Gas Migration External

In this section we describe the load case "Gas Migration Extgrad" available in Oliasoft WellDesign™.

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Gas Migration External is a collapse load case, where the unknown is the external pressure profile of the tubing.

NOTE!
Note: In this documentation we denote any tubular as tubing. All calculations however encompass any tubular, such as tubings, casings, liners, tie-backs etc. //: # (end - note)

Summary

Gas migration is a collapse load for production. The scenario looks at gas bubbles migrating up the cement from the reservoir without space to expand, resulting in hanger pressure equal to the reservoir pressure. The external pressure profile from the hanger to the shoe is the hanger pressure with hydrostatic mud from the hanger to the casing shoe.

Override wellhead pressure is applied from hanger to the casing shoe to simulate pressure communication in the cement.

Illustrating Pressure Profile Graph

Printable Version

Oliasoft Technical Documentation - Gas Migration

Inputs

The following inputs define the gas migration load case:

1. The true vertical depth (TVD) along the wellbore as a function of measured depth. Alternatively, the wellbore described by a set of survey stations, with complete information about measured depth and inclination.
2. The true vertical depth / TVD of
   1. The hanger of the tubing, TVD$_{hanger}$.
   2. The shoe of the tubing, TVD$_{shoe}$.
   3. The shoe of the prior tubing, TVD$_{prior\;shoe}$.
3. The temperature profile of the wellbore, T.
4. The packer fluid density, $\rho_{pf}$.
5. The reservoir pressure, P$_{res}$.
6. The fracture pressure at prior shoe, P$_{f@ps}$.
7. The mud weight/density, $\rho_{mud}$.
8. The gravitational constant, $g$.
9. Whether or not to limit the external pressure at prior shoe by the fracture pressure there.

Calculation

If the limit the external pressure at prior shoe is enabled, and $P_{ps} > P_{f@ps}$, then the external pressure is given by

$P_{e} = P_{f@ps} - g\rho_{mud}(TVD_{prior\;shoe}-TVD), \;\;\;\; TVD\; \epsilon \;[TVD_{hanger}, TVD_{shoe}]\;\;\;\; (3)$

else it is given by

$P_{e} = P_{res} - g\rho_{mud}(TVD-TVD_{hanger}), \;\;\;\; TVD\; \epsilon \;[TVD_{hanger}, TVD_{shoe}]\;\;\;\; \qquad (4)$