Net-Pay Cutoff

The two most critical issues in the determination of net pay are permeability to reservoir fluids and water saturation.

In many cases analysts use other criteria for net pay, such as porosity and shale volume. Although porosity is an important input in the net-pay determination process, if the researcher has a good permeability model, a separate porosity criterion is unnecessary. In many cases the use of a porosity criterion alone for net pay can lead to erroneous calculations of net pay because permeability is a function of both porosity and irreducible water saturation.

If the irreducible Bulk Volume Water (BVW) (the product of porosity and water saturation) is not a constant value within the reservoir, the porosity-permeability relationship will not be consistent either. High irreducible BVW zones require higher porosity values to achieve the same permeability as lower porosity low irreducible BVW zones. Clay volume is an input to the process as well, refining both porosity and water saturation calculations. It is unnecessary to have clay volume as a separate net-pay criterion after permeability and Sw models have been calibrated.