Leakage mechanisms
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Five major seal failure mechanisms form the basis for this section's discussion on seal leakage.
Seal failure mechanisms
A given seal can leak by one of several mechanisms, as defined by Watts.[1]
| Seal Type | Seal Failure Mechanism |
|---|---|
| Intact membrane | Capillary pressure (created by the height of the underlying petroleum column) exceeds seal capillary displacement pressure. This seal type does not fracture during deformation. |
| Fractured membrane | Capillary pressure (created by the height of the underlying petroleum column) exceeds displacement pressure of fracture porosity in the seal. |
| Hydrofractured | Total fluid pressure (capillary pressure plus water pressure) exceeds minimum compressive stress of the seal; seal fails by natural hydraulic fracturing. |
| Micropermeability | Leakage is caused by low displacement pressures of oil-wet seals or by capillary pressure of the reservoir exceeding displacement pressure of the seal in a water-wet seal. Accumulations are preserved for geologically significant time if leakage rate is low. Rate of leakage is controlled by seal effective permeability. |
| Diffusive | Gas is lost by dissolving in water and diffusing through interstitial water of the seal. |
Comparison of mechanisms
The first three mechanisms listed in the table above have minimum pressure criteria to be exceeded before leakage occurs. Even after leakage, an economic column of petroleum may remain. The last two mechanisms can destroy an accumulation, given enough time.
See also
References
- ↑ Watts, N., L., 1987, Theoretical aspects of cap-rock and fault seals for single- and two-phase hydrocarbon columns: Marine and Petroleum Geology, vol. 4, p. 274–307., 10., 1016/0264-8172(87)90008-0
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