**2. LSW/gas hybrid EOR technique**

The application of hybrid LSW/gas flooding has recently attracted the attention of different researchers. Various injection schemes have been studied using experimental and simulation approaches. LSW can be injected into water alternating gas (WAG) or simultaneous water alternating gas (SWAG) modes. Moreover, gas may be injected after completion of LSW injection to improve the total oil recovery. CO2 is generally used as the injection gas in this hybrid approach as it is cheap, has a lower minimum miscibility pressure (MMP), and provides environmentally positive results. Injection of LSW affects the solubility of gas in water and, consequently, the gas/oil interaction and oil recovery. There are two different observations during the hybrid injection of gas and LSW. Some researchers reported benefits from the application of this hybrid method, while others did not observe any incremental recovery compared to continuous gas injection (CGI).

Experimental and modeling studies show that the solubility of gas, especially CO2, in brine increases with decreasing salinity of water due to the salting out effect [2–5]. Hence, higher CO2 solubility results in lower amount of free gas available to come into contact with the oil and improve oil mobility, which reduces the oil recovery. On the other hand, many other experiments resulted in higher oil recovery at lower water salinities, as higher solubility increases the gas diffusion in water, which affects the waterfront, mobility ratio, and stability of the injection fluid. Improvement in these parameters leads to lower fingering of the injected fluid that increases its contact with previously bypassed oil droplets in the porous media. Therefore, dissolved gas in water changes the shape of the waterfront which contacts unswept oil more easily than free gas.

In both carbonate and sandstone formations, hybrid LSW/gas injection can be beneficial if it can alter the mobility of the injected fluid. Aleidan and Mamora [6] investigated different schemes of water/CO2 injection such as SWAG and WAG at different salinities to study the effect on oil recovery in carbonate core samples. They observed higher oil recovery for both injection schemes when switching to LSW, as shown in **Figure 1**. More dissolution of gas in brine reduces the mobility of the injected fluid. An important observation in this study was that the LSW alone was not effective; hence, the observed incremental oil recovery was due to the hybrid method and the synergy between gas injection and LSW injection.

The same trend was observed by Jiang et al. [7]. They investigated the effect of water salinity on the performance of WAG during miscible flooding in sandstones using a highly viscous crude oil. Higher solubility of gas in low-salinity water controls the mobility of the water and reduces the mobility ratio of water and viscous oil. Hence, they observed better oil recovery, as shown in **Figure 2**.

*Hybrid EOR Methods Utilizing Low-Salinity Water DOI: http://dx.doi.org/10.5772/intechopen.88056*

### **Figure 1.**

*Enhanced Oil Recovery Processes - New Technologies*

**2. LSW/gas hybrid EOR technique**

contacts unswept oil more easily than free gas.

injection (CGI).

The idea of combining two (or more) EOR methods, known as hybrid methods,

has been investigated recently to promote the activation of several oil recovery mechanisms to increase the ultimate oil recovery, tackle operational challenges, reduce environmental damage, and lower the production costs. Hybrid methods can be optimized for different injection scenarios to achieve the highest feasible recoveries. LSW flooding has been found to be effective when combined with gas injection (mainly CO2), surfactant and/or polymer flooding, nanofluid injection, and hot water injection, each of which can improve the oil recovery through several mechanisms such as mobility control, wettability alteration, IFT reduction, etc. Experimental and modeling studies reviewed in this chapter have found that LSW hybrid methods can

provide up to 30% original oil-in-place (OOIP) incremental oil recovery.

The application of hybrid LSW/gas flooding has recently attracted the attention of different researchers. Various injection schemes have been studied using experimental and simulation approaches. LSW can be injected into water alternating gas (WAG) or simultaneous water alternating gas (SWAG) modes. Moreover, gas may be injected after completion of LSW injection to improve the total oil recovery. CO2 is generally used as the injection gas in this hybrid approach as it is cheap, has a lower minimum miscibility pressure (MMP), and provides environmentally positive results. Injection of LSW affects the solubility of gas in water and, consequently, the gas/oil interaction and oil recovery. There are two different observations during the hybrid injection of gas and LSW. Some researchers reported benefits from the application of this hybrid method, while others did not observe any incremental recovery compared to continuous gas

Experimental and modeling studies show that the solubility of gas, especially CO2, in brine increases with decreasing salinity of water due to the salting out effect [2–5]. Hence, higher CO2 solubility results in lower amount of free gas available to come into contact with the oil and improve oil mobility, which reduces the oil recovery. On the other hand, many other experiments resulted in higher oil recovery at lower water salinities, as higher solubility increases the gas diffusion in water, which affects the waterfront, mobility ratio, and stability of the injection fluid. Improvement in these parameters leads to lower fingering of the injected fluid that increases its contact with previously bypassed oil droplets in the porous media. Therefore, dissolved gas in water changes the shape of the waterfront which

In both carbonate and sandstone formations, hybrid LSW/gas injection can be beneficial if it can alter the mobility of the injected fluid. Aleidan and Mamora [6] investigated different schemes of water/CO2 injection such as SWAG and WAG at different salinities to study the effect on oil recovery in carbonate core samples. They observed higher oil recovery for both injection schemes when switching to LSW, as shown in **Figure 1**. More dissolution of gas in brine reduces the mobility of the injected fluid. An important observation in this study was that the LSW alone was not effective; hence, the observed incremental oil recovery was due to the hybrid method and the synergy between gas injection and LSW injection.

