Abstract: A clay-based or clay-free aqueous thixotropic wellbore fluid having improved fluid loss control, desirable flow characteristics and low shale sensitivity for use in drilling a well comprising water or a brine base including an effective amount of an additive comprising a crosslinked potato starch, a heteropolysaccharide derived from a carbohydrate by bacteria of the genus Xanthomonas, and hydroxyethylcellulose or carboxymethylcellulose, is disclosed. This drilling fluid has been found to be nondamaging to the formations through which the well is drilled.

Abstract: A lost circulation material is described wherein a mixture of flexible flakes, fibers and granular particles, preferably shredded paper, mineral wool and calcium carbonate, respectively, which, when maintained in a carrying liquid being circulated through a well during drilling operations, acts to stop the loss of drilling fluid into permeable or fractured formations. The seal on the formation can be degraded by contact with an acidizing acid, such as hydrochloric acid, allowing a potential hydrocarbon producing formation to be reopened for production after a seal has been effected using the material of the invention.