Experimental artifacts caused by wettability variations in chalk

Alteration of the wettability is often a part of the restoration process for core plugs or used to obtain a desired wettability in outcrop plugs. If the procedure does not provide a uniform wettability state in the plug, experimental artifacts can result. For immersion under crude oil to alter wettability, it was observed that the wettability distribution in a chalk plug was nonuniform. Capillary pressure curves by the direct measurement of saturation method were used to determine the wettability variation within the plug. This plug did not imbibe water because its exterior was rendered almost oil-wet, while the interior remained strongly water-wet. Similarly, for preparation methods that flow crude oil into a plug, the wettability could vary from the end of the sample to the interior or to the other end. The distribution of water and oil in the plug from a subsequent waterflood would then vary with the pressure imposed by the flow direction. Consequently, a forced displacement waterflood could recover different amounts of oil depending upon the direction of the waterflood. A neural net was used to capture the behavior of forced displacement for various wettabilities from measured capillary pressure curves and then used to predict waterflood results.