Abstract

A representative description of residual oil saturation is one of the fundamental requirements for studying the behaviour of a field under waterflooding.

Presented is a case history for describing waterflood residual oil saturation for the Buffalo field, offshore Australia. The aim is to give a practical framework for correlating laboratory results for residual oil saturation. The new methodology correlates residual oil saturation for a particular facies with initial oil saturation, permeability and when appropriate also wettability, and as a function of displacement velocity. As a non-conventional feature, the wettability parameter used is based on relative permeability endpoints.

It is demonstrated that correlation with permeability can follow both, a more conventional, positive relationship in case of a heterogeneous pore structure and a negative relationship in case of a more homogeneous formation. It is also shown how alteration in wettability, both in-situ due to drilling mud contamination and in the laboratory due to variation in ageing can impact the results.

The strength of the methodology is that it can be applied to very diverse situations and simple nomograms have been developed to better visualise the results. The methodology builds on the ideas of Land1 and can be universally applied using basic, readily available data. The method is also very suitable for comparative studies of different facies of the same field or in comparing different fields. Additional examples are cited from the Gulf of Mexico to demonstrate the applicability of the method.

Introduction

Waterflood residual oil saturation (both aquifer influx and water injection) can be expressed by using different, in many cases alternative parameters. Describing the prevailing conditions during flooding, these essentially fall into three independent categories: rock properties, fluid properties and the overall flow situation in the field or type of test in the laboratory. In combination, the above properties are also responsible for rock-fluid and fluid-fluid interaction, which together with the basic parameters determine the initial and final fluid saturation. Figure 1 attempts to outline this rather complex situation, which not only has led to difficulties in selecting appropriate parameters but also to elusion, to date, of a universal predictive theory or methodology for the determination of residual oil saturation.

There are three approaches to the problem: theoretical, laboratory and empirical, or a combination of these. In the study presented here, correlations have been developed by selecting the minimum number of parameters to honour the laboratory data and without resorting to additional, specialised laboratory testing. In addition to selecting suitable parameters, there were also two other, commonly encountered challenges. First of all, there was the choice of laboratory experiments, hopefully simulating reservoir conditions (in terms of the displacement process) or at least a method which allows the translation of laboratory results to field conditions. Secondly, there was the associated problem of core preparation, aiming at core conditions (mainly wettability) which are representative of unaltered, in-situ reservoir conditions.

The specific objective of the Buffalo laboratory data review was to gain some insight into the relatively high average residual oil saturation of 38percent at reservoir displacement velocities. In order to determine the possible causes for this high saturation, various analyses and studies were reviewed: routine core, petrographic, log, special core and crude properties.

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