In accordance with one or more embodiments, this disclosure describes a viscoelastic fluid for a subterranean formation comprising: viscoelastic surfactant comprising the general formula:

##STR00001##

where R.sub.1 is a saturated or unsaturated hydrocarbon group of from 17 to 29 carbon atoms, R.sub.2 and R.sub.3, are each independently selected from a straight chain or branched alkyl or hydroxyalkyl group of from 1 to 6 carbon atoms; R.sub.4 is selected from H, hydroxyl, alkyl or hydroxyalkyl groups of from 1 to 4 carbon atoms; k is an integer of from 2-20; m is an integer of from 1-20; and n is an integer of from 0-20; brine solution; and at least one nanoparticle viscosity modifier comprising a particle size of 0.1 to 500 nanometers, or 0.1 to 100 nanometers.

In accordance with one or more embodiments, this disclosure describes a viscoelastic surfactant fluid for a subterranean formation comprising: brine solution; at least one polyacrylamide viscosity modifier with a weight averaged molecular weight (Mw) from 250,000 g/mol to 40,000,000 g/mol; and a viscoelastic surfactant according to formula (I):

##STR00001##

where R.sub.1 is a saturated or unsaturated hydrocarbon group of from 17 to 29 carbon atoms, R.sub.2 and R.sub.3 are each independently selected from a straight chain or branched alkyl or hydroxyalkyl group of from 1 to 6 carbon atoms; R.sub.4 is selected from H, hydroxyl, alkyl or hydroxyalkyl groups of from 1 to 4 carbon atoms; k is an integer of from 2-20; m is an integer of from 1-20; and n is an integer of from 0-20.

Methods for separating fluids with a barrier pill within a downhole environment are provided. The method includes introducing a barrier pill fluid into the wellbore containing a first fluid to form the barrier pill on top of the first fluid in the wellbore and introducing a second fluid into the wellbore. The barrier pill separates the first fluid and the second fluid. The barrier pill includes a viscoelastic surfactant and an aqueous fluid, such as a brine containing water and about 5 wt % to about 50 wt % of a salt.

Compositions and methods are provided for delayed breaking of viscoelastic surfactant gels inside subterranean formations. Breaking is accomplished without mechanical intervention or use of a second fluid. The delayed breaking agent is a hydrophobically modified alkali swellable emulsion polymer, which can be a copolymer comprising acidic monomers, nonionic monomers, and associative monomers. The viscoelastic surfactant can be a zwitterionic surfactant, and can be selected from the group consisting of sultaines, betaines, and amidoamine oxides.

A fracturing fluid comprising: water; a water-soluble salt; and a viscoelastic surfactant, wherein the viscoelastic surfactant: increases the viscosity of the fracturing fluid, wherein the viscosity is increased to at least a sufficient viscosity that proppant are suspended in the fracturing fluid; and is in at least a sufficient concentration such that the viscoelastic surfactant spontaneously forms micelles. A method of fracturing a subterranean formation comprising: introducing the fracturing fluid into a well, wherein the well penetrates the subterranean formation; and creating one or more fractures within the subterranean formation with the fracturing fluid.

Embodiments of the present disclosure are directed to a method of producing a viscoelastic surfactant (VES) fluid, the VES fluid comprising desulfated seawater. The method of producing the VES fluid comprises adding an alkaline earth metal halide to seawater to produce a sulfate precipitate. The method further comprises removing the sulfate precipitate to produce the desulfated water. The method further comprises adding a VES and one or more of a nanoparticle viscosity modifier or a polymeric modifier to the desulfated seawater. Other embodiments are directed to VES fluids that maintain a viscosity greater than 10 cP at temperatures above 250° F.

There is a viscoelastic fluid. The fluid has one or more cationic surfactants selected from the group consisting of certain quaternary salts, certain amines, and combinations thereof; one or more anionic polymers/anionic surfactants; one or more of certain zwitterionic/amphoteric surfactants; and water. There is also a method of fracturing a subterranean formation. The viscoelastic fluid is pumped through a wellbore and into a subterranean formation at a pressure sufficient to fracture the formation. There is also a method for gravel packing a subterranean formation.

A method for realizing balanced displacement of crude oil by injection and production optimization coordinated chemical flooding which comprises the following steps: determining the median size and elastic modulus of viscoelastic particles according to the average permeability of the reservoir; optimizing the concentration ratio of the total concentration of the chemical agent; the physical parameters of each layer are counted, and the hierarchical system is combined according to the entropy weight algorithm and the cluster analysis method based on the gravity center method; the optimal section slug volume ratio of the single well injected with two slugs under the heterogeneity of the permeability of the strata is calculated; and the objective function is established by combining the coefficient of variation of remaining oil saturation, the effect of chemical flooding and the cost, and the numerical simulator is used to optimize the objective function.

A subterranean treatment fluid may include an invert emulsion including a clarified diutan-based gelled aqueous internal phase and a mineral oil-based external phase, and an acid composition. The acid composition may include at least one acid selected: formic acid, acetic acid, propionic acid, lactic acid, glycolic acid, and combinations thereof, at least one acid-generating compound selected from: an ester; an aliphatic polyester; an ortho ester; a poly(ortho ester); an ortho ether; a poly(ortho ether); a lactide; a poly(lactide); a glycolide; a poly(glycolide); an ε-caprolactone; a poly(ε-caprolactone); a hydroxybutyrate; a poly(hydroxybutyrate); an anhydride; a poly(anhydride); an aliphatic carbonate; an aliphatic polycarbonate; an amino acid; a poly(amino acid), and combinations thereof, or a combination of the at least one acid and the at least one acid-generating compound. The subterranean treatment fluid may have a density less than water.

A viscoelastic surfactant (VES) for a self-diverting acid under high temperature has a structural formula shown as formula (I), wherein, n is saturated hydrocarbon with 2 to 8 carbon atoms; R.sub.1 is saturated or unsaturated hydrocarbon with 18 to 28 carbon atoms; R.sub.2 and R.sub.3 are independently methyl, ethyl or hydrogen, and R.sub.2 and R.sub.3 can be the same or different; and X.sup.− is any one of Cl.sup.−, Br.sup.−, CO.sub.3.sup.2−, SO.sub.4.sup.2−, HCOO.sup.− and CH.sub.3COO.sup.−. The method for preparing the surfactant includes subjecting a fatty acid and an organic amine to acid-amine condensation to obtain an intermediate. The intermediate reacts with a metal hydride to obtain a fatty amine. Then, an acid solution is used to protonate the fatty amine to obtain an ultra-long-chain viscoelastic cationic surfactant. The present invention also provides use of the surfactant as a thickener for a self-diverting acid.