Natural hydraulic fracturing of top seals

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Critical elements of the trap
Chapter Evaluating top and fault seal
Author Grant M. Skerlec
Link Web page
Store AAPG Store

Fracturing and consequent loss of top seal integrity can occur by increasing pore pressure. High pore pressure can overcome the normal stresses that keep fractures closed. Similar fracturing is artificially induced during leak-off tests, well stimulations, and subsurface waste disposal.[1]

Importance of hydraulic fracturing

High pore pressure has fractured the top seal and lost hydrocarbons in several basins, including the North Sea[2][3][4][5] the Norwegian Sea[6] and the Malay basin.[7] The process is undoubtedly more widespread. Loss of top seal integrity due to natural hydraulic fracturing also appears to control the risk economics and vertical distribution of hydrocarbons in the Gulf Coast.[8][9][10]

Theoretical fracture pressure, pf

The overpressure required to cause fracturing is traditionally calculated by determining the theoretical fracture pressure, Pf:[11]

where:

  • Pf = theoretical fracture pressure
  • σ3 = effective least principal stress or confining pressure
  • p = pore pressure
  • α = poroelastic constant, assumed to be 1 in most analyses[12]

Fracture pressure is the fluid pressure necessary to overcome the normal stress that keeps the fractures closed.

Calculating pf

Use the steps outlined in the following table to calculate Pf.

Step Action Method
1 Calculate σ1, overburden stress. Use density logs to calculate overburden stress. For example, a 1-cm cube with a density of 2.4 g/cm3 exerts an overburden stress of 2.4 g/cm2 at the base of the cube.
2 Determine ν, Poisson's ratio. Calculate the ratio from leak-off tests. Take care since leak-off tests may report the pressure value either prior to or after the fracture pressure point.[13] Leak-off tests are also commonly taken where casing has been set and may reflect the mechanical properties of the cement casing rather than the wall rock. Alternatively, Poisson's ratio can be estimated from available laboratory data.[14] Poisson's ratio increases with depth to approach a maximum of 0.5.
3 Determine p, pore pressure. Pore pressure can be determined from measurements or regional pressure maps or estimated from burial history.[15] It may be necessary to predict paleopore pressure.
4 Calculate σ3 , effective confining pressure. Solve the equation
5 Calculate Pf , theoretical fracture pressure. Solve the equation . The fracture pressure is commonly expressed as a gradient, and the equation becomes , where Z is depth.

Other ways to calculate pf

Variations on this equation as well as empirical relationships are common.[11][16][13][17][18] An alternative method of determining the principal stresses and fracture gradient is through the use of borehole deformation.[19][20]

See also

References

  1. ↑ Evans, D. M., 1966, The Denver area earthquakes and the Rocky Mountain Arsenal disposal well: The Mountain Geologist, vol. 3, no. 1, p. 23–36.
  2. ↑ Skerlec, G. M., 1982, Risking top seals in the Central Graben: Exxon Production Research Company internal report.
  3. ↑ Skerlec, G. M., 1990, SEALS: A short course for risking top seal and fault seal: Franklin, Pennsylvania, SEALS International, 600 p.
  4. ↑ Caillet, G., 1993, The caprock of the Snorre field (Norway): a possible leakage by hydraulic fracturing: Marine and Petroleum Geology, vol. 10, no. 1, p. 42–50, DOI: 10.1016/0264-8172(93)90098-D.
  5. ↑ Leith, T. L., I. Kaarshad, J. Connan, J. Pierron, and G. Caillet, 1993, Recognition of caprock leakage in the Snorre field, Norwegian North Sea: Marine and Petroleum Geology, vol. 10, no. 1, p. 29–41, DOI: 10.1016/0264-8172(93)90097-C.
  6. ↑ Ungerer, P., J. Burrus, B. Doligez, P. Y. Chenet, and F. Bessis, 1990, Basin evaluation by integrated two-dimensional modeling of heat transfer, fluid flow, hydrocarbon generation, and migration: AAPG Bulletin, vol. 74, no. 3, p. 309–335.
  7. ↑ Scharr, G., 1976, The occurrence of hydrocarbons in overpressured reservoirs of the Baram delta, offshore Sarawak, Malaysia: Fifth Annual Convention, Indonesian Petroleum Association, Proceedings, p. 163–169.
  8. ↑ Fertl, W. H., and W. G. Leach, 1988, Economics of hydrocarbon reserves in overpressured reservoirs below 18,000 feet in south Louisiana: SPE paper 18146, 16 p.
  9. ↑ Leach, W. G., 1993a, Fluid migration, HC concentration in south Louisiana Tertiary sands: Oil & Gas Journal, vol. 91, no. 11, p. 71–74.
  10. ↑ Leach, W. G., 1993b, Maximum hydrocarbon window determination in south Louisiana: Oil & Gas Journal, vol. 91, no. 13, p. 81–84.
  11. ↑ 11.0 11.1 Hubbert, M. K., and D. G. Willis, 1957, Mechanics of hydraulic fracturing: JPT, vol. 9, no. 6, p. 153–168.
  12. ↑ Engelder, T., and A. Lacazette, 1990, Natural hydraulic fracturing, in N. Barton and O. Stephansson, eds., Rock Joints: Rotterdam, A. A. Balkema, p. 35–43.
  13. ↑ 13.0 13.1 Eaton, B. A., 1969, Fracture gradient prediction and its application in oilfield operations: Trans. AIME, October, p. 1353–1360.
  14. ↑ Lama, R. D., and V. S. Vutukuri, 1978, Handbook of Mechanical Properties of Rocks: Rockport, MA, Trans. Technical Publications.
  15. ↑ Mann, D. M., and A. S. Mackenzie, 1990, Prediction of pore fluid pressures in sedimentary basins: Marine and Petroleum Geology, v. 7, no. 1, p. 55-65.
  16. ↑ Matthews, W. R., and J. Kelly, 1967, How to predict formation pressure and fracture gradient from electric and sonic logs: Oil & Gas Journal, vol. 65, no. 8, p. 92–106.
  17. ↑ Breckles, I. M., and H. A. M. Van Eekelen, 1982, Relationship between horizontal stress and depth in sedimentary basins: Journal of Petroleum Technology, vol. 34, no. 9, p. 2191–2199, DOI: 10.2118/10336-PA.
  18. ↑ Brennan, R. M., and M. R. Annis, 1984, A new fracture gradient prediction technique that shows good results in Gulf of Mexico abnormal pressure: SPE paper 13210, 6 p.
  19. ↑ Bell, J. S., 1990, Investigating stress regimes in sedimentary basins using information from oil industry wireline logs and drilling records, in A. Hurst, M. A. Lovell, and A. C. Morton, eds., Geological Applications of Wireline Logs: Geological Society London Special Publication 48, p. 305–325.
  20. ↑ Evans, C. J., and N. R. Brereton, 1990, In situ crustal stress in the United Kingdom from borehole breakouts, in A. Hurst, ed., Geological Applications of Wireline Logs: Geological Society of London Special Publication 48, p. 327–338.

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