Why mobile water increases

QUESTION

Working on sealed area without water injection after some production time simulation model may show water production increase.

ANSWER

Rock compressibility effects cause PV decrease during production, which in turn leads to higher SW resulting in mobile water.
As pressures in gridblocks change its pore volume decreases due to compressibility and since water is not mobile initially, then only hydrocarbons exit the gridblock. That means a greater fraction of the volume of the gridblock is water, which means higher SW.