1. . 
Solids added to a drilling fluid to bridge across the pore throat or fractures of an exposed rock thereby building a filter cake to prevent loss of whole mud or excessive filtrate. Bridging materials are commonly used in drilling fluids and in lost circulation treatments. For reservoir applications, the bridging agent should be removable-common products include calcium carbonate (acid-soluble), suspended salt (water-soluble) or oil-soluble resins. For lost-circulation treatments, any suitably sized products can be used, including mica, nutshells and fibers. These products are more commonly referred to as lost-circulation material (LCM).
Synonyms: bridging agent
2. . 
Material of a coarse, fibrous or flaky composition used to form an impermeable barrier across a formation interface or perforation. Bridging materials are most commonly used when lost circulation occurs during drilling. They are also used in workover operations in preparation for killing a well when the kill fluid is likely to be lost to the perforations. The selection of an appropriate bridging material is critical during workover operations since the barrier should be completely removed in preparation for placing the well back on production.