Water-wet rocks

Wettability. The wettability of the porous medium refers to its preference for one or the other fluid in becoming wet. It is defined as the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids (7). In a rock-oil-brine system, it is a measure of the preference that the rock has for either the oil or the water. A water-wet rock is preferentially wetted by the water phase, and similarly for an oil-wet system, the rock primarily makes contact with the oil phase. 

Berg (1975) amongst others, has described separate phase hydrocarbon migration from a rock pore through an adjacent pore throat in a water-wet rock under hydrostatic conditions. The following outline of the migration process is largely based on his work. 

Figure 4.3 Migration of an oil globule through pore throats in a water-wet rock (after Berg, 1975. Reprinted by permission of the American Association of Petroleum Geologists).

In 1959, Wagner and Leach (11) suggested that increased oil recovery could be obtained by changing wettability of rock material from oil-wet to water-wet. Melrose and Bradner (7) and Morrow (12) also suggested that for optimal recovery of residual oil by a low interfacial tension flood, the rock structure should be water-wet. Previous investigators (13,14) have used sodium hydroxide to make the reservoir rock water-wet. Slattery and Oh (15) have shown that intermediate wettability may be less desirable than either oil-wet or water-wet rocks. Since, chemical floods satisfy many of these conditions, they have been considered promising for enhanced recovery of oil. The mechanism of oil displacement in porous media has been reviewed by Bansal and Shah (16) and more recently by Taber (17). 

Gas injection can also recover oil by reducing oil viscosity and residual oil saturation, even when miscibility is not achieved. Reduction in viscosity is more significant if the oil viscosity is large, and this process is attractive in viscous or semiviscous reservoirs, especially when accompanied by some other improved recovery mechanism. Residual oil saturation in three-phase flow in water-wet rock is very low (essentially zero), even at very low capillary numbers. Two main problems in such a process are the low relative permeabilities and sweep efficiencies. This process can be implemented in a highly dipping reservoir to take... 

Relative permeability curves were also determined for the displacement of oil by water following the polymer/oil tests. Figure 5.53 compares the relative permeability data for the oil and water phases before (with the subscript 1) and after (with subscript p) polymer contact. RRF in the figure denotes in the text. In the water-wet rocks, there was little difference between the residual oil saturation obtained before and after polymer contact, as would be expected. Oil... 

Oil droplets (oil globules) are trapped at pore throats by capillary forces, commonly observed in strongly water-wet rocks. 

Mixed-Wet Systems. At high IFT conditions, oil from oil-filled rock surrounded by water will be expelled from the vertical rock surface and from the top and bottom surfaces in line with a counter-current flow mechanism governed by capillary forces [97]. The imbibition rate and the oil production rate are much faster from a water-wet rock than from a mixed-wet rock. The size of the core is very important for the production profile. For bigger blocks, the oil production plateau will be higher for the mixed-wet case compared to the water-wet case [97], contrary for smaller blocks. The block size in the reservoir may therefore be important, and lab experiments on small sized rock may lead to a too pessimistic recovery estimate [98]. 

A problem in the WAG process is that injected water blocks contact between the injected gas phase and resident oil. This reduces displacement efficiency at the pore scale i.e., it results in a larger ROS. This effect has been found to be a strong function of rock wettability and more detrimental in water-wet rocks. 8... 

In summary, both physical modeling and computer simulation have shown that WAG performance is dependent on rock wettability. Oil trapping in water-wet rocks is a significant negative factor. Volumetric sweep can be improved with WAG injection, but a corresponding delay in production response can adversely s ect the economies of the process. 

FIGURE 2.35 Wettability (a) definition of the angle 0 and interfacial tension terms (b) water-wet rock (water-oil system) (c) oU-wet rock (water-oil system). 

Drainage is the displacement of a wetting phase by a non-wetting phase—the non-wetting saturation increases. For a water-wet rock, the water saturation decreases. 

For the water in a water-wet rock, the surface relaxation will usually dominate. A bulk relaxation correction must be made when there are iron, manganese, chromum, nickel, or other paramagnetic ions in the mud filtrate. 

Water in vugs will relax at its bulk rate, modified by diffusion effects. Similarly, oil in water-wet rock will relax at its bulk rate with a diffusion effect. 

In water-wet rocks, water adheres to grain surfaces and builds up a more or less continuous phase in the rock. In oU-wet rocks, the non-conducting oil becomes the continuous fluid phase, and the water occurs mostly as isolated droplets. In this case, the resistivity is much higher and the satmation exponent is much greater than 2 than in case of water wet. A discussion of wettability influences on electrical properties is given by Anderson (1986) and Sharma et al. (1991). 

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