In one aspect, this invention relates to the use of thermoset nanocomposite particles as components of drilling fluid, drill-in fluid, completion fluid, and workover fluid additive packages to reduce fluid losses to a formation and / or to enhance a wellbore strength. In another aspect, this invention relates to the use particles of specific gravity ranging from about 0.75 to about 1.75 as components of drilling fluid, drill-in fluid, completion fluid, and workover fluid additive packages to reduce fluid losses to a formation and / or to enhance a wellbore strength. Using embodiments of the invention, reduction of fluid loss and / or enhancement of wellbore strength may be achieved while working with water-based, oil-based, invert emulsion, or synthetic drilling muds. The currently most preferred embodiments of the invention use substantially spherical thermoset nanocomposite particles, possessing a specific gravity from approximately 1.02 to approximately 1.15 wherein the matrix is a terpolymer of styrene, ethylvinylbenzene and divinylbenzene, and wherein carbon black particles possessing a length that is less than about 0.5 microns in at least one principal axis direction are incorporated as a nanofiller.