Introduction and Literature Review
1.5 Different Types of WAG Flooding
Chapter 1 Introduction and Literature Review
(b) Direct Thickener: The use of direct thickeners like soluble polymers that significantly increase CO2 viscosity is sometimes used for mobility control during CO2 flooding. The thickened CO2 gas injected without water improves the displacement efficiency without the water blocking problems and corrosion issues associated with WAG.
(c) CO2-foams: Foam has been used to control gas mobility and improve oil recovery during gas EOR processes. Foam exhibits various favorable attributes which makes it an attractive method for improving oil recovery. Foam reduces the apparent viscosity of the gas and lowers the relative permeability of the liquid, making the mobility ratio favorable. Additionally, foam reduces CO2 mobility by a greater fraction in high-permeability cores than in lower-permeability cores [55, 56]. This unique property of foam is termed as selective mobility reduction which assists in smoothening heterogeneities [57]. The stronger foam generated in the high-permeability zones behaves like a more viscous fluid which diverts fluid to low-permeability zones of the reservoir, thus providing better mobility control to improve Evo [58]. The presence of surfactant during foam flooding, on the other hand, helps to mobilize residual oil by lowering the oil-water IFT value.
Chapter 1 Introduction and Literature Review
held residual oil due to the high oil-water IFT and consequently, the microscopic displacement efficiency is low. Moreover, in the case of viscous oil reservoirs due to the unfavorable mobility ratio, viscous fingering of injected water and early injection gas breakthrough occurs. Thus, major areas of the reservoir with residual oil remain unswept by the injected fluids resulting in low oil recovery [61]. Other concerns associated with CO2-WAG injection are difficulty in controlling gas/CO2 breakthrough as the WAG process matures, huge volumes of water injection delay the project duration, corrosion, and water injectivity loss.
Various studies to improve the performance of CO2-WAG and overcome its limitations have led to the development of the chemically-enhanced-water alternated gas (CEWAG) method. This method combines the benefits of both gas and chemical EOR methods. Different types of chemicals like surfactants, alkalis, co-surfactants, salts, polymers, co-solvents, and nanoparticles are used based on the specific application [57, 62]. The mechanism is referred to as surfactant-alternated-gas (SAG) flooding when surfactants are added to water during the WAG injection process resulting in foam formation in the pore spaces of the reservoir rock [63, 64]. Previous studies have reported improvement in Evo and significant increase in oil recovery by SAG flooding compared to continuous CO2 injection and CO2-WAG injection [65-68].
The higher oil recovery obtained by SAG injection can be attributed to a number of factors like the reduction of oil-water IFT due to the presence of surfactants, better mobility control due to foam formation, and mutual mass transfer between the fluids.
Foam increases the apparent viscosity of CO2 gas thereby reducing its mobility. Thus channeling and viscous fingering problems are alleviated significantly. Additionally, foam also decreases the permeability to water due to the higher trapped gas saturation in pore spaces of the reservoir rock [69]. Although foam can be formed in the reservoir by
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the co-injection or alternate injection of gas and surfactant solutions, the alternate method is preferred over co-injection due to its characteristic advantages [66, 70, 71].
SAG minimizes contact between the water and gas/CO2 in the surface facilities and pipelines reducing corrosion. SAG injection also increases gas injectivity due to changing saturation near the well-bore [72]. Additionally, SAG injection can reduce gravity override problems without increasing injection well pressures that cannot be done with continuous co-injection method [73].
However. surfactants injected with CO2 during the SAG process are susceptible to be adsorbed by the clay minerals in the rock matrix which reduces the efficiency of the foam process [74]. Traditionally, alkali has been used to decrease the adsorption of anionic surfactants onto the reservoir rock. Alkali acts as a sacrificial agent for anionic surfactants by fixing the surface charge to negative values. The negative charge of the surface causes electrostatic repulsion between the rock surface and the surfactant, leading to a significant decrease in adsorption of surfactant [75]. Adding alkali also assists in converting the naturally occurring naphthenic acids in crude oils to produce in situ surfactant (soaps). The combination of injected surfactants and in-situ soaps generated helps to form microemulsion, which exhibits ultra-low oil-water IFT (<0.01 mN/m) thereby mobilizing capillary held residual oil for increasing oil recovery. In association with the CO2 gas, the alkali-surfactant (AS) combination in the chemical slug results in the formation of strong/ stable in-situ foam in the reservoir. This process of alternate injection of gas/CO2 and AS slug is variably referred to as alkaline- surfactant-foam (ASF) flooding, low tension gas (LTG) process, alkali surfactant gas (ASG) injection and alkaline-surfactant-alternated-gas/CO2 (ASAG) flooding in the literature [76-81]. Fig. 1.4 shows a schematic representation of the displacement process in continuous gas injection (CGI), WAG, SAG, and ASAG flooding.
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CO2 Flooding WAG Flooding SAG Flooding ASAG Flooding
Poor Evo Better Evo Improved Evo Best Evo
Viscous fingering Lower mobility ratio Foam formation Stronger foam
Gravity overriding Affected by gravity Gravity segregation low Ultra-low IFT Poor mobility control segregation and Heterogeneities smoothen Better Edo
Early gas breakthrough heterogeneity Higher Ero Highest Ero
Heterogeneity effects
Fig. 1.4: Schematic representation of the displacement process in CGI, WAG, SAG, and ASAG injection scheme [[82], modified]
CO2 Oil Water/CO2 Oil Surfactant/CO2 Oil
Alkali-Surfactant
/CO2 Oil
Chapter 1 Introduction and Literature Review