Abstract: A fluid system has been discovered using graded calcium carbonate particle sizes and a modified lignosulfonate. Optionally, a thixotropic polymer such as a welan or xanthum gum polymer is used to keep the CaCO.sub.3 and lignosulfonate in suspension. It is important that the calcium carbonate particles be distributed across a wide size range to effectively prevent filtration or fluid loss into the formation. Further, the lignosulfonate must be polymerized to an extent effective to reduce its water solubility. The modified lignosulfonate (lignin sulfonate) is necessary for the formation of a filter cake essentially on the surface of the well bore. Since the filter cake particles do not invade the well bore due to the action of the modified lignosulfonate, no high pressure spike occurs during the removal of the filter cake which indicates damage to the formation and well bore surface and typically reduces overall permeability of the formation.