EOR Methods

Oil & Gas Science and Technology – Rev. IFP, Vol. 63 (2008), No. 1, pp. 9-19

Copyright © 2007, Institut français du pétrole

DOI: 10.2516/ogst:2007060

Enhanced Oil Recovery – An Overview

S. Thomas

PERL Canada Ltd., Canada

e-mail: sarathomas@shaw.ca

Résumé — Récupération assistée du pétrole : panorama — Près de 2,0 × 1012 barils (0,3 × 1012 m3)

de pétrole conventionnel et 5,0 × 1012 barils (0,8 × 1012 m3) de pétrole lourd resteront dans les réservoirs

du monde entier lorsque les méthodes de récupération traditionnelles auront été épuisées. Une grande

partie de ce pétrole serait récupéré grâce à des méthodes de Récupération Assistée du Pétrole (EOR), qui

fait partie du projet général de Récupération Améliorée du Pétrole (IOR). Le choix de la méthode et la

récupération escomptée dépendent de nombreuses considérations économiques et technologiques. Cet

article étudie les méthodes EOR qui ont été testées sur le terrain. Certaines ont été une réussite commerciale,

tandis que d’autres sont d’un intérêt essentiellement académique. Les raisons en sont discutées.

L’article examine les méthodes de récupération du pétrole thermique et non thermique. Elles sont présentées

de façon équilibrée, en prenant en compte le succès commercial sur le terrain. Seules quelques

méthodes de récupération ont connu une réussite commerciale, tels que les processus d’injection de

vapeur dans les pétroles lourds et les sables bitumineux (si le réservoir offre des conditions favorables

pour de telles applications) et de dioxyde de carbone miscible pour les réservoirs de pétrole léger.

D’autres méthodes de récupération ont été testées, et ont même permis d’augmenter la récupération

d’huile mais comportent des limites inhérentes. Les technologies EOR actuelles sont présentées dans une

perspective appropriée, soulignant les raisons techniques au manque de réussite. Les méthodes d’amélioration

de la récupération de pétrole, en particulier celles visant à diminuer la saturation interstitielle du

pétrole, ont fait l’objet d’une attention particulière dans les laboratoires et sur le terrain. Les nombreux

documents qui traitent du sujet donnent l’impression qu’il est relativement simple d’augmenter la récupération

de pétrole au-delà de la récupération secondaire (en assumant que le réservoir se prête à une récupération

primaire et secondaire). Il s’avère que ce n’est pas le cas. De nombreux réservoirs adaptés à l’injection

de vapeur et au dioxyde de carbone ont déjà été exploités et arrivent à maturité. D’autres

méthodes EOR rencontrent des limites qui ne sont pas liées à des facteurs économiques. La récupération

du pétrole supplémentaire est complexe et coûteuse, et s’est révélé probante seulement pour quelques

processus et ce, dans des conditions astreignantes. Néanmoins, l’EOR continuera d’avoir une place

importante dans la production pétrolière, en raison de l’intensification de la demande en énergie et de

l’offre limitée. Un important travail de recherche doit être mené à bien pour développer des technologies

de récupération sur les deux tiers du pétrole qui ne sera pas récupéré dans les réservoirs. Des références

clés sont indiquées.

Abstract — Enhanced Oil Recovery: An Overview — Nearly 2.0 × 1012 barrels (0.3 × 1012 m3) of

conventional oil and 5.0 × 1012 barrels (0.8 × 1012 m3) of heavy oil will remain in reservoirs worldwide

after conventional recovery methods have been exhausted. Much of this oil would be recovered by

Enhanced Oil Recovery (EOR) methods, which are part of the general scheme of Improved Oil Recovery

(IOR). The choice of the method and the expected recovery depends on many considerations, economic

as well as technological. This paper examines the EOR methods that have been tested in the field. Some

Molecular Structures of Heavy Oils and Coal Liquefaction Products

Structure moléculaire des huiles lourdes et produits de liquéfaction du charbon

IFP International Conference

Rencontres Scientifiques de l’IFP

Oil & Gas Science and Technology – Rev. IFP, Vol. 63 (2008), No. 1

1 IOR VS. EOR

The terms EOR and IOR have been used loosely and

interchangeably at times. IOR, or improved oil recovery, is a

general term which implies improving oil recovery by any

means. For example, operational strategies, such as infill

drilling and horizontal wells, improve vertical and areal

sweep, leading to an increase in oil recovery. Enhanced oil

recovery, or EOR, is more specific in concept, and it can be

considered as a subset of IOR. EOR implies a reduction in

oil saturation below the residual oil saturation (Sor).

