Method of preparing a drilling fluid comprising structured surfactants

A method for preparing a drilling fluid consisting essentially of a pourable aqueous composition containing an aqueous structured surfactant, together with at least one weighting agent, said method comprising

• (a) obtaining a concentrate which consists essentially of water and, in a concentration sufficient to form a mobile G-phase,
• (i) a water soluble, polyvalent metal salt of an alkyl ether acid selected from the group consisting of C.sub.8 -C.sub.20 alkyl polyalkleneoxy sulphuric acid and C.sub.8 -C.sub.20 alkyl polyalkyleneoxy carboxylic acid having in each case from 1 to 50 ethyleneoxy groups;
• (ii) a polyvalent metal salt of a C.sub.8 -C.sub.20 alkyl benzene sulphonic acid;
• and (iii) said at least one weighting agent;
• and
• (b) adding to said concentrate sufficient water to form a spherulitic composition;
• wherein said water soluble polyvalent metal salt of an alkyl ether acid (i) and said polyvalent metal salt of a C.sub.8 -C.sub.20 alkyl benzene sulphonic acid (ii) are surfactant components, present to form said aqueous structured surfactant;
• said at least one weighting agent consists essentially of a water soluble inorganic polyvalent metal salt selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide, barium chloride and ferric chloride, or a water soluble inorganic polyvalent metal base selected from the group consisting of calcium hydroxide and barium hydroxide;
• and
• said spherulitic composition contains at least 2% by weight of said water soluble polyvalent metal salt (i), at least 1% by weight of said polyvalent metal salt (ii) and at least about 34% by weight of said at least one weighting agent.