Incorporating Hydraulic Units Concepts in Saturation-Height Modeling in a Gas Field

Abstract

Abstract A comprehensive petrophysical study in a gas field has demonstrated the effective application of saturation-height functions for calculating water saturation. The results show that linking depositional and diagenetic rock fabric to hydraulic units, and then linking the hydraulic units to zones with similar core capillary pressure relationships, improved the accuracy of the models. In the study field, saturation-height functions provided accurate water saturation, and they can potentially overcome uncertainties associated with log interpretation, using Archie or shaly sand models. The saturation-height models were developed from core capillary pressure (Pc) data to calculate water saturation versus depth, which is independent of logs. Consequently, the core-based saturation height functions can be useful in the calibration of log-based petrophysical models. Capillary pressure curves from special core analysis (SCAL) studies were distributed into corresponding hydraulic units (HUs), based on the calculated flow zone indicators. Saturation-height function was then developed for each HU and used to calculate water saturation in the study field. The most accurate saturation model that evolved is a function of only porosity and height above free water level. It is a modification of Cuddy's1 saturation function, which relates the bulk volume water (BVW) to height above free water level. Cuddy referred to the function as FOIL, but did not define the acronym. Like FOIL, the Modified FOIL function, that has been developed, does not require permeability in its application, and performed better than the Leverett J-function in this field. Modified FOIL model that has evolved out of rigorous core-log integration will provide more accurate petrophysical interpretation in shaly sands and thin bed units. Introduction Several models have been used to calculate water saturation from logs in this case study gas field. These include deterministic Archie2 equation, Waxman-Smits3, and an optimizing Dual-water (D-W) model. The Archie equation which assumes a nonconductive matrix is written as: