Introduction and Literature Review
1.7 Oil Recovery Mechanisms of Other EOR Methods
ASAG process was designed to integrate the favorable attributes of chemical (alkaline and surfactant) and gas /CO2 flooding, which in turn makes the oil recovery mechanism quite complex. The oil recovery mechanisms of alkaline and surfactant chemical EOR processes are summarized below:
1.7.1 Alkaline Flooding
In this EOR method, alkaline solutions are injected into an oil reservoir during or post waterflooding. The oil recovery mechanisms of alkaline flooding primarily include reduction of oil-water IFT, wettability reversal, and emulsification with entrapment of oil [6, 86-90]. Depending on the nature of the crude oil and the reservoir rock, each of the mechanisms may play more or less important roles when alkaline solutions are injected under different reservoir conditions. Alkaline flooding is not recommended for carbonate reservoirs because the alkali solution reacts with calcium ions causing hydroxide precipitation which leads to formation damage [91].
Crude oils with acid number greater than 0.5mg KOH/g in crude oil are known as acidic crudes are suitable for alkaline EOR [92]. The mechanism of alkali-crude oil
Drive
water CO2 AS CO2 Foam Oil
Bank Extra
oil recovery AS CO2
Impermeable rock Cap Rock
Impermeable rock
Injector Producer
Injected fluids Oil + water +gas
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reaction in alkaline flooding is shown in Fig. 1.6. Sheng [93] reported that a highly oil- soluble single pseudo-acid component (HA) is assumed to be present in oil. This pseudo- acid component partitions into the aqueous phases upon contact with water i.e.
o w
HA HA (1.4) where HAo and HAw are the acid species, A is long organic chain and subscript ‘o’ and ‘w’
represents oleic and aqueous phase respectively. HAw dissociates into its components as:
+ -
HAw H + A (1.5) Further, upon alkali (Na2CO3) hydrolysis OH- ions are produced which react with HAw to form oil soaps NaA (which acts as a soluble anionic surfactant). The overall reaction is:
w 2 3 2
HA + Na CO NaA + H O (1.6) Another important oil recovery mechanism of alkaline flooding is wettability reversal. In oil-wet reservoirs, the addition of alkali increases the pH of injected water which causes the rock wettability reversal from oil-wet to water-wet. As a result, the water- oil relative permeability ratio and the water-oil mobility ratio are reduced, that improves the oil displacement efficiency [94, 95]. In the case of water-wet reservoirs, the non- wetting residual oil in discontinuous form can be converted to a continuous wetting phase through wettability reversal under specific conditions of pH, salinity and reservoir temperature. Moreover, water droplets in the continuous oil phase increase the pressure gradient of the flow. Thus, the capillary held residual oil is mobilized and higher recovery is obtained [86].
The commonly used alkalis are NaOH, Na2CO3, Na4O4Si, BNaO, (NH4)2CO3, and NH4OH. Na2CO3 has been reported to be a better candidate for alkaline flooding. Alkali consumption during flooding can be reduced by the use of Na2CO3 and breakthrough times are also minimized. With Na2CO3, the mineral dissolution and ion exchange are also considerably lower compared with NaOH and Na4O4Si [96]. Cheng [97] reported that
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formation damage by precipitates of CO3 is less severe due to their smaller sizes compared to SiO4 and OH precipitates. Moreover, Na2CO3 has also been found to be less corrosive when compared to NaOH and Na4O4Si for sandstone reservoirs [98].
Fig. 1.6: Schematic diagram of alkaline flooding mechanism illustrating the reaction between alkali (NaOH) and acid component of crude oil [99]
1.7.2 Surfactant Flooding
In surfactant flooding, solutions containing surface-active agents are injected into the reservoir for the purpose of mobilizing trapped residual oil [100]. The primary oil recovery mechanism of surfactant flooding are lowering of oil-water IFT due to the adsorption of surfactants on the liquid-liquid interface and changing the reservoir rock wettability [101]. IFT reduction plays a very important role in surfactant flooding and is affected by many factors like the type of surfactants and their concentrations, solvents, salinity, composition of crude oil and reservoir conditions [102]. A correctly designed surfactant system interacts with brine and crude oil to form microemulsions at the interface of oil and water reducing IFT to ultra-low value [103]. In this respect, the phase behavior study of microemulsion is very important in surfactant-based EOR application to evaluate surfactant formulations and as an indicator of ultra-low IFT. Microemulsion phase
ROCK H2O
OIL HAo
A- Na+ OH-
NaOH H M
HAw A-+ H+
HAo H2O
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behavior can be described by Winsor type I, type II, and type III. A change in the phase behavior can be brought about by changing the variable such as salinity, surfactant structure, temperature, and pressure [104]. For an ionic surfactant, microemulsion phase behavior is particularly affected by the salinity or concentration of electrolyte [87].
At low salinity, type I or oil-in-water microemulsions occur and are characterized by coexistence with nearly pure excess oil phase. Whereas, at very high salinity, type II or water-in-oil microemulsions are formed, which are characterized by coexistence with an excess brine phase. In between the type I and type II regions, a narrow intermediate salinity range exists in which oil and water microemulsions are formed as a middle phase and coexist with both excess oil and excess water phases. These are referred to as type III microemulsions and the salinity as optimal salinity. Type III microemulsion and optimal salinity are of great importance in surfactant flooding because of the existence of ultra-low IFT [105, 106]. With low IFT, the residual oil droplets are able to easily flow through the pore throats as the capillary trapping is reduced (Fig. 1.7). These oil droplets travel forward and merge with the oil down the stream leading to the formation of the oil bank [102].
Additionally, surfactant solutions also cause the altering of reservoir rock wettability to more water-wet condition by increasing the spontaneous imbibition of water into matrix blocks, which helps to increase the oil recovery [107]. The wettability mechanism is more prominent in carbonate reservoirs which are likely to be oil-wet.
Fig. 1.7: Mechanism of oil recovery by surfactant flooding illustrating the effect of low IFT on residual oil [[108], modified]
Trapped oil
Trapped oil produced IFT reduced due to
surfactant flooding
Oil is trapped Trapped oil is produced
Rock surface Rock surface
Rock surface Rock surface
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