Sep. 11, 2014  3  US 2014/0256604 Al  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.  [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.  [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.  [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.  [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  Viscoelastic Surfactants for Increasing Viscosity  Damage to Permeability  Breaking Fluid Viscosity or Gel  Viscoelastic Surfactants in Acidizing 