Patent ID: 7112557

Claim:
A drilling fluid comprising a polar base oil and complimentary fatty acid surfactants blended or combined such that the base oil and the surfactants comprise the oil based layer of an invert emulsion, wherein the base oil and the surfactants comprise about 90 volume percent of the emulsion and calcium chloride brine comprises the water phase of the emulsion, wherein said complimentary fatty acid surfactants are selected from the group consisting of sorbitan esters, sorbitan ester derivatives, ethoxylated sorbitan esters, ethoxylated sorbitan ester derivatives, and combinations thereof, and wherein at least one of the complimentary fatty acid surfactants is more water soluble than another and at least one of the complimentary fatty acid surfactants is more oil soluble than another, such that the surfactants form micelles in quantities providing a staggered arrangement and denser concentration in the palisade layer of the drilling fluid than either surfactant would form alone, such that the drilling fluid has sufficient electrical conductivity to facilitate resistance-based wellbore logging while also having utility as an oil-based drilling fluid.