Changes

Therefore, at a solid-liquid boundary interface, the molecules of the liquid are subjected to opposing forces of attraction; in the first instance, the liquid is attracted by its own molecules and secondly by the molecules of the solid across the boundary. The degree to which force is dominant controls what is termed the wettability.<ref name=Vavra>Vavra, C. L., J. G. Kaldi, and R. M. Sneider, 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0006/0000/0840.htm Geological applications of capillary pressure: A review]: AAPG Bulletin, v. 76, no. 6, p. 840–850.</ref> For instance, glass is water wet, in that water will spread across the surface of a glass plate as a thin sheet. The adhesive attraction of the water for the glass is greater than the cohesive attraction of the water molecules for each other. A liquid such as mercury will form globules on a glass surface and is nonwetting. The cohesive attraction of the mercury molecules for each other is greater than the adhesive attraction of glass and mercury ([[:File:M91FG24.JPG|Figure 1]]).

Therefore, at a solid-liquid boundary interface, the molecules of the liquid are subjected to opposing forces of attraction; in the first instance, the liquid is attracted by its own molecules and secondly by the molecules of the solid across the boundary. The degree to which force is dominant controls what is termed the wettability.<ref name=Vavra>Vavra, C. L., J. G. Kaldi, and R. M. Sneider, 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0006/0000/0840.htm Geological applications of capillary pressure: A review]: AAPG Bulletin, v. 76, no. 6, p. 840–850.</ref> For instance, glass is water wet, in that water will spread across the surface of a glass plate as a thin sheet. The adhesive attraction of the water for the glass is greater than the cohesive attraction of the water molecules for each other. A liquid such as mercury will form globules on a glass surface and is nonwetting. The cohesive attraction of the mercury molecules for each other is greater than the adhesive attraction of glass and mercury ([[:File:M91FG24.JPG|Figure 1]]).

Where a reservoir rock is water wet, the water forms a thin film over most of the grain surfaces and will also fill the smaller pores. The oil or gas will occupy the remaining, more central volume of the pore system. Conversely, in a reservoir that is oil wet, it is the oil that covers the grain surface and occupies the smaller pores; the water is located centrally within the pore structure (Anderson, 1986).

Where a reservoir rock is water wet, the water forms a thin film over most of the grain surfaces and will also fill the smaller pores. The oil or gas will occupy the remaining, more central volume of the pore system. Conversely, in a reservoir that is oil wet, it is the oil that covers the grain surface and occupies the smaller pores; the water is located centrally within the pore structure.<ref name=Anderson86>Anderson, W. G., 1986, Wettability literature survey: 1. Rock/oil/brine interactions and the effects of core handling on wettability: Journal of Petroleum Technology, SPE 13932, v. 38, no. 10, p. 1125–1144.</ref>

Most reservoirs were water wet before oil migration started; the major mineral phases in reservoirs such as quartz, carbonate and dolomite are all water wetting prior to coming in contact with oil (Abdallah et al., 2007). Following oil migration, sandstone reservoirs can end up as predominantly water wet, predominantly oil wet, or more frequently in a mixed-wettability state, that is, somewhere in between oil wet and water wet. Carbonate reservoirs are commonly described as showing mixed wettability tending to oil wet (Treiber et al., 1972; Chilingar and Yen, 1983). The degree of wettability can vary even within a single reservoir. The rocks in the reservoir will show a variety of mineral types, each mineral with its own wetting characteristics. Other variables affecting wettability include the wetting nature of the numerous compounds comprising crude oil and the degree to which polar compounds from the oil are absorbed onto the rock surface (Anderson, 1986).

Most reservoirs were water wet before oil migration started; the major mineral phases in reservoirs such as quartz, carbonate and dolomite are all water wetting prior to coming in contact with oil (Abdallah et al., 2007). Following oil migration, sandstone reservoirs can end up as predominantly water wet, predominantly oil wet, or more frequently in a mixed-wettability state, that is, somewhere in between oil wet and water wet. Carbonate reservoirs are commonly described as showing mixed wettability tending to oil wet (Treiber et al., 1972; Chilingar and Yen, 1983). The degree of wettability can vary even within a single reservoir. The rocks in the reservoir will show a variety of mineral types, each mineral with its own wetting characteristics. Other variables affecting wettability include the wetting nature of the numerous compounds comprising crude oil and the degree to which polar compounds from the oil are absorbed onto the rock surface.<ref name=Anderson86 />

Waterfloods produce more efficient sweeps in water-wet reservoirs than in oil-wet systems. Water forced to move through a water-wet pore system will displace the oil from the center of the pores relatively efficiently ([[:File:M91FG25.JPG|Figure 2]]). Water will also be drawn into the smaller pores, displacing oil into the main flow pathways. In an oil-wet sandstone, the oil forms a film around the sand grains and water will move through the center of the pores, particularly the larger connected pores. The pathway for the water here is less tortuous than in water-wet sandstones, and the water will move through the rock more quickly, bypassing a large volume of oil. Rapid water breakthrough to the production wells typically occurs, and oil rates will drop significantly once this happens. Nevertheless, the film of oil around the grains can survive as a continuous path to a production well after water has broken through. Because of this, a continuous flow of oil can still be maintained in oil-wet reservoirs by injecting large volumes of water (Anderson, 1987).

Waterfloods produce more efficient sweeps in water-wet reservoirs than in oil-wet systems. Water forced to move through a water-wet pore system will displace the oil from the center of the pores relatively efficiently ([[:File:M91FG25.JPG|Figure 2]]). Water will also be drawn into the smaller pores, displacing oil into the main flow pathways. In an oil-wet sandstone, the oil forms a film around the sand grains and water will move through the center of the pores, particularly the larger connected pores. The pathway for the water here is less tortuous than in water-wet sandstones, and the water will move through the rock more quickly, bypassing a large volume of oil. Rapid water breakthrough to the production wells typically occurs, and oil rates will drop significantly once this happens. Nevertheless, the film of oil around the grains can survive as a continuous path to a production well after water has broken through. Because of this, a continuous flow of oil can still be maintained in oil-wet reservoirs by injecting large volumes of water.<ref>Anderson, W. G., 1987, Wettability literature survey: 6. The effects of wettability on waterflooding: Journal of Petroleum Technology, SPE 16471, v. 39, no. 12, p. 1605–1622.</ref>

==Buoyancy forces in reservoir fluids==

==Buoyancy forces in reservoir fluids==