Saturation cannot be lower than residual

[rolk]

There is a problem with having saturations lower than the residual saturation; the transport solver will alter the mass balance to get into the range of the rel.perm. curve.

Clearly, this is unphysical; I can have an aquifer filled with 100% brine which has no CO2, i.e. S_co2 = 0 initially, but which has a residual saturation, i.e. an amount of any CO2 that enters the cell later will be trapped.

Consider the following test case to elicit the problem: It consists of 5 blocks of unit size in a regular grid. There is an injector and a producer, but I am injecting with a rate of 0, so there should be no saturation change.

I run with a command like this (using commit adf421a of opm-core):

bin/sim_2p_incomp 458_ressat.param > 458_ressat.out

using these input files:

• 
• 

and these are the output files obtained:

• 
• 
• 

Notice that the saturation of CO2 is increased to slightly above the residual
saturation, in all blocks. If I change the lower bound of the rel.perm. curve,
then there is a corresponding change in the saturation.

[atgeirr]

Still valid, not resolved, not working on it, will keep open as a reminder but remove from release milestone.