The effect of five typical oil-based whole-muds and their filtrates on the wettability and permeability of sandstone cores was evaluated. A comparison is made between the wettability alterations observed for whole muds with those observed in an earlier study with individual mud components. The combined USBM/Amott index test was used to measure wettability changes. All the muds tested made the water-wet core samples significantly less water-wet and greatly reduced the permeability. The whole drlling muds have less of an effect on the wettability than their corresponding filtrates because of the presence of filter cakes which effectively prevent infiltration of large quantities of drilling fluid. Under static filtrations conditions, the wettability alteration was observed to occur up to distances of three inches from the rock-mud contact surface. The extent of alteration was greatest for the upstream sample and rapidly reduced with the depth of invasion.

Various core-cleaning procedures suggested in the past were tested for their effectiveness in restoring the core to its original wettability condition. A three step solvent cleaning method showed the best results. A 50/50 mixture of toluene and methanol also proved reasonably successful. Toluene used alone was, however, not effective. In all cases the permeability could not be restored. The use of alcohols as solvents is also of some concern from the point of view of clay destabilization and mobilization.