Chemistry In Enhanced Oil Recovery- An Overview
Authors: Boon J.A. 
Published: March 2010

Abstract
During the primary production stage of a reservoir, oil moves to producing wells under the influence of "natural forces" provided by mechanisms such as expansion, compaction, gas drive,' natural water drive and solution gas drive. During the secondary recovery stage, water is injected at appropriate points into the reservoir to displace the oil toward the producing wells. Because of bypassing and capillary forces, 40% to 70% of the original oil in place is left behind. Further, many reservoirs are not amenable to waterflooding. The term ‘Enhanced Oil Recovery’ refers to all processes (other than water flooding) in which energy and chemicals are supplied to the reservoir to establish pressure gradients, modify interfacial tensions and wettability, adjust the mobility of the driving fluid, alter the permeability of selected zones and change fluid properties in such a way that oil flows toward the producing well in a controlled manner(1).
Incremental recovery potential on a world scale amounts to 32 × 109 m3, whereas new discoveries are estimated at 32 –64 × 109 m3(2). The EOR potential in Canada is 0.64 × 109 m3, as compared to remaining conventional reserves of 1.0 × 109 m3(2).

Background Theory
The residual oil saturation that can be achieved is determined by both macroscopic and microscopic displacement efficiency. The former is reduced by large permeability variations in the reservoir and an unfavourable mobility ratio: if the displacing fluid is more mobile than the displaced fluid, viscous fingering occurs. The latter refers to the entrapment of oil blobs at the pore-size level by capillary forces. The most important parameters characterizing these forces are interfacial tension γ, contact angle θ and capillary pressure Pc= Po - Pw (Fig. 1). Both macroscopic and microscopic phenomena can be manipulated by controlling the chemistry at the oil-water-sand interfaces and in aqueous solutions, hence the great importance of chemistry to enhanced oil recovery.

Contact Angle
Contact angle influences fluid distribution, displacement processes and oil-water relative permeabilities. Recently, Teletzke et al. (3), on the basis of gradient theory predicted that when a solid phase is contacted with two liquid phases that coexist near their critical temperature, one of the two will perfectly wet the solid (Fig. 2). They also predicted that a first-order film transition occurs at the "critical surface temperature" Tcs; two wetting films of different thickness are in equilibrium with each other (Fig. 2). At the "critical wetting temperature" Tcw, perfect wettability is lost and the fluid-fluid interface appears to intersect the solid surface at an angle 8 (Fig. 2). Gradient theory assumes that the Helmholtz free- energy is composed of a homogeneous fluid term, a species interaction term in regions where density gradients occur and a term that accounts for molecular interactions between fluid species and the solid. There is good experimental evidence for the existence of Tcw but empirical proof for the film-film transition at Tcw has not yet been reported.