Relative Permeability Coupled Saturation-Height Models on the Basis of Hydraulic (Flow) Units in a Gas Field

Abstract

Summary Saturation/height functions on the basis of unique flow units have been developed as part of an integrated petrophysical analysis of a gas field. Furthermore, coupling the saturation/height functions with appropriate relative permeability models has effectively quantified hydrocarbon saturation, classified producibility of intervals, and defined critical water saturation. The results show that linking depositional and diagenetic rock fabric with flow units and then linking the flow units with zones that have similar core capillary pressure and relative permeability relationships have enhanced the utility of the saturation models. The saturation/height functions provided more-accurate water saturation in the study field, and potentially they can overcome uncertainties associated with log interpretation by use of Archie or shaly-sand models. The saturation/height models were developed from core capillary pressure (Pc) data to calculate water saturation vs. depth, which is independent of logs. The relative permeability models were obtained from special-core analysis (SCA). Consequently, the core-based saturation/height functions can be useful in the calibration of log-based petrophysical models and with relative permeability can also be used to estimate water/gas ratios (WGRs) and critical water saturation. Capillary pressure and relative permeability curves from SCA studies were distributed into corresponding flow units, on the basis of the calculated flow-zone indicators(FZIs). Saturation/height functions were then developed for each unit and were used to calculate water saturation in the study field. The most accurate flow-unit-based saturation model that evolved is a function only of porosity and of height above the free-water level; it does not require permeability in its application; and it performed better than the Leverett J-function in this field. Coupled with hydraulic unit (HU)-based relative permeability curves, the saturation models may provide more comprehensive petrophysical interpretation in gas-bearing formations and may highlight potential differences in reservoir producibility.