An experimental study of the relationship between wettability and oil production characteristics

Wettability tests have been performed using two different North Sea sandstones and three different fluid systems composed of a NaCl-brine and pure n-decane, or n-decane with additives. Oil displacement experiments by water injection with composite cores of the same types of sandstone, and the same fluid systems, have also been done. It has been shown that it is possible to modify the wettability characteristics of the two sandstones from water-wet to neutral-wet, and further to oil-wet, by addition of small amounts of organic acid or organic base to the oil. Wettability indices obtained by the Amott and USBM tests are consistent. Water injection into two composite sandstone cores with fluid systems giving the three different states of wettability mentioned above, responded as expected. The water-wet cores had the highest oil recoveries at water break through. The non-water-wet systems all exhibited a significant tail production of oil. The highest ultimate oil recoveries were obtained for the neutral-wet systems, and the lowest recoveries were given by the oil-wet systems. Change in wettability by addition of organic acids or bases to the oil is likely to be due to adsorption of the additive on the surface of the rock. The mechanism of wettability alteration is thus similar to what can be obtained by the addition of a water-soluble surfactant to the brine. The only difference is that an additional interphase mass transfer step is involved for the oil-soluble, practically water-insoluble, additive to reach the rock surface by diffusion.