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

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA 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, saturationheight 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's 1 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.

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

“…From these experiments, the intrinsic formation resistivity factor (F*), intrinsic cementation exponent (m*), intrinsic saturation exponent (n*), and volume concentration of exchangeable cations (Q v ) used for the Waxman-Smits shalysand water-saturation model can be estimated. Experimental details and examples are given in Amabeoku et al (2005a).…”

“…Extensive laboratory measurements conducted by Amabeoku et al (2005a) and Efnik et al (2006) show variability of the saturation (n) and cementation exponents (m) vertically within the well and from well to well. This makes the use of single-valued (average) parameters untenable.…”

“…Amabeoku et al (2005a) first incorporated HU zonation to overcome the initial limitations of the FOIL function (Cuddy 1993) and observed an improvement in water-saturation calculation. Mitchell 1 had suggested the FOIL water-saturation model could be modified to fit the core capillary pressure data by adjusting the porosity term.…”

“…Further improvement in water saturation was obtained with the data from the gas field under study when a porosity exponent was introduced. The resulting empirical equation was presented by Amabeoku et al (2005a) as the Modified FOIL function: A, B, and C are parameters that are obtained by robust nonlinear regression and other optimization schemes.…”

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

“…Amabeoku, et al 3 incorporated HU zonation to overcome initial limitations of Cuddy's 1 function and presented a modification to improve water saturation calculation. The resulting equation is expressed as: where A, B, and C are fitting parameters.…”