Permeability to Reservoir Fluids

Permeability is the ability of fluids to move in the reservoir. The net-pay cutoff should be based on how effective permeability is to reservoir fluids under reservoir conditions. Most core permeability data concern air-at-surface conditions, and adjustments should be made to these values to reflect in situ conditions. In dry gas reservoirs, reasonable estimates can be obtained from core data analyzed under net overburden stress conditions using Klinkenberg corrections. This type of analysis is not generally cost prohibitive, and not all samples need it. Excellent correlations can commonly be obtained on a subset of the total core data set that allows the net overburden Klinkenberg corrections to be applied to all samples.

Correlations can also be done in oil reservoirs; however, significantly more expense and effort are required. Determining the values of effective permeability to oil and water is a useful exercise. Part of this effective-permeability determination process is estimating irreducible water saturation and relative permeability to oil and water. These issues will be discussed in the next section. In the absence of special core analysis in oil zones, a reasonable calibration can be made to pressure transient data. In this case it is important to use the value of permeability-thickness from the well test and to match the log-derived permeability thickness, rather than the assumed permeability value. The permeability value from a well test requires an input of net pay that may or may not be valid before proper calibration.