Core plugs of argillaceous sandstone from the Wilcox Formation of South Texas were examined by petrographic methods and tested for oil permeabilities before and after exposure to a variety of drilling fluids and filtrates. This was done to determine why some drilling fluids are more damaging to permeabilities than others. The clay minerals in these sandstones, which are mostly diagenetic, include chlorite, kaolinite, and pore-bridging mixed-layer illite-smectite. The greatest permeability damage was caused by gelligno-sulfonate-based fluids, which caused pore throat blockage by dispersion and migration of the mixed-layer clays. Much less damage wascaused by the PHPA polymer/KCl-based fluids, which apparently stabilized the fibrous illite-smectite.

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