The Losseal family of reinforced composite mat pills are designed to reduce nonproductive time caused by lost circulation during drilling and cementing operations. The pills are an energized blend of fibers and optimally sized solids that create strong impermeable grids to stop drilling or cementing fluids from flowing into permeable thief zones. The technology is suitable for a wide range of environments, including natural fractures, dolomite, and carbonate zones. The Losseal system can be designed to pump through bottomhole assembly nozzles as 10/32 in., eliminating the need to pull out of hole. Traditional lost circulation fiber materials have often required information about fracture width for effective bridging and plugging. The Losseal design combines fibers with different mechanical properties and high solids content. This engineered design makes it less sensitive to fracture sizes because the particle size of the solids is optimized to plug the network fibers.