The same trend was observed by Jiang et al. [7]. They investigated the effect of water salinity on the performance of WAG during miscible flooding in sandstones using a highly viscous crude oil. Higher solubility of gas in low-salinity water controls the mobility of the water and reduces the mobility ratio of water and viscous

oil. Hence, they observed better oil recovery, as shown in **Figure 2**.

**2**

*Recovery factor of oil and cumulative water production at different salinity levels (0, 6, and 20%) (i.e., 0 wt.%-oil and 0 wt.%-water are oil production and water production by water injection with salinity of 0%, respectively): (a) SWAG, (b) WAG [6].*

### **Figure 2.**

*The effect of brine salinity (in ppm) during hybrid LSW/gas WAG injection on oil recovery at secondary (waterflooding) and tertiary modes [7].*

Kumar et al. [8] experimentally studied the effect of hybrid smart water/CO2 flooding into sandstone samples with high clay content. 0.5 NaCl, 0.5 KCl, and 0.5 wt% MgCl2 solution were used as smart brines for the hybrid method. They observed that smart water alternating gas injection controlled the mobility ratio of the fluids injected and reduced fluid channeling and oil bypassing effects. Hence, smart water alternating immiscible gas flooding recovered more oil than the stand-alone smart water and continuous gas injection (CGI) in their experiments. It should be noted that the rock samples in their study were initially water wet; therefore, wettability alteration by smart water was not the dominant mechanism during the hybrid EOR.

If the initial mobility ratio of water and oil is favorable, application of the hybrid method does not provide any benefit, as it does not affect the flow front stability. In these cases, salting out effect becomes more dominant, which reduces the amount of free gas contacting oil during the hybrid method. Hence, switching to LSW alternating gas (LSWAG) reduces the oil recovery. In [7], the LSWAG method was used to improve the recovery of a model oil composed of *n*-decane and *n*-hexadecane in sandstones. Due to the low viscosity of the oil and the homogeneity of the small artificial cores used in their experiments, the initial sweep efficiency was high, and LSWAG did not have a noticeable effect on oil recovery as shown in **Figure 3**. Hence, LSWAG is not recommended at these conditions.

Another factor that should be considered for an effective hybrid LSW/gas method is the performance of CGI. Wherever the recovery by stand-alone gas injection is high, application of the hybrid method does not provide noticeable incremental oil. For example, Al-Shalabi et al. modeled the hybrid injection of LSW and miscible CO2 into carbonates using UTCOMP simulator. In their studies, the recovery factor by CGI was 98.9%, and shifting to the hybrid method just changed it to 99.7% by viscous fingering control [9]. Hence, in similar conditions, where the dominant mechanism is the miscibility of the gas, application of the hybrid method is not recommended.

An important parameter that affects the success of the hybrid LSW/gas method is the initial wettability of the rock. Wettability alteration from oil wet to water wet is considered as one of the main reasons for the positive performance of LSW, especially in sandstones [10]. Hence, if the initial wettability of the rock is water wet, LSW and consequently the hybrid method do not work. In this condition, the salting out effect controls the recovery mechanism during hybrid injection. Ramanathan et al. experimentally studied seawater alternating gas (SeaWAG) and LSWAG injection to recover oil from water-wet sandstone [11]. The recovery factor in LSWAG was lower than SeaWAG as the rocks were initially strongly water wet. Conversely, in an aged oil wet core, recovery by WAG changed from 76 to more than 97% when the water utilized changed from seawater to low-salinity brine, as shown in **Figure 4**.

The importance of the initial wettability was also confirmed by Teklu et al. [1]. They did several contact angle measurements for carbonate, sandstone, and shale samples at different initial wettability conditions, as shown in **Figure 5**, in which, case A shows the initial contact angle of an oil-wet disk, while case D shows the original water-wet condition of rock before aging by the oil. B and C cases show alteration in the contact angle by sample aging in seawater and CO2 and in lowsalinity brine and CO2, respectively. LSW/CO2 is more effective in the presence of CO2 because the wettability of the rock changes toward a more hydrophilic surface. As shown, this mechanism is more effective when the initial wettability is oil wet.

In [13], Al-Abri et al. experimentally studied the hybrid immiscible CO2 and smart water injection into sandstone core samples. They investigated the synergistic effects between gas injection and different ions in the water samples. Three synthetic brines were used in their work which contained 5000 ppm NaCl, MgCl2, and KCl, respectively. Considerable improvement in oil recovery was observed, as shown in