Changes

  | part    = Critical elements of the trap

  | part    = Critical elements of the trap

  | chapter = Evaluating top and fault seal

  | chapter = Evaluating top and fault seal

  | frompg  = 10-1

  | frompg  = 10-57

  | topg    = 10-94

  | topg    = 10-58

  | author  = Grant M. Skerlec

  | author  = Grant M. Skerlec

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

[[Fracture|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 [[stimulation]]s, and subsurface waste disposal ([[Evans, 1996]]).

[[Fracture|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 [[stimulation]]s, and subsurface waste disposal.<ref>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.</ref>

==Importance of hydraulic fracturing==

==Importance of hydraulic fracturing==

High pore pressure has fractured the top seal and lost hydrocarbons in several basins, including the North Sea<ref name=ch10r70>Skerlec, G., M., 1982, Risking top seals in the Central Graben: Exxon Production Research Company internal report.</ref><ref name=ch10r71>Skerlec, G., M., 1990, SEALS: A short course for risking top seal and fault seal: Franklin, Pennsylvania, SEALS International, 600 p.</ref><ref name=ch10r11>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: [http://www.sciencedirect.com/science/article/pii/026481729390098D 10.1016/0264-8172(93)90098-D].</ref><ref name=ch10r48>Leith, T., L., Kaarshad, I., Connan, J., Pierron, J., Caillet, G., 1993, Recognition of caprock leakage in the Snorre field, Norwegian North Sea: Marine and Petroleum Geology, vol. 10, no. 1, p. 29–41, DOI: [http://www.sciencedirect.com/science/article/pii/026481729390097C 10.1016/0264-8172(93)90097-C].</ref> the Norwegian Sea<ref name=ch10r84>Ungerer, P., Burrus, J., Doligez, B., Chenet, P., Y., Bessis, F., 1990, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0074/0003/0000/0309.htm 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.</ref> and the Malay basin.<ref name=ch10r66>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.</ref> 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.<ref name=ch10r30>Fertl, W., H., Leach, W., G., 1988, Economics of hydrocarbon reserves in overpressured reservoirs below 18,000 feet in south Louisiana: [https://www.onepetro.org/conference-paper/SPE-18146-MS SPE paper 18146], 16 p.</ref><ref name=ch10r46>Leach, W., G., 1993a, Fluid migration, HC concentration in south Louisiana Tertiary sands: Oil & Gas Journal, vol. 91, no. 11, p. 71–74.</ref><ref name=ch10r47>Leach, W., G., 1993b, Maximum hydrocarbon window determination in south Louisiana: Oil & Gas Journal, vol. 91, no. 13, p. 81–84.</ref>

High pore pressure has fractured the top seal and lost hydrocarbons in several basins, including the North Sea<ref name=ch10r70>Skerlec, G. M., 1982, Risking top seals in the Central Graben: Exxon Production Research Company internal report.</ref><ref name=ch10r71>Skerlec, G. M., 1990, SEALS: A short course for risking top seal and fault seal: Franklin, Pennsylvania, SEALS International, 600 p.</ref><ref name=ch10r11>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: [http://www.sciencedirect.com/science/article/pii/026481729390098D 10.1016/0264-8172(93)90098-D].</ref><ref name=ch10r48>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: [http://www.sciencedirect.com/science/article/pii/026481729390097C 10.1016/0264-8172(93)90097-C].</ref> the Norwegian Sea<ref name=ch10r84>Ungerer, P., J. Burrus, B. Doligez, P. Y. Chenet, and F. Bessis, 1990, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0074/0003/0000/0309.htm 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.</ref> and the Malay basin.<ref name=ch10r66>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.</ref> 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.<ref name=ch10r30>Fertl, W. H., and W. G. Leach, 1988, Economics of hydrocarbon reserves in overpressured reservoirs below 18,000 feet in south Louisiana: [https://www.onepetro.org/conference-paper/SPE-18146-MS SPE paper 18146], 16 p.</ref><ref name=ch10r46>Leach, W. G., 1993a, Fluid migration, HC concentration in south Louisiana Tertiary sands: Oil & Gas Journal, vol. 91, no. 11, p. 71–74.</ref><ref name=ch10r47>Leach, W. G., 1993b, Maximum hydrocarbon window determination in south Louisiana: Oil & Gas Journal, vol. 91, no. 13, p. 81–84.</ref>

