|As the reservoir depletes, the internal pressure of the reservoir drops. Since the overburden pressure remains constant, the frame pressure is increased. The increasing net stress on the rock tends to reduce the pore space and hence the porosity.
Although these values are small, they become more important in reservoirs that are under saturated when calculations are made to define oil-in-place or aquifer constants that will be used to predict future pressure performance.
Ignoring pore volume compressibility in material balance calculations results in over estimation of oil-in-place and calculation of excessive volumes of aquifer influx.
It is a cost effective way to determine rock compressibility. Typical values range from 3 x 10-6 to 25 x 10-6 pv/pv/psi. This means that for each million barrels of pore space, a reduction of 3 to 25 barrels would occur for each psi of reservoir pressure drop.