Viscoelastic Acid

Abstract

An acidic viscoelastic surfactant fluid composition is formulated with an acid, for example hydrochloric acid, a cationic surfactant, for example a quaternary amine, and a break-enabling salt, for example NH 4 Cl. The fluid composition has low viscosity as formulated, undergoes an increase in viscosity as the acid is consumed when it reacts with an acid-reactive material, and returns to approximately the initial viscosity as the last of the acid is consumed. The fluid composition is used as a diverting fluid or as the main treatment fluid in oilfield treatments such as matrix acidizing and acid fracturing.

1 - 25 . (canceled) 
     
     
         26 . A fluid composition comprising
 a. an amount of a surfactant effective to increase the viscosity of the fluid, said surfactant comprising a quaternary ammonium salt of the amine corresponding to the formula:
   R 1 N + (R 2 )(R 3 )(R 4 )X −   
 in which R 1  is a group that has from about 14 to about 26 carbon atoms and may be branched or straight chained, aromatic, saturated or unsaturated, and may include a carbonyl, an amide, a retroamide, an imide, a urea, or an amine; R 2 , R 3 , and R 4  are each independently hydrogen or a C 1  to about C 6  aliphatic group which may be the same or different, branched or straight chained, saturated or unsaturated and one or more than one of which may be substituted with a group that renders the R 2 , R 3 , and R 4  group more hydrophilic; the R 2 , R 3  and R 4  groups may be incorporated into a heterocyclic 5- or 6-member ring structure which includes the nitrogen atom; the R 2 , R 3  and R 4  groups may be the same or different; one of R 2 , R 3 , and R 4  may be hydrogen; and X −  is an anion; 
   b. from about 3 to about 28 percent of an acid; and   c. from about 1 to about 10 percent of a salt,   wherein the viscosity of the fluid composition decreases, as the acid content of the fluid composition decreases by reaction with an acid-reactive material to less than 2 percent, by a percentage greater at a shear rate of 1 sec −1  than at a shear rate of about 100 sec −1  to about 170 sec −1 .   
     
     
         27 . The composition of  claim 26  wherein R 1  comprises from about 18 to about 22 carbon atoms and may comprise a carbonyl, an amide, or an amine; R 2 , R 3 , and R 4  comprise from 1 to about 3 carbon atoms; and X −  is a halide. 
     
     
         28 . The composition of  claim 27  wherein R 1  comprises from about 18 to about 22 carbon atoms and may comprise a carbonyl, an amide, or an amine, and R 2 , R 3 , and R 4  are the same as one another and comprise from 1 to about 3 carbon atoms. 
     
     
         29 . The composition of  claim 26  wherein said viscoelastic surfactant further comprises an amine. 
     
     
         30 . The composition of  claim 29  wherein said amine has the structure:
   R 1 N(R 2 )(R 3 )   and R 1 , R 2 , and R 3  are as defined in claim  1 .   
     
     
         31 . The composition of  claim 29  wherein said amine is present at a concentration of between about 0.01 and about 1 percent. 
     
     
         32 . The composition of  claim 26  wherein said viscoelastic surfactant system further comprises a polysulfonic acid. 
     
     
         33 . The composition of  claim 26  wherein R 1  comprises from about 18 to about 22 carbon atoms, and may comprise a carbonyl, an amide, or an amine, and R 2 , R 3 , and R 4  comprise from 1 to about 3 carbon atoms and may be substituted with a group that renders the R 2 , R 3 , and R 4  group more hydrophilic. 
     
     
         34 . The composition of  claim 33  wherein the surfactant is erucyl bis-(2-hydroxyethyl)methyl ammonium chloride. 
     
     
         35 . The composition of  claim 26  wherein the acid is selected from the group consisting of hydrochloric acid, a mixture of hydrochloric and hydrofluoric acids, fluoroboric acid, nitric acid, phosphoric acid, maleic acid, citric acid, acetic acid, formic acid, and mixtures thereof. 
     
     
         36 . The composition of  claim 35  wherein the acid is hydrochloric acid. 
     
     
         37 . The composition of  claim 26  wherein the salt is selected from the group consisting of lithium, sodium, ammonium, and potassium chlorides and nitrates and mixtures thereof. 
     
     
         38 . The composition of  claim 37  wherein the salt is ammonium chloride. 
     
     
         39 . The composition of  claim 37  wherein the salt is present in an amount of from about 4 to about 5 weight percent. 
     
     
         40 . The composition of  claim 26  wherein the surfactant is present in an amount of from about 1 to about 4 weight percent active material.