Seal ductility

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Predicting the occurrence of oil and gas traps
Chapter Evaluating top and fault seal
Author Grant M. Skerlec
Link Web page
Store AAPG Store

The most important mechanical property for evaluating seal integrity is ductility. Ductile rocks make good top seals; brittle rocks make poor top seals. Shales and salt are two of the most ductile rock types and, not surprisingly, two of the most common top seals.[1]

What is ductility?

Ductility is the amount of strain a seal can withstand before brittle failure and the loss of top seal integrity. Rocks with an extremely high ductility can deform without brittle failure. On the other hand, rocks with low ductility can accommodate only a small amount of strain before fracturing. A seal can be brittle but unfractured; a seal can be ductile but fractured. Fracture depends upon whether the strain exceeds the seal ductility.

Variables that control ductility

Seal ductility is controlled by at least nine different variables. The table below lists these variables and briefly notes how they control ductility.

Variable Control on ductility
Lithology Grain mineralogy and cement type control ductility. Brittle seals include dolostone, quartzite, anhydrite, and some shales. Ductile seals include halite, some shales, and some limestones.
Composition Not all limestones or shales have the same ductility. Compositional variations such as total organic carbon[2] and clay mineralogy[3] change ductility.
Confining pressure Increasing confining pressure increases ductility.
Pore pressure Increasing pore pressure decreases ductility.
Fluid composition The presence or absence of fluids and their composition affects ductility.
Temperature Increasing temperature increases ductility.
Strain rate High strain rates decrease ductility.
Time Ductility changes with time as seals undergo burial and diagenesis.
Compaction state Ductility decreases with progressive compaction and diagenesis.

See also

References

  1. Grunau, H., R., 1987, A world-wide look at the cap-rock problem: Journal of Petroleum Geology, vol. 10, no. 3, p. 245–266., 10., 1111/jpg., 1987., 10., issue-3
  2. Chong, K., P., Hoyt, P., M., Smith, J., W., Paulsen, B., Y., 1980, Effects of strain rate on oil shale fracturing: International Journal of Rock Mechanics, vol. 17, no. 1, p. 35–43., 10., 1016/0148-9062(80)90004-2
  3. Corbett, K., Friedman, M., Spang, J., 1987, Fracture development and mechanical stratigraphy of Austin Chalk, Texas: AAPG Bulletin, vol. 71, no. 1, p. 17–28.

External links

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Seal ductility
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