HPHT well

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In High Presure High Temperature (HPHT) environments, high differential pressures often lead to the use of high strength, thick-walled, and occasionally non-standard, casing.

High temperatures compound the design problem by reducing the yield strength of the casing steel, by causing significant thermal linear expansion of the steel, and by generating high pressures in sealed annuli due to thermal expansion of the trapped fluid.

The following areas should be given particular attention. They have been divided into factors which affect the load experienced by the casing, and factors which influence the capacity of the casing to resist loads.

Casing loading

  • The effects of high tensile or compressive axial forces on the ability of the casing to resist collapse and burst pressures. Although the effects occur in all wells, they are more significant in HP/HT wells because of the high forces and pressures involved. A triaxial stress analysis is therefore required.
  • Build up of annulus pressures due to thermal expansion of fluid in sealed annuli which cannot be bled off, e.g. subsea wellheads. Such pressures, which may become very high with continued production, can be estimated using computer programs.
  • High buckling potential due to the linear expansion resulting from large temperature increases during deeper drilling and subsequent testing/production. The increase in internal fluid density during deeper drilling adds to this potential.
  • The testing programme to ensure the casing is capable of withstanding the anticipated burst loads need careful design when combination strings are used. The axial loading resulting from use of retrievable test packers should be checked as part of the pressure test preparation.

Casing load bearing capacity

  • High H2S partial pressures (sour conditions). Due to the high pressures involved, the partial pressure for H2S which defines sour conditions (0.05 psia) is achieved for relatively low H2S concentrations. Because of the high temperatures in the well, sour-rated tubulars are usually only required at shallow depths, however, due to the high pressure loads heavy wall pipe could result.
  • The requirement for gas-tight connections operating at high temperatures and differential pressures is not easily satisfied. Only suitably qualified connections should be selected.
  • Effects of dimensional tolerances on casing performance influences casing selection. Rather than use non-standard casing sizes, the desired casing rating may be achieved by a tightening of manufacturing tolerances.
  • Reduction of casing material yield strength at high temperatures. The capacity of the casing should be downrated according to the downhole operating temperature.
  • Several drilling liners might be required to allow for deeper drilling. This is because the anticipated mud gradients to balance the high pore pressures are usually close to the formation breakdown gradient.

Comments  

#1 anhtuan123 2017-08-15 05:21
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I am really grateful to you.
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