SPE Reservoir Evaluation & Engineering
Volume 11, Number 6, December 2008, pp. 1013-1028

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 (P_c ) 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 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. 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.

Introduction

Models used to calculate water saturation from logs in this gasfield case study include the deterministic Archie equation (Archie 1942), Waxman and Smits (1968), and an optimizing dual-water (DW) model presented by Eyvazzadeh et al. (2003).

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.

The presence of illite, even though in small quantities, has necessitated the use of the DW model routinely to calculate porosity and water saturation in this field. Illite is filament-like, nonswelling clay that coats grain surfaces. It is thought that the DW model, which was optimized for this formation, provides more-accurate water saturation. The model uses as input all available logs, mineral analyses, and electrical parameters, and it solves for clay-bound water and free fluids in the flushed and unflushed zones of the wellbore.

The relationship between capillary pressure (P_c ) and water saturation offers a means to estimate water saturation vs. depth, which is independent of wireline logs and provides the ability to calibrate log-derived saturations. Saturation/height models, if implemented successfully, would also minimize, or eliminate, the uncertainties associated with electrical-parameter measurements. Some uncertainties that can impact the accuracy of electrical parameters include electrode configuration, saturation and resistivity equilibration, and incomplete core cleaning. The experimental protocols should also be designed to determine intrinsic saturation and cementation exponents (n* and m*, respectively) and not apparent properties in shaly formations such as those discussed in this paper.

© 2008. Society of Petroleum Engineers

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History

• Original manuscript received: 25 June 2006
• Meeting paper published: 24 September 2006
• Revised manuscript received: 16 May 2008
• Manuscript approved: 1 September 2008
• Version of record: 29 December 2008