Sep.  11, 2014  8  US 2014/0256604 Al  [0134]  YES fluids form asymmetric rod-shaped aggrega› tions, known as micelles that entangle on the application of  shear and hence develops high viscosity. On the dilution or in  contact with oil, these rod-shaped micelles are transformed  into more symmetrical spherical micelles, which results in  low viscosity, referred to as "breaking" of the viscosified  fluid. The same effect is observed with increase in the tem› perature. The shape of the aggregation depends upon various  factors such as the chemical structure of the surfactant (e.g.,  the balance of the sizes of the hydrophobic tail and hydro› philic head), the concentration of the surfactant, nature of  counter ions, ionic salt concentration, co-surfactants, solubi› lized components (if any), pH, and temperature.  [0135]  YES fluids exhibit viscosification as the acid spends  due to which they function as excellent diverting agents and  help in achieving deeper acid penetration. This viscosifica› tion is due to the increase in salinity of the system as acid  spends on carbonate or dolomite formations releasing either  CaC1$_{2 }$or a mixture of CaC1$_{2 }$ and MgC1$_{2 }$in the system. With an  increase in the salinity, the surfactant molecules rearrange  themselves into asymmetric rod-shaped micelles that get  entangled on the application of shear and hence develop high  viscosity.  [0136]  It  is important to inhibit the corrosion on the steel  surfaces in an oil well when they are contacted with acidizing  fluids. The higher rate of corrosion caused by using strongly  acidic fluids at higher temperature is another challenge, how› ever, because the incorporation of corrosion inhibitors tends  to reduce the viscosity of a YES fluid.  [0137]  A YES cannot be used at temperature higher than  93°  C. (200°  F.). The viscosity ofVES fluids containing YES  is reduced in presence of the previously-used corrosion  inhibitors for YES fluids. Without necessarily being limited  by any theory, it is believed that this effect is due to the prior  use of a cationic YES in combination with a cationic corro› sion inhibitor.  [0138]  According to the invention, an organic component  having at least 4 carbon atoms and a non-cationic corrosion  inhibitor are used with a cationic viscoelastic surfactant in a  treatment fluid; wherein the pH of the treatment fluid is less  than 0.5. The non-cationic corrosion inhibitor can be the  anionic organic component. Most preferably, no cationic cor› rosion inhibitor is used in a treatment fluid or method accord› ing to the invention.  [0139]  Without being limited by any theory, it is believed  the thinning ofVES fluids can be avoided by altering inter› molecular interactions between the surfactant molecules. In  general, there is repulsion between the charged polar (hydro› philic) parts of the surfactant molecule, whereas there is ten› dency for the hydrophobic part of the surfactant molecule to  reduce contact with the water molecules. In an attempt to  avoid water, the hydrophobic parts of the surfactant mol› ecules cause the molecules to come closer to each other  forming aggregates, whereas repulsion due to charged hydro› philic part of the molecules increases the spacing between the  molecules. This balance not only determines the spacing  between the molecules but also shape of the aggregation such  as spherical, rod, lamellar or vesicles.  [0140]  Reducing the repulsion decreases the spacing  between the molecules and gradually changes the shape of the  aggregation from spherical to rod-like and then to vesicles or  lamellar. The repulsion due to charged species can be reduced  by having another organic molecule with opposite charge that  can associate in between the YES molecules. This results in  the increase in the viscosity of the fluid.  [0141]  On the other hand, addition of organic molecules  with the same charge disturbs the molecular spacing and  hence reduces the viscosity. Viscoelastic surfactants such as  YES are usually cationic in nature. These have been conven› tionally used with corrosion inhibitors that also contain one or  more organic cations. At the same time, inclusion of a corro› sion inhibitor in a YES fluid is imperative to counteract the  effect of corrosion at higher temperatures and cannot be  avoided. This has obstructed the usage of viscoelastic fluids at  higher temperatures because it is difficult to get adequate  viscosity in presence of the corrosion inhibitor. Thus identi› fication of a corrosion inhibitor that is compatible with the  YES fluid is extremely important.  [0142]  It  is believed that anionic organic molecules can be  used to reduce the charge repulsion in cationic YES mol› ecules, thereby increasing the aggregation number of the  micelles. For example, anionic molecules of a corrosion  inhibitor can interact in synergistic way with cationic surfac› tant in YES and reduces the charge repulsion between the  cationic surfactant molecules. This enables the aggregation of  the surfactant molecules to remain rod-shaped, even at higher  temperatures above 93°  C. (200°  F.), at which temperature  rates of corrosion tend to also be higher.  [0143]  As used herein, an ionic bond is considered a bond  wherein the ionic character is greater than the covalent char› acter.  [0144]  As used herein, "ionic," "cationic," "anionic," and  "non-ionic" are used to describe the organic portion of an  organic molecule or salt thereof. An "ionic" organic portion  would form an ionic bond with another element or another  portion of the molecule. Of course, it should be understood  that "cationic" and "anionic" are types of ionic species. A  non-ionic species would not dissociate into ionic species in a  neutral pH aqueous solution.  [0145]  The degree of "anionic" character of an organic  portion of a molecule can be defined in term of the pKa(l) of  the acid form of the molecule (as opposed to a conjugate  base). As used herein, weak organic acids (Bronsted Lowry)  have a pKa(l) in the range of -1.75 to 7.  [0146]  In addition, as used herein, a "non-cationic" mol› ecule refers to the organic portion of an organic molecule or  salt thereof that may be either anionic or non-ionic.  [0147]  Preferably, other than the cationic viscoelastic, the  fluid includes less than a concentration of one or more other  cationic organic molecules that would interfere with the  building of viscosity upon the spending of the acid and  increase in salinity of the fluid. More preferably, for example,  it is believed that the concentration of any one or more other  cationic organic molecules should be less than 0.5% by  weight of the aqueous phase ( excluding the weight of dis› solved inorganic salts).  [0148]  In an embodiment according to the invention, 20%  HCl spent acid fluid retained its high viscosity up to 121 °  C.  (250°  F.) with formulation comprising a cationic YES and an  anionic corrosion inhibitor. The same fluid also showed low  corrosion loss in the standard corrosion test at 121 °  C. (250°  F.) using P-110 coupons. With the inclusion of the corrosion  inhibitor with organic anionic molecules, the current fluid  system can be used in stimulation of carbonate reservoirs up  to 121 °  C. (250°  F.) with effective corrosion control.  General Approach 