A set of experiments was completed that clearly shows that drilling muds can cause large irreversible damage to fractures and dramatically reduce the productivity of wells producing from a natural fracture network. The use of sized fibers as an additive to drilling muds can significantly reduce both the depth and extent of formation damage. Granular additives (such as CaCO3) are much less effective. The use of acid soluble fibers is, therefore, recommended for reducing formation damage in fractured reservoirs.