Patent ID: 8895481

Claim:
A method of treating a wellbore, the method comprising introducing an acid treatment fluid through the wellbore, the treatment fluid comprising from 0.5 to 5.5% by weight of a zwitterionic viscoelastic surfactant, an acid in an amount more than 15% by weight to provide the treatment fluid with an initial pH of ≦0 and a rheology enhancer selected from at least one of a polyvinyl ester, a polyvinyl acetate, a polyacrylate and a sulfonate-containing polymer, and combinations thereof, the zwitterionic viscoelastic surfactant comprises a surfactant or mixture of surfactants having the formula: RCONH—(CH 2 ) a (CH 2 CH 2 O) m (CH 2 ) b —N + (CH 3 ) 2 —(CH 2 ) a′ (CH 2 CH 2 O) m′ (CH 2 ) b′ COO − wherein R is an alkyl group that contains from about 17 to about 23 carbon atoms which may be branched or straight chained and which may be saturated or unsaturated: a, b, a′, and b′ are each from 0 to 10 and m and m′ are each from 0 to 13, a and b are each 1 or 2 if m is not 0 and (a+b) is from 2 to 10 if m is 0: a′ and b′ are each 1 or 2 when m′ is not 0 and (a′+b′) is from 1 to 5 if m′ is 0; (m+m′) is from 0 to 14; and CH 2 CH 2 O may also be OCH 2 CH 2 , and, wherein the treatment fluid has a high viscosity at the initial pH and a lower viscosity when the pH of the treatment fluid is raised to an intermediate pH of from greater than 0 to about 5; and wherein: the treatment fluid has at least one property of (1) and (2), wherein (1) is a viscosity at the intermediate pH of less than about 20 mPas at a shear rate of 100 s −1 , and (2) is the viscosity at the intermediate pH is decreased by as much as 70% or more compared to the high viscosity of the fluid at the initial pH.