Factors Affecting the Permeability Cutoff

Type of completion is an issue because lower permeability zones having hydraulic fractures can be economically produced, whereas identical zones having no stimulation may not be. Many of the low-permeability gas zones in the U.S. had little or no net pay before hydraulic fracturing was introduced, although today these same reservoirs are responsible for a good deal of the total gas produced. If the zone cannot be stimulated, the amount of skin damage expected becomes a factor.

Lower permeability zones that can be drilled without skin (using techniques such as underbalanced drilling) can be economically produced, whereas equivalent permeability zones with high skin damage may not. In the case of hydrocarbon zones close to water zones, one zone may contain net pay if sufficient stress contrast exists between the hydrocarbons and the water, although an identical zone may not be considered net pay because a hydraulic fracture would result in uneconomic water production.

The viscosity of the produced fluid is also an issue, gas reservoirs requiring permeabilities to produce economically that are significantly lower than those of oil reservoirs. Reservoir pressures and flowing well-bore pressures are also an issue. Zones having high reservoir pressures and high drawdowns can be produced economically at lower permeabilities than can lower pressure and drawdown zones having equivalent permeabilities.

All of these factors go into the permeability cutoff for net-pay determination. A detailed discussion of these is beyond the scope of this exercise, and it is assumed here that this value has been determined by the reservoir engineer and petrophysicist for the field. If additional background is needed for establishing permeability and Sw criteria for net pay, a review of the references is recommended.