Recovery of oils retained due to capillary forces (after a

waterflood in light oil reservoirs), and oils that are immobile

or nearly immobile due to high viscosity (heavy oils and tar

sands) can be achieved only by lowering the oil saturation

below Sor. Miscible processes, chemical floods and steambased

methods are effective in reducing residual oil saturation,

and are hence EOR methods. The main focus of this

paper is on EOR methods.

The target of EOR varies considerably for the different

types of hydrocarbons. Figure 1 shows the fluid saturations

and the target of EOR for typical light and heavy oil reservoirs

and tar sands. For light oil reservoirs, EOR is usually

applicable after secondary recovery operations, and the EOR

target is ~45% OOIP. Heavy oils and tar sands respond

poorly to primary and secondary recovery methods, and the

bulk of the production from such reservoirs come from EOR

methods.

2 RECOVERY OF RESIDUAL OIL

Mobilization of residual oil is influenced by two major

factors: Capillary Number (Nc) and Mobility Ratio (M).

Capillary Number is defined as Nc = vμ/σ, where v is the

Darcy velocity (m/s), μ is the displacing fluid viscosity (Pa.s)

and σ is the interfacial tension (N/m). The most effective and

practical way of increasing the Capillary Number is by

10

of these have been commercially successful, while others are largely of academic interest. The reasons

for the same are discussed. The paper examines thermal and non-thermal oil recovery methods. These

are presented in a balanced fashion, with regard to commercial success in the field. Only a few recovery

methods have been commercially successful, such as steam injection based processes in heavy oils and

tar sands (if the reservoir offers favourable conditions for such applications) and miscible carbon dioxide

for light oil reservoirs. Other recovery methods have been tested, and even produced incremental oil, but

they have inherent limitations. The current EOR technologies are presented in a proper perspective,

pointing out the technical reasons for the lack of success. Methods for improving oil recovery, in particular

those concerned with lowering the interstitial oil saturation, have received a great deal of attention

both in the laboratory and in the field. From the vast amount of literature on the subject, one gets the

impression that it is relatively simple to increase oil recovery beyond secondary (assuming that the reservoir

lends itself to primary and secondary recovery). It is shown that this is not the case. Many reservoirs

suitable for steam injection and carbon dioxide have already been exploited and are approaching

maturity. Other EOR methods suffer from limitations that have little to do with economics. Recovering

incremental oil is complex and costly, and has been successful only for a few processes under exacting

conditions. Nevertheless, EOR will continue to have an important place in oil production, in view of the

escalating energy demand and the tight supply. It is suggested that much research is needed to develop

technologies for recovering over two-thirds of the oil that will remain unrecovered in reservoirs. Key

references are indicated.

(Assuming Soi = 85% PV and Sw = 15% PV)

Light oils Heavy oils Tar sands

Water

EOR Target

100% OIP

Water

EOR Target

45% OIP

Secondary

30% OIP

Primary

25% OIP

Water

EOR Target

90% OIP

Secondary

5% OIP

Primary

5% OIP

Figure 1

EOR targt for different hydrocarbons.

S Thomas / Enhanced Oil Recovery – An Overview

reducing σ, which can be done by using a suitable surfactant

or by the application of heat. An approximation of the effect

of Capillary Number on residual oil saturation is shown in

Figure 2. Capillary number at the end of a waterflood is

~10-7. A 50% reduction in residual oil saturation requires that

the Capillary Number be increased by 3 orders of magnitude.

Capillary number in a miscible displacement becomes infinite,

and under such conditions, residual oil saturation in the

swept zone can be reduced to zero if the

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