Changes

Potentiometric elevations are mapped and contoured to determine the change in Potentiometric elevation per unit distance, called the ''Potentiometric gradient''. The hydrodynamic tilt of a fluid contact can be estimated from the Potentiometric gradient and fluid densities by the following relationship<ref name=pt06r56>Hubbert, M. K., 1953, Entrapment of petroleum under hydrodynamic conditions: AAPG Bulletin, v. 37, p. 1954–2026.</ref><ref name=pt06r21>Dahlberg, E. C., 1982, Applied Hydrodynamics in Petroleum Exploration: New York, Springer Verlag, 161 p.</ref>:

Potentiometric elevations are mapped and contoured to determine the change in Potentiometric elevation per unit distance, called the ''Potentiometric gradient''. The hydrodynamic tilt of a fluid contact can be estimated from the Potentiometric gradient and fluid densities by the following relationship<ref name=pt06r56>Hubbert, M. K., 1953, Entrapment of petroleum under hydrodynamic conditions: AAPG Bulletin, v. 37, p. 1954–2026.</ref><ref name=pt06r21>Dahlberg, E. C., 1982, Applied Hydrodynamics in Petroleum Exploration: New York, Springer Verlag, 161 p.</ref>:

:<math>h_{\rm A}  = 2369/0.433 - 5160 = 311 \mbox{ ft}</math>

:<math>h_{\rm A}  = 2369/0.433 - 5160 = 311 \mbox{ ft}</math>

===Reservoir heterogeneity===

===Reservoir heterogeneity===

[[file:fluid-contacts_fig4.png|thumb|{{figure number|4}}Effect of reservoir heterogeneity on fluid contacts. (a) [[Capillary pressure]] curves for facies A and B within the reservoir. The dashed line corresponds to the saturation trend of the well In part (b). Sharp changes in saturation correspond to elevations of facies changes. (b) Oil-water contact corresponding to capillary pressure curves. The free water surface (''f''_w) is the same for all facies, but the different displacement pressure results in different oil-water contact elevations (thick arrows). The transition zones will also have different thicknesses due to different [[relative permeability]] characteristics not shown here. The vertical line is the well position corresponding to the saturation profile shown in part (a).]]

[[file:fluid-contacts_fig5.png|left|thumb|{{figure number|5}}Irregular contact caused by semipermeable barriers in a reservoir. (a) Capillary behavior of the reservoir and barriers A, B, and C. (b) Fluid contact elevations result from charging of the reservoir from the left. Each compartment of the reservoir has a different free water surface and oil-water contact. The displacement pressure of bed A causes the contact elevation difference between contacts 1 and 2. The displacement pressure of fault B results in the elevation difference between contacts 1 and 3. The displacement pressure of the mineralized fracture C results in the difference in elevation between contacts 3 and 4. The gas column is not thick enough to invade across the fault.]]

Reservoir rocks may have substantially different pore structures in different parts of a field. These heterogeneities may result in significant variations in hydrocarbon-water contacts, especially in low permeability reservoirs ([[:file:fluid-contacts_fig4.png|Figure 4]]). Where all reservoir facies are very porous, heterogeneity of depositional environments does not significantly affect fluid contact elevation.

Reservoir rocks may have substantially different pore structures in different parts of a field. These heterogeneities may result in significant variations in hydrocarbon-water contacts, especially in low permeability reservoirs ([[:file:fluid-contacts_fig4.png|Figure 4]]). Where all reservoir facies are very porous, heterogeneity of depositional environments does not significantly affect fluid contact elevation.

===Semipermeable fluid barriers===

===Semipermeable fluid barriers===

Semipermeable barriers can divide a reservoir into compartments with different fluid contacts even if the capillary properties of the reservoir rock are the same on both sides of the barrier. Semipermeable barriers can include faults, mineralized fractures, or semipermeable beds. The resulting pool has horizontal fluid contacts, but the contacts occur at different elevations on different sides of the barrier (Figure 5). The elevation difference between fluid contacts is related to the displacement pressure of the semipermeable barrier.<ref name=pt06r151>Watts, N. L., 1987, Theoretical aspects of cap-rock and fault seals for single- and two-phase hydrocarbon columns: Marine and Petroleum Geology, v. 4, p. 274–307., 10., 1016/0264-8172(87)90008-0</ref> Whereas fault or mineralized fracture compartmentalization is not readily recognized without detailed mapping, semipermeable beds are commonly recognized and the reservoir is separated into different pools corresponding to the different fluid contacts. Once the position of the barrier is mapped and the elevations of the contact on either side are determined from control wells, the fluid contacts can be mapped as horizontal surfaces within each compartment of the pool. Limited communication across semipermeable barriers is possible during production from the pool.

Semipermeable barriers can divide a reservoir into compartments with different fluid contacts even if the capillary properties of the reservoir rock are the same on both sides of the barrier. Semipermeable barriers can include faults, mineralized fractures, or semipermeable beds. The resulting pool has horizontal fluid contacts, but the contacts occur at different elevations on different sides of the barrier ([[:file:fluid-contacts_fig5.png|Figure 5]]). The elevation difference between fluid contacts is related to the displacement pressure of the semipermeable barrier.<ref name=pt06r151>Watts, N. L., 1987, Theoretical aspects of cap-rock and fault seals for single- and two-phase hydrocarbon columns: Marine and Petroleum Geology, v. 4, p. 274–307., 10., 1016/0264-8172(87)90008-0</ref> Whereas fault or mineralized fracture compartmentalization is not readily recognized without detailed mapping, semipermeable beds are commonly recognized and the reservoir is separated into different pools corresponding to the different fluid contacts. Once the position of the barrier is mapped and the elevations of the contact on either side are determined from control wells, the fluid contacts can be mapped as horizontal surfaces within each compartment of the pool. Limited communication across semipermeable barriers is possible during production from the pool.

 

==Anomalously thick transition zones==

==Anomalously thick transition zones==