BEFORE YOU BEGIN: LOG ANAYSIS MODEL VALIDATION

Whenever possible the log analysis models should be tied to real information from cores or production data. In this case the petrophysicist has used core porosity, XRD shale volume, irreducible water saturation, and Klinkenberg corrected net overburden permeability data as "reality checks" for the log analysis models. The specific models that were calibrated to this data were the porosity model, shale volume model, water saturation model, and permeability models. Figure 2 below shows this agreement between core measured values and the log calculations.

Figure 2. Log-derived values of PHIE, Sw, Vshale; and permeability agree reasonably well with core data.

NET PAY CRITERIA SELECTION

Once the calibrated model is in place, the petrophysicist and reservoir engineer need to determine what will produce economic quantities of hydrocarbons. In this case (a tight-gas reservoir in South Texas) they have determined that permeabilities over 0.001 md will provide economic flow rates. They have also determined that water saturations over 60 percent will result in uneconomic water-production volumes.