Changes

For an oil field, the capillary-bound water comprises a continuous column of water within the oil leg, which will have a hydrostatic pressure gradient controlled by the water density. The oil is located in the remaining pore space as a continuous phase and will have a pressure gradient controlled by the (lower) oil density (Figure 26). Although oil and water can coexist in the same localized volume of rock, the pressures acting on the two fluids are different. The difference in pressure between the oil and water phases increases with height above the free-water level. The free-water level is the level at which the water-hydrocarbon interface would theoretically stand in a large open hole drilled through the oil column (Schowalter, 1979). In this situation, only gravity and buoyancy forces control the fluid distribution in the borehole.

For an oil field, the capillary-bound water comprises a continuous column of water within the oil leg, which will have a hydrostatic pressure gradient controlled by the water density. The oil is located in the remaining pore space as a continuous phase and will have a pressure gradient controlled by the (lower) oil density (Figure 26). Although oil and water can coexist in the same localized volume of rock, the pressures acting on the two fluids are different. The difference in pressure between the oil and water phases increases with height above the free-water level. The free-water level is the level at which the water-hydrocarbon interface would theoretically stand in a large open hole drilled through the oil column (Schowalter, 1979). In this situation, only gravity and buoyancy forces control the fluid distribution in the borehole.

[[FIGURE 26.]] Water saturation decreases with height in an oil column. The volume of water is a function of the balance of capillary forces pulling the water up from the oil-water interface and the force of gravity acting together with the density contrast between the reservoir fluids, tending to pull the water down.

[[File:M91FG26.JPG|thumb|300px|{{figure number|26}}Water saturation decreases with height in an oil column. The volume of water is a function of the balance of capillary forces pulling the water up from the oil-water interface and the force of gravity acting together with the density contrast between the reservoir fluids, tending to pull the water down.]]

As the buoyancy pressure increases with height above the free-water level, the oil phase will displace more water from increasingly smaller pore volumes. The effect of this is that hydrocarbon saturations increase with height above the hydrocarbon-water contact. The relationship between capillary and buoyancy forces thus controls the static distribution of fluids in oil and gas pools. Knowledge of these relationships is fundamental to the accurate calculation of hydrocarbon volumes within a reservoir.

As the buoyancy pressure increases with height above the free-water level, the oil phase will displace more water from increasingly smaller pore volumes. The effect of this is that hydrocarbon saturations increase with height above the hydrocarbon-water contact. The relationship between capillary and buoyancy forces thus controls the static distribution of fluids in oil and gas pools. Knowledge of these relationships is fundamental to the accurate calculation of hydrocarbon volumes within a reservoir.

With increasing injection pressure, more and more mercury is forced into the rock. The shape of the curves on a capillary pressure plot reflects the grain sorting and the connection of pores and pore throats. The longer the plateau shown by the capillary curve, the better the reservoir quality. Poorly sorted, fine-grained sediment with narrow pore throats will retain water to higher pressures than coarser grained, better sorted sediments. A homogenous reservoir rock can be represented by a single capillary pressure curve. By contrast, a heterogenous reservoir will have a family of rock types, each with its own capillary pressure curve (Figure 27).

With increasing injection pressure, more and more mercury is forced into the rock. The shape of the curves on a capillary pressure plot reflects the grain sorting and the connection of pores and pore throats. The longer the plateau shown by the capillary curve, the better the reservoir quality. Poorly sorted, fine-grained sediment with narrow pore throats will retain water to higher pressures than coarser grained, better sorted sediments. A homogenous reservoir rock can be represented by a single capillary pressure curve. By contrast, a heterogenous reservoir will have a family of rock types, each with its own capillary pressure curve (Figure 27).

[[FIGURE 27.]] The shape of the curves on a capillary pressure plot reflects the grain sorting and the connection of pores and pore throats within the various rock types. The longer the plateau shown by the capillary curve, the better is the reservoir quality of the rock (from Sneider et al., 1977). Reprinted with permission from the Society of Petroleum Engineers.

[[File:M91FG27.JPG|thumb|300px|{{figure number|27}}The shape of the curves on a capillary pressure plot reflects the grain sorting and the connection of pores and pore throats within the various rock types. The longer the plateau shown by the capillary curve, the better is the reservoir quality of the rock (from Sneider et al., 1977). Reprinted with permission from the Society of Petroleum Engineers.]]

Petrophysicists will use capillary pressure curves as the basis for deriving a water saturation versus height relationship for a reservoir (Vavra et al., 1992).

Petrophysicists will use capillary pressure curves as the basis for deriving a water saturation versus height relationship for a reservoir (Vavra et al., 1992).