Gas Migration

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

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Gas Migration is a collapse load case, where the unknown is the 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 internal pressure profile is given by the hydrostatic pressure from the packer fluid from RKB to the casing shoe. //: # (end - summary)

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. //: # (end - inputs)

Calculation

The internal pressure profile is simply the hydrostatic pressure from the packer fluid, i.e.

$P_{i} = g\rho_{pf}TVD, \;\;\;\;\;\; TVD\; \epsilon \;[TVD_{hanger}, TVD_{shoe}]\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; (1)$

It is assumed that the external pressure at the wellhead/hanger is equal to the reservoir pressure, $P_{res}$. By this assumption and the mud weight, it follows that the pressure at the prior shoe, $P_{ps}$, is

$P_{ps} = P_{res} + g\rho_{mud}(TVD_{prior\;shoe}-TVD_{hanger}) \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; (2)$

External profile

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

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