Fluid invasion into productive zones is recognized as detrimental to well productivity. Filtrate and solids invasion can cause irreversible formation damage and permeability reduction. Nondamaging acid-soluble solids arc added to drill-in fluids to promote pore plugging and minimize fluid penetration. Development of less invasive, nondamaging fluid formulations requires knowledge of filtration mechanisms of solids-containing polymeric solutions in porous media.