In view of the increasing scarcity of energy resources, wells are being drilled to progressively greater depths for the production of liquid and gaseous hydrocarbons. Consequently, the drilling equipment is being subjected to higher temperatures and pressures. Economical exploitation of the reservoir is possible only with the application of drilling and completion methods which do not damage the formation. The damage potential of drilling fluids and treatment fluids is assessed on the basis of return permeability tests. For obtaining more conclusive results from return permeability tests and for increasing the reliability of their interpretation in the future, systematic investigations on the reproducibility of the results were of special importance. In the course of this third project phase, several return permeability tests were performed on each of two different model cores under constant conditions in the circulation system constructed at the institute. On the one hand, Obernkirchen sandstone was employed as model material; on the other hand, a synthetic material consisting of sintered borosilicate glass was used for the other model core. At least five tests were performed on the model core of each material, in order to evaluate the results for their reproducibility. A further important task was the selection of a suitable core material for future tests.