Laboratory water floods of fractional oil-wet sands have been conducted to determine the effect of wettability, viscosity and initial fluid saturation on oil recovery. These floods were made using a sand-packed lucite model which represented a quarter five-spot pattern. The fractional oil-wet sands used in this study were mixtures of sand grains which were 100 per cent oil-wet and 100 per cent water-wet. A concentrated silicone solution was used to obtain the oil-wet sand.
In most instances it was found that increasing the fractional oil wetness of a sand increased the residual oil. The total oil produced increased with decreases in the oil-water viscosity ratio and increases in the initial oil saturation. The greatest residual oil saturations occurred for zero per cent connate water, high oil-water viscosity ratios and large fractional oil-wet sands.
Most investigators agree that rock wettability has a definite effect on the recovery of oil by water flooding petroleum reservoirs. As early as 1925, Nutting concluded from his study that clean silica grains would be preferentially wet by fresh water in the presence of oil. However, he further indicated that this did not hold true if the silica-water-oil mixture was given time to age. After aging he found that part of the silica remained wet with water and the rest became oil-wet.
Kennedy, Burja and Boykin studied the effect of wettability on oil recovery measured by the sessile drop method. Surface-active chemicals were used to control the wetting properties of sandstone cores. Their results indicated that for constant interfacial tension between oil and brine, higher recoveries or lower residual oil References and illustrations at end of-paper. saturations were obtained in systems of neutral wettability. Moore and Slobod also showed that a maximum oil recovery should be obtained at intermediate wettability. These authors proposed that in an oil-wet medium the viscous forces are opposed by the capillary forces, and the resulting displacement will leave oil in the smaller pores and as a film on the sand grains. In a system of intermediate wettability, the capillary and viscous forces are more nearly equal and a minimum oil saturation should result.
Newcombe, McGhee and Rzasa have shown from their study of unconsolidated sands treated with a silicone that lower residual oil saturations are obtained in water-wet systems if all other factors remain constant.
Warren and Calhoun made a rather extensive study using glass spheres coated with silicones. They found that certain dimensionless factors may - be used to estimate the oil recovered by water flooding partially oil-wet systems. For their system the oil recovery at breakthrough decreased as the porous media became more water-wet.
Bobek, Mattax and Denekas made laboratory and field studies to develop methods which could be used to measure rock wettability and to investigate factors which may alter the wettability of reservoir rock. They concluded from their investigations that wettability can have an important influence on oil recovery by water flooding and that oil recoveries from water-wet rock were about 15 per cent of the original oil-in-place higher than those from oil-wet rock. It was also concluded from their study that the preferential wettability of a reservoir rock is largely controlled by compounds absorbed at the rock surface.
Wagner and Leach conducted water flood tests using synthetic cores and oil and water having wetting characteristics similar to those of reservoir fluids.
