Beside their worldwide abundance, oil recovery from fractured carbonate reservoirs is
commonly low. Such reservoirs are usually oil-wet, thus, water
ooding leads into early breakthrough
and low recovery due to the high conductivity of the fracture network, negative capillary pressure of
the matrix and, consequently, the poor spontaneous imbibitions of water from fractures into the matrix
during the course of water
ooding. In such problematic reservoirs, changing the wettability of the matrix
toward water-wetness can improve spontaneous imbibition by changing the sign and, thus, the direction of
capillary forces, resulting in an improvement of water
ood eciency and, consequently, oil recovery. A
study of this technique on the most signicant Iranian oil-producing reservoir, Asmari, seems necessary.
Some surfactants of dierent ionic charges have been examined in this study. Asmari reservoir rock
samples were used and the petrophysical and mineralogical properties of the rock samples were determined
by both thin section analysis and core
ooding techniques. Interfacial tension measurements have been
done to decide surfactant solution concentrations. Capillary pressure measurements were conducted both
before and after wettability alteration. Amott-Harvey and USBM wettability indices were determined.
Among the surfactants, a cationic one could best raise the level of spontaneous imbibition. Favourable
changes in the wettability indices were observed.