==Theoretical fracture pressure, p_f==

==Theoretical fracture pressure, p_f==

The overpressure required to cause [[Fracture|fracturing]] is traditionally calculated by determining the theoretical fracture pressure, P_f:<ref name=ch10r39>Hubbert, M., K., Willis, D., G., 1957, [http://www.depts.ttu.edu/gesc/Fac_pages/Yoshinobu/4361_5361_Folder/2013-readings/Hubbert%20and%20Willis,%201972%20mechanics%20of%20hydr%20frac.pdf Mechanics of hydraulic fracturing]: JPT, vol. 9, no. 6, p. 153–168.</ref>

The overpressure required to cause [[Fracture|fracturing]] is traditionally calculated by determining the theoretical fracture pressure, P_f:<ref name=ch10r39>Hubbert, M. K., and D. G. Willis, 1957, [http://www.depts.ttu.edu/gesc/Fac_pages/Yoshinobu/4361_5361_Folder/2013-readings/Hubbert%20and%20Willis,%201972%20mechanics%20of%20hydr%20frac.pdf Mechanics of hydraulic fracturing]: JPT, vol. 9, no. 6, p. 153–168.</ref>

:<math>\mbox{P}_{\rm f} = \alpha \sigma_{3} + \mbox{p}</math>

:<math>\mbox{P}_{\rm f} = \alpha \sigma_{3} + \mbox{p}</math>

* σ₃ = effective least principal stress or confining pressure

* σ₃ = effective least principal stress or confining pressure

* p = pore pressure

* p = pore pressure

* α = poroelastic constant, assumed to be 1 in most analyses<ref name=ch10r25>Engelder, T., Lacazette, A., 1990, Natural hydraulic fracturing, in Barton, N., Stephansson, O., eds., Rock Joints: Rotterdam, A., A. Balkema, p. 35–43.</ref>

* α = poroelastic constant, assumed to be 1 in most analyses<ref name=ch10r25>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.</ref>

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

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

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! Step

! Step || Action || Method

! Action

! Method

|-

|-

| 1

| 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.

| 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

| 2 || Determine ν, [http://en.wikipedia.org/wiki/Poisson%27s_ratio Poisson's ratio].

| Determine ν, [http://en.wikipedia.org/wiki/Poisson%27s_ratio 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.<ref name=ch10r23>Eaton, B. A., 1969, Fracture gradient prediction and its application in oilfield operations: Trans. AIME, October, p. 1353–1360.</ref> 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.<ref name=ch10r45>Lama, R. D., and V. S. Vutukuri, 1978, Handbook of Mechanical Properties of Rocks: Rockport, MA, Trans. Technical Publications.</ref> Poisson's ratio increases with depth to approach a maximum of 0.5.

| 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.<ref name=ch10r23>Eaton, B., A., 1969, Fracture gradient prediction and its application in oilfield operations: Trans. AIME, October, p. 1353–1360.</ref> 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.<ref name=ch10r45>Lama, R., D., Vutukuri, V., S., 1978, Handbook of Mechanical Properties of Rocks: Rockport, MA, Trans. Technical Publications.</ref> Poisson's ratio increases with depth to approach a maximum of 0.5.

|-

|-

| 3

| 3 || Determine p, pore pressure.

| Determine p, pore pressure.

| Pore pressure can be determined from measurements or regional pressure maps or estimated from burial history.<ref name=Man_Mackenzie1990>Mann, D. M., and A. S. Mackenzie, 1990, [http://www.sciencedirect.com/science/article/pii/026481729090056M Prediction of pore fluid pressures in sedimentary basins]: Marine and Petroleum Geology, v. 7, no. 1, p. 55-65. </ref> It may be necessary to predict paleopore pressure.

| Pore pressure can be determined from measurements or regional pressure maps or estimated from burial history.<ref name=Man_Mackenzie1990>Mann, D. M., and A. S. Mackenzie, 1990, [http://www.sciencedirect.com/science/article/pii/026481729090056M Prediction of pore fluid pressures in sedimentary basins]: Marine and Petroleum Geology, v. 7, no. 1, p. 55-65. </ref> It may be necessary to predict paleopore pressure.

|-

|-

| 4

| 4 || Calculate σ₃ , effective confining pressure.

| Calculate σ₃ , effective confining pressure.

| Solve the equation <math>\sigma_{3} = (\sigma_{1} - \mbox{p}) \left(\frac{\mbox{v}}{1 - \mbox{v}}\right)</math>

| Solve the equation <math>\sigma_{3} = (\sigma_{1} - \mbox{p}) \left(\frac{\mbox{v}}{1 - \mbox{v}}\right)</math>

|-

|-

| 5

| 5 || Calculate P_f , theoretical fracture pressure.

| Calculate P_f , theoretical fracture pressure.

| Solve the equation <math>\text{P}_{\text{f}} = \alpha \sigma_{3} + \text{p}</math>. The fracture pressure is commonly expressed as a gradient, and the equation becomes <math>\frac{\text{P}_\text{f}}{\text{Z}} = \alpha \left(\frac{\sigma_3}{\text{Z}}\right) + \frac{\text{p}}{\text{Z}}</math>, where Z is depth.

| Solve the equation <math>\text{P_{\text{f}}} = \alpha \delta_3 + \rho</math>. The fracture pressure is commonly expressed as a gradient, and the equation becomes P_f /Z = α σ₃ /Z + p/Z, where Z is depth.

|}

|}

==Other ways to calculate p_f==

==Other ways to calculate p_f==

Variations on this equation as well as empirical relationships are common.<ref name=ch10r39 /><ref name=ch10r55>Matthews, W., R., Kelly, J., 1967, How to predict formation pressure and fracture gradient from electric and sonic logs: Oil & Gas Journal, vol. 65, no. 8, p. 92–106.</ref><ref name=ch10r23 /><ref name=ch10r8>Breckles, I., M., Van Eekelen, H., A., M., 1982, Relationship between horizontal stress and depth in sedimentary basins: Journal of Petroleum Technology, vol. 34, no. 9, p. 2191–2199, DOI: [https://www.onepetro.org/journal-paper/SPE-10336-PA 10.2118/10336-PA].</ref><ref name=ch10r9>Brennan, R., M., Annis, M., R., 1984, A new fracture gradient prediction technique that shows good results in Gulf of Mexico abnormal pressure: SPE paper 13210, 6 p.</ref> An alternative method of determining the principal stresses and fracture gradient is through the use of borehole deformation.<ref name=ch10r4>Bell, J., S., 1990, [http://sp.lyellcollection.org/content/48/1/305.abstract Investigating stress regimes in sedimentary basins using information from oil industry wireline logs and drilling records], in Hurst, A., Lovell, M., A., Morton, A., C., eds., Geological Applications of Wireline Logs: Geological Society London Special Publication 48, p. 305–325.</ref><ref name=ch10r28>Evans, C., J., Brereton, N., R., 1990, [http://sp.lyellcollection.org/content/48/1/327.abstract In situ crustal stress in the United Kingdom from borehole breakouts], in Hurst, A., ed., Geological Applications of Wireline Logs: Geological Society of London Special Publication 48, p. 327–338.</ref>

Variations on this equation as well as empirical relationships are common.<ref name=ch10r39 /><ref name=ch10r55>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.</ref><ref name=ch10r23 /><ref name=ch10r8>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: [https://www.onepetro.org/journal-paper/SPE-10336-PA 10.2118/10336-PA].</ref><ref name=ch10r9>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.</ref> An alternative method of determining the principal stresses and fracture gradient is through the use of borehole [[deformation]].<ref name=ch10r4>Bell, J. S., 1990, [http://sp.lyellcollection.org/content/48/1/305.abstract 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.</ref><ref name=ch10r28>Evans, C. J., and N. R. Brereton, 1990, [http://sp.lyellcollection.org/content/48/1/327.abstract 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.</ref>

==See also==

==See also==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Evaluating top and fault seal]]

[[Category:Evaluating top and fault seal]]