Abstract

The invention provides a method and fluid for drilling or servicing a well in a subterranean formation, particularly containing lost circulation zones or depleted, low pressure reservoirs. A method of decreasing the density of the fluid circulated in a borehole, decreasing the invasion of fluid into formations contacted by the fluid, or decreasing the lost circulation potential of the fluid is provided by using as the fluid an aqueous liquid having dispersed therein a polymer which increases the low shear rate viscosity of the fluid to the extent that the shear thinning index of the fluid is at least about 10, a surfactant, and aphrons, wherein the aphrons are preferably generated by the turbulence and pressure drop as the fluid exits the drill bit in the vicinity of the formation, the fluid having a low shear rate viscosity of at least 10,000 centipoise. The fluid additionally contains an oligosaccharide mixture composed of pentosans and/or hexosans containing from one to about 10 combined sugar units selected from the group consisting of arabinose, mannose, galactose, glucose, xylose, cellobiose, and mixtures thereof. Preferably the oligosaccharide mixture is the water soluble portion of thermally hydrolyzed lignocellulose.