Method for reducing the viscosity of viscosified fluids for applications in natural gas and oil fields

1. A method of reducing the viscosity of a viscosified treatment fluid comprising the steps of:

(i) providing a viscosified treatment fluid containing a water soluble polymeric gelling agent wherein the viscosified treatment fluid is prepared using natural occurring brines selected from the group consisting of formation water, produced water, and/or flow back from a well after a stimulation process, and mixtures thereof, wherein the natural occurring brine has a total organic carbon (TOC) in the range of 310-5000 mg/l, a total dissolved solids (TDS) salt of at least 10,000 ppm, said total dissolved solids (TDS) salt includes Fe(II) salt, the water soluble polymeric gelling agent comprising

(I) structural units of formula (I)

wherein

R1, R2 and R3 independently are hydrogen or C 1 -C 6 -alkyl,

(II) up to 95% by weight structural units of formula (II)

wherein

R4 is hydrogen or C 1 -C 6 -alkyl,

R5 is hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

A is a covalent C—S bond or a two-valent organic bridging group,

(V) up to 20% by weight structural units of formula (V)

wherein

R17 is hydrogen or, C 1 -C 6 -alkyl, and

R18 and R19 are independently of one another hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

B is a covalent C—P bond or a two-valent organic bridging group,

with the proviso that the percentage of the structural units of formulae (I), (II), and (V) refer to the total mass of the copolymer and the percentage of the structural units of formulae (I), (II), and (V) amounts to 100%,

(ii) providing a breaker composition consisting of a non-oxidizing peroxide selected from the group consisting of:

(1) percarbonates including t-amyl-2-ethyl hexyl percarbonate, t-butyl-2-ethyl hexyl percarbonate,

(2) peroxyl dicarbonates including diisopropyl peroxydi carbonate, di sec-butyl peroxydicarbonat, di t-butyl peroxydicarbonate, di n-butyl peroxydicarbonate, di (2-ethylhexyl) peroxyperdicarbonate, di (4-t-butyl)cyclohexyl peroxydicarbonate, dicetyl peroxydicarbonate,

(3) diacyl peroxide including diisobutyryl peroxide, dilauroyl peroxide, didecanoyl peroxide, dibenzoyl peroxide,

(4) dialkyl peroxides including di t-butylperoxide, dicumylperoxide, cumyl-t-butyl peroxide, 2,5-di methyl-2,5-di(t-butylperoxy)hexane, 1,1-di(t-butylperoxy)cyclohexane, 1,1-di(t-amylperoxy)cyclohexane, 1,1-di(t-butylperoxy)3,3,5-trimethylcyclohexane,

(5) hydroperoxides including cumyl hydoperoxide, i-propylcumyl hydroperoxide, t-butyl hydroperoxide, t-amyl hydroperoxide, 1,1,3,3-tetramethyl hydroperoxide, and

(6) esters of peracids including esters from peracetic acid, perpivalic acid, perbutanoic acid, perdecanoic acid, perheptanoic acid, per(2-ethylhexanoic) acid, perbenzoic acid with t-butanol, t-amyl alcohol, 2-ethyl hexanol, and does not oxidize iodide to iodine,

(iii) mixing said viscosified treatment fluid and said breaker composition and injecting the mixture of viscosified treatment fluid and breaker composition into a natural gas or oil reservoir,

(iv) allowing said viscosified treatment fluid and said breaker composition to interact whereby the viscosity of the viscosified treatment is reduced.

2. The method of claim 1 wherein the water soluble polymeric gelling agent is selected from the group consisting of polymers containing:

(i) 10 to 90% by weight of structural formula I

(ii) 0 to 80% by weight of structural formula II

(v) 0 to 20% by weight of structural formula V

referred to the total mass of the polymer.

3. The method of claim 1 wherein a quantity of the non-oxidizing peroxide as breaker ranges from 0.001 to 5% by weight, referred to a total mass of aqueous polymer solution.

4. The method of claim 1 wherein the viscosified treatment fluid further comprises a multivalent metal ion or multivalent metal ion complex comprising an ion selected from group IIIA, IVB, VB, VIB, IIVB and/or VIIIB of the periodic table of elements.

5. The method of claim 4 , wherein the multivalent metal ion or multivalent metal ion complexes comprises zirconium, aluminium, titanium, boron, chromium and/or iron.

6. The method of claim 1 wherein the viscosified treatment fluid is injected as hydraulic fracturing fluid.

7. The method of claim 6 , wherein the viscosified treatment fluid is injected at a temperature below 90° C.