The most accurate way to simulate Gas Cap expansion (or shrinkage) is full-field 3D Dynamic Flow Model where Gas Cap expansion is treated as one of the fluid phases and accounts of geological heterogeneities, gas fluid properties, relperm properties and heat exchange with surrounding rocks.

Unfortunately, in many practical cases the detailed information on the Gas Cap is not available.

Besides many practical applications require only knowledge of one element of the Gas Cap expansion process – a pressure support and not the sweep in the invaded zones. 

This allows building a Gas Cap Drive @model using simple analytical methods.