New Insight Into Eromanga Basin Oil Saturations

Adams, S.J. (WellEval.com Limited)

OnePetro 

Abstract

The Eromanga Basin is an established Australian producing region with oil and gas found in several different Formations. At the request of an Operator, a project was undertaken to construct saturation-height functions for all the Eromanga reservoir units with a secondary objective being to define residual hydrocarbon saturations.

Initial investigations revealed many reservoirs with residual hydrocarbon columns, the significance of which had not been well understood. The residual hydrocarbons implied that imbibition, rather than drainage, capillary pressure curves were representative of water saturations in the reservoir. This insight suggested higher oil-in-place and reserves volumes than previously assumed since mobile hydrocarbons are present very close to the pressure derived Free-Water Level in imbibition systems.

When individual hydrocarbon Fields were considered, there were insufficient special core analyses to derive meaningful residual hydrocarbon or saturation-height relationships. However, on the basin scale, a significant volume of measurements had been acquired over a period of 22 years, albeit using different laboratories and a variety of measurement techniques. With knowledge of the measurement techniques and Formations sampled, the data were combined in such a way that consistent datasets were obtained for end-point relative permeabilities and drainage and imbibition capillary pressure curves.

Interpretation of these datasets produced residual oil saturation and drainage and imbibition saturation height relationships. These relations were tested against those log-derived water saturations considered most reliable by the Operator, showing excellent matches. The model developed successfully described the water saturation distributions in the reservoirs tested in a manner not previously possible. Indeed, the use of the drainage and imbibition saturation-height functions together with residual hydrocarbon relationships provides a powerful tool to determine both static and dynamic fluid contacts, while checking the validity of wireline log-based water saturations.

Introduction

At the request of an Operator, a review has been undertaken of all the available Special Core Analyses (SCAL) for the Jurassic Oil reservoirs found in the Eromanga Basin of Australia. The primary objective of this study was to construct appropriate saturation-height functions for oil volume quantification and reservoir modelling. A secondary objective was the identification of residual oil saturations from suitable core analyses.

The Jurassic reservoir units involved were the Adori, Basal-Jurassic, Birkhead, Hutton, McKinlay, Murta, Namur and Westbourne Formations.

History

The Eromanga Basin is an established producing area, with many fields and a large database of wireline log measurements and core analyses collected over more than 20 years.

Despite a number of different Operators and a history of production in the area, the significance of the residual oil found below the pressure derived free-water levels (FWL) of many fields had not been fully recognised.

The signs that imbibition may be significant in reservoirs include:

  • presence of "residual oil" below the pressure derived FWL at discovery,

  • dry oil production from close to a FWL,

  • sharper log-derived transition zones than the reservoir permeability suggests.

In addition, as oil fields are produced, water sweeps through sections of reservoir. These sections have gone or are undergoing water imbibition.