What is Net Pay? Definition and Cutoffs
Any discussion of net, netpay or cutoffs should start with a definition to clarify and unify the approach of different disciplines – a common cause of cutoff confusion. In the opinion of this author:
Netpay definition: The objective of Netpay is to delineate intervals containing fluids which make a contribution to production during the life of the field. Fluids which move or experience a change in saturation or pressure during production will make a finite, if small, contribution to production.
Hence if cutoffs are used the root criteria should be fluid movement, that is permeability, and saturation to imply which phase will move, not porosity or vshale. In the opinion of this author cutoffs are not necessary if permeability has been evaluated to within about 1/2 log cycle and correctly averaged in the reservoir static and dynamic models, indeed they will usually add error. Large errors in simulation are frequently caused by cutoffs – to blank out large volumes of rock, typically more than 50% GRV, as zero storage, zero permeability is usually more wrong than to at least approximately quantify their bulk volume hydrocarbon and permeability. This is especially true in low permeability, dry gas reservoirs.
Net Pay Cutoff SCAL Calibration
At last, a cut-off you can justify! Have you or your friendly petrophysicist ever used relative permeability data to determine a value for your oil reservoir’s netpay cut-off? No? But relative permeability data defines the conditions at which oil becomes mobile … at which an oil saturated rock becomes PAY.