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

Wettability tests have been performed using two different North Sea sandsto nes and three different fluid systems composed of a NaCl-brine and pure n-d ecane, or n-decane with additives. Oil displacement experiments by water in jection with composite cores of the same types of sandstone, and the same f luid systems, have also been done. It has been shown that it is possible to modify the wettability characteristics of the two sandstones from water-we t to neutral-wet, and further to oil-wet, by addition of small amounts of o rganic acid or organic base to the oil. Wettability indices obtained by the Amott and USBM tests are consistent. Water injection into two composite sa ndstone cores with fluid systems giving the three different states of wetta bility mentioned above, responded as expected. The water-wet cores had the highest oil recoveries at water break through. The non-water-wet systems al l exhibited a significant tail production of oil. The highest ultimate oil recoveries were obtained for the neutral-wet systems, and the lowest recove ries were given by the oil-wet systems. Change in wettability by addition o f organic acids or bases to the oil is likely to be due to adsorption of th e additive on the surface of the rock. The mechanism of wettability alterat ion is thus similar to what can be obtained by the addition of a water-solu ble surfactant to the brine. The only difference is that an additional inte rphase mass transfer step is involved for the oil-soluble, practically wate r-insoluble, additive to reach the rock surface by diffusion. (C) 1999 Else vier Science B.V. All rights reserved.