Manganese tetraoxide is used as a weighting material in drilling deep wells (> 90 lb/ft3). Removal of the filter cake that contains Mn 3O4 is a difficult task. This is mainly because Mn 3O4 is a strong oxidizing agent. Extensive lab work was done to: 1) study the reaction of glycolic acid with Mn3O4 particles (D50 =1 μm) as a function of time at 190°F in a batch reactor, 2) examine the solubility of the filter cake using a modified HP/HT filter press, 3) determine the compatibility and thermal stability of cleaning fluids with α-amylase using an HP/HT visual cell, 4) characterize the filter cake using computer tomography (CT), and 5) assess the retained permeability of limestone and sandstone cores. The results following the removal of the filter cake showed that 4 wt% glycolic acid (4 g Mn3O 4 to 200 ml acid solution) dissolved 75 wt% of Mn3O 4 particles at 190°F. Glycolic acid (5 wt%) had a removal efficiency of 85 to 90% of manganese tetraoxide-based filter cake after 20 hrs soaking time. The retained permeability was 100% for the Indiana limestone core and 125% for the Berea sandstone cores. CT results showed that glycolic acid was able to stimulate the cores and no formation damage was observed.