A series of drilling mud invasion experiments were conducted on Berea sandstone cores to measure the extent and depth of formation damage by mud filtrates and mud particles. A specially designed dynamic filtration core holder was used in this study. Pressure taps along the core holder allowed us to measure the permeabilities for 3 sections of the core while circulating mud. Seven kinds of muds with different salinities and different dynamic filtration rates were circulated across the face of the core for 10 hours. After mud circulation, return permeabilities for brine and oil were measured. The effect of bentonite concentration, the effective particle size, degree of flocculation, polymer additive concentration, and oil saturation in the core were shown to influence the extent of formation damage.