Spontaneous imbibition of water into oil-wet carbonates

Contrary to sandstone reservoirs, the oil-wetting nature of carbonate reservoirs appears to increase as the reservoir temperature decreases. Thus, a carbonate reservoir at Tres<50 °C containing an oil with an acid number (AN) of about 1 mg KOH/g oil is very likely to behave as oil-wet. If, in addition, the reservoir is fractured with low permeability matrix blocks, water flooding of the reservoir is not recommended unless a wettability alteration process is possible. Normally, the oil recovery from this type of reservoirs by natural pressure depletion is low, and therefore the potential for improved oil recovery (IOR) is very high. The present paper reviews some recent work focused on improved oil recovery from oil-wet carbonates using surface-active chemicals to promote wettability alteration. Water will then spontaneously imbibe into the matrix blocks, and the oil recovery can be markedly increased. llowing topics are discussed:• ve affinities of crude oil components towards carbonates, and how to prepare homogeneous oil-wet cores from water-wet outcrop material; ttability alteration mechanism using CTAB, emphasizing ion-pair formation as a key factor; lative influence of capillary and gravity forces on the fluid flow during the imbibition process; ficiency of commercially available technical products at low price suitable for field applications.