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 T-res < 50 degreesC 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. The following topics are discussed: Relative affinities of crude oil components towards carbonates, and how to prepare homogeneous oil-wet cores from water-wet outcrop material; The wettability alteration mechanism using CTAB, emphasizing ion-pair formation as a key factor; The relative influence of capillary and gravity forces on the fluid flow during the imbibition process; The efficiency of commercially available technical products at low price suitable for field applications. (C) 2003 Elsevier Science B.V. All rights reserved.