARISON OF PRECIPITATION TIMES OF SPENT AQUEOUS HYDROCHLORIC ACID
 CONTAINING VARIOUS CONCENTRATIONS OF IRON (Fe+++) AND VARIOUS SEQUESTERIN
 G ADDITIVES PRECIPITATION TIME VS. IRON CONCENTRATION     2000 4000 6000 8
 000 10,000 15,000 20,000 25,000 30,000 40,000 50,000 60,000   Citric
 Milli- Milli- Milli- Milli- Milli- Milli- Milli- Milli- Milli- Milli-
 Milli- Milli-  Levulinic Acid, Lbs. Acetic grams grams grams grams grams
 grams grams grams grams grams grams grams  Acid, % by per 1,000 Acid, %
 by per per per per per per per per per per per per  Volume of Gallons of
 Volume of Liter Liter Liter Liter Liter Liter Liter Liter Liter Liter
 Liter Liter Solution Solution Solution Solution (Fe+++) (Fe+++) (Fe+++)
 (Fe+++) (Fe+++) (Fe+++) (Fe+++) (Fe+++) (Fe+++) (Fe+++) (Fe+++) (Fe+++)
   1 2.0 0 0 72 HRS 48 HRS 24 HRS16 HRS 0.5 HRS -- -- -- -- -- -- -- 2 0
 0 2.0 24 HRS 24 HRS 12 HRS  6 HRS   2 HRS -- -- -- -- -- -- -- 3 1.0 50
 0 -- -- -- -- -- 720 HRS 144 HRS 0.5 HRS -- -- -- -- 4 0 50 1.0 -- -- --
 -- -- 720 HRS 192 HRS 18 HRS 0.5 HRS -- -- -- 5 2.0 100 0 -- -- -- -- --
 -- -- -- -- 720 HRS 720 HRS 0.5 HRS 6 0 100 2.0 -- -- -- -- -- -- -- --
 -- 720 HRS 192 HRS HRS
 
    
     From Table II it can be seen that for iron concentrations of up to 8000 milligrams per liter that levulinic acid alone is more effective as an iron sequestering additive in a spent hydrochloric acid solution than acetic acid alone. In spent solutions containing iron concentrations of up to an amount in excess of about 20,000 milligrams per liter it can be seen that the sequestering additive of this invention is quite effective in preventing precipitation, but that for iron concentrations of greater than about 15,000 to an amount less than about 30,000 milligrams per liter, a sequestering additive comprised of citric acid and acetic acid is more effective than the sequestering additive of the present invention. However, at iron concentrations of from about 40,000 to an amount in excess of about 50,000 milligrams per liter, the levulinic acid-citric acid sequestering additive of the present invention is at least equal to and in some instances is more effective in preventing iron compound precipitation than an acetic acid-citric acid sequestering additive.