fluids. Low reservoir pressure in certain areas of a subterra› nean formation or a rock matrix or a proppant pack of high permeability may permit that portion to accept larger amounts of certain treatment fluids. It may be difficult to obtain a uniform distribution of the treatment fluid through› out the entire zone. For instance, the treatment fluid may preferentially enter portions of the zone with low fluid flow resistance at the expense of portions of the zone with higher fluid flow resistance. Matrix diversion is different from zonal diversion between different zones. 

[0029] Similar fluids and methods can be used for "leak-off control" in acid fracturing and for "diversion" in matrix acidizing. Such a method or acidic fluid may be termed a "leak-off control acid system" or a "self-diverting acid sys› tem" depending upon its use and purpose. 

[0030] Increasing the viscosity or gelling ofa fluid can help divert subsequently introduced fluid from higher permeabil› ity to lower permeability portions of a zone. This can be useful for leak-off control in acid fracturing or matrix diver› sion in matrix acidizing. 

[0031] A viscosity-increasing agent is sometimes referred to in the art as a viscosifying agent, viscosifier, thickener, gelling agent, or suspending agent. In general, any of these refers to an agent that includes at least the characteristic of increasing the viscosity of a fluid in which it is dispersed or dissolved. There are several kinds of viscosity-increasing agents and related techniques for increasing the viscosity of a fluid. 

## Breaking Fluid Viscosity or Gel 

[0032] After a treatment fluid is placed where desired in the well and for the desired time, the viscous fluid or gel usually must be removed from the wellbore or the formation to allow for the production of oil or gas. To accomplish this removal, the viscosity of the treatment fluid must be reduced to a very low viscosity, preferably near the viscosity of water, for opti› mal removal from the zone of the subterranean formation. 

[0033] Reducing the viscosity of a viscosified fluid is referred to as "breaking" the fluid. Chemicals used to reduce the viscosity of fracturing fluids are called "breakers." 

[0034] No particular mechanism is necessarily implied by the term. A breaker or breaking mechanism should be selected based on its performance in the temperature, pH, time, and desired viscosity profile for each specific treatment. 

## Damage to Permeability 

[0035] In well treatments using viscous well fluids, the material for increasing the viscosity of the fluid can damage the permeability of the proppant pack or the matrix of the subterranean formation. For example, a treatment fluid can include a polymeric material that is deposited in the fracture or within the matrix. By way of another example, the fluid may include surfactants that lead to sludge formation or change the wettability of the formation in the region of the fracture. 

## Viscoelastic Surfactants for Increasing Viscosity 

[0036] Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liq› uids, or that between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants. 

[0037] Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups ("tails") and hydrophilic groups ("heads"). Therefore, a sur› factant contains both a water-insoluble ( or oil soluble) portion and a water-soluble portion. 

[0038] A "surfactant package" can include one or more different chemical surfactants. 

[0039] In a water phase, surfactants form aggregates, such as micelles, where the hydrophobic tails form the core of the aggregate and the hydrophilic heads are in contact with the surrounding liquid. These aggregates can be formed in vari› ous shapes such as spherical or cylindrical micelles or bilay› ers. 

[0040] As used herein, the term "micelle" includes any structure that minimizes the contact between the lyophobic ("solvent-repelling") portion of a surfactant molecule and the solvent, for example, by aggregating the surfactant molecules into structures such as spheres, cylinders, or sheets, wherein the lyophobic portions are on the interior of the aggregate structure and the lyophilic ("solvent-attracting") portions are on the exterior of the structure. 

[0041] Certain types of surfactants can impart viscosity and elasticity to a fluid. Such a surfactant is referred to as a "viscoelastic surfactant" ("YES"). When used as a viscosity› increasing agent, the molecules ( or ions) of the surfactant associate to form micelles of a certain mi cellar structure ( e.g., rod-like, worm-like, vesicles, etc., which are referred to herein as "viscosifying micelles") that, under certain condi› tions (e.g., concentration, ionic strength of the fluid, etc.) are capable of, inter alia, imparting increased viscosity to a par› ticular fluid or forming a gel. Certain viscosifying micelles may impart increased viscosity to a fluid such that the fluid exhibits viscoelastic behavior ( e.g., shear thinning properties) due, at least in part, to the association of the surfactant mol› ecules contained therein. 

## Viscoelastic Surfactants in Acidizing 

[0042] The various types of cross-linked polymeric fluids that are commonly used in the treatment. However, such cross linked fluids are known to leave solid residue after the treat› ment and thereby damage the formation. 

[0043] There are certain YES fluids that develop viscosity after the acid starts to spend. This results in better diversion that can be considered as another advantage of the YES fluid. The acid diversion is very important in acid stimulation treat› ment to enhance oil production by creating better wormholes. It also increases the depth of penetration of acid into the reservmr. 

[0044] The viscoelastic surfactant fluids are gaining impor› tance due to their less-damaging nature towards the formation as compared to crosslinked polymer fluids. The YES fluids develop viscosity by aggregation of surfactants molecules that shows similar properties of polymer fluid. The YES fluid breaks down easily on dilution or contact with oil, thereby leaving negligible residue in the reservoir. The viscosity of a YES fluid depends on various factors such as the structure of the surfactant, nature of the counter ion, temperature, and presence of water-insoluble components. 

[0045] The main limitation of YES fluids is the steep decrease in viscosity with increase in temperature that limits it application for high-temperature reservoirs. At present, known YES fluids can work only up to about 93 ° C. (200° F.) and cannot be used for higher temperatures. Hence there was 

