Viscoelastic compositions

A wellbore service fluid comprising an aqueous viscoelastic surfactant having the structure of formula

• R1—X—(CR5R6)m-A⊖B⊕
• where (i) R1 is a saturated or unsaturated, linear or branched aliphatic chain of at least 18 carbon atoms;
• (ii) X is an R7N(CO), or (CO)NR7 or —(C═O)O— or —O(C═O)—group;
• (iii) m is at least one;
• (iv) R5, R6 and R7 are each independently hydrogen;
• a linear or branched, saturated aliphatic chain of at least 1 carbon atom;
• or a linear or branched, saturated aliphatic chain of at least 1carbon atom with one or more of the hydrogen atoms replaced by a hydroxyl group,
• (v) A⊖ is a sulfonate anionic group;
• and
• (vi) B⊕ is hydrogen or a monovalent cation;
• the amount of the anionic viscoelastic surfactant which is present in the wellbore fluid being sufficient that the fluid is an aqueous viscoelastic gel.

A method for preparing a wellbore fluid which is predominantly aqueous comprising:

• admixing an anionic viscoelastic surfactant of formula I:
• R—X—(CR5R6)m—SO3−B+ (I)
• in which:
• R is a saturated or unsaturated, linear or branched aliphatic hydrocarbon chain comprising from 18 to 22 carbon atoms, including mixtures thereof and/or optionally incorporating an aryl group
• X is —(C═O)N(R7)—, —N(R7)(C═O)—, —N(R7)—, —(C═O)O— or —O(C═O)—;
• R5 and R6 are the same or different and are each independently hydrogen, a linear saturated aliphatic hydrocarbon chain of at least 1 carbon atom, a branched saturated aliphatic hydrocarbon chain of at least 2 carbon atoms, or a linear saturated aliphatic hydrocarbon chain of at least 1carbon atom or a branched aliphatic hydrocarbon chain of at least 2 carbon atoms with one or more of the hydrogen atoms replaced by a hydroxyl group;
• or
• when X is —N(R7)(C═O)— or —O(C═O)—, the group (CR5R6) may include a COO−group;
• R7 is a hydrogen, a linear saturated aliphatic hydrocarbon chain of at least 1 carbon atom, a branched saturated aliphatic hydrocarbon chain of at least 2 carbon atoms, a linear saturated aliphatic hydrocarbon chain of at least 1 carbon atom or a branched saturated aliphatic hydrocarbon chain of at least 2 carbon atoms with one or more of the hydrogen atoms replaced by a hydroxyl group, or a cyclic hydrocarbon group;
• B+ is hydrogen or a monovalent cation and
• m is an integer of from 1 to 4;
• in the form of monomeric unit, a dimer or oligomer;
• with an alcohol or amine additive to form a liquid concentrate and thereafter mixing that concentrate with water to form the wellbore fluid, the amount of the anionic viscoelastic surfactant which is present in the wellbore fluid being sufficient that the fluid is a viscoelastic gel.