WETTABILITY AND ITS EFFECT ON OIL-RECOVERY

Reservoir wettability is not a simply defined property. Classification of reservoirs as water-wet or oil-wet is a gross oversimplification. Various procedures for measuring wettability have been proposed. In this paper, the adopted scales of reservoir wettability and their relationships to interface boundary conditions are considered together with the dramatic effects that wettability can have on oil recovery. Reservoir wettability can cover a wide spectrum of conditions. Systems of intermediate or mixed wettability are quite common, whereas VSWW systems may be a rarity. Contact-angle behavior of crude-oil/brine systems falls into two main classes distinguished by either small or large hysteresis. Adhesion behavior of crude oils is strongly dependent on pH. Methods in most general use for measuring the wettability of COBR systems (the Amott test, the USBM test, and imbibition rate measurements) are strongly dependent on capillary pressure. In all three tests, decrease in interface curvature to zero is a key condition. Laboratory waterfloods on COBR systems show that oil recovery is optimum at neutral wettability. Agreement in results for fresh core with those for restored core provides the best indication that reservoir wettability has been duplicated in the laboratory. ROS values obtained in the laboratory for mixed-wettability systems and those determined by in-situ measurements show that it is possible for waterfloods to achieve very high displacement efficiencies.