Many return permeability tests are performed with extremely water-wet cores. This is a result of using outcrop core or solvent-cleaned reservoir core. Core is often prepared to a similar liquid saturation as in the reservoir with a matching viscosity mineral oil as a substitute for the main reservoir production phase. As a result the core remains in a water-wet condition. Exposing highly water-wet rock to an oil-based filtrate has the ability to alter mineral surface wettability due to the presence of emulsifiers. The subsequent reduction in permeability can be interpreted as damage, although there is uncertainty that such reduction would take place if the wettability was correct at the start of the test. The pass criteria for drilling fluids are often set as a minimum percentage return permeability. Unrepresentatively high wettability alteration in the laboratory may lead to wrong conclusions.

This paper presents data and observations which display the differences between the use of water-wet and mixed-wet, aged core and the impact on return permeability results. Return permeability results are presented by allowing cores to naturally alter wettability in the presence of reservoir fluids prior to the exposure of oil-based drilling fluids. These data are presented in comparison to duplicate tests on strongly water-wet cores. The difference in return permeability results when comparing the two start conditions is significant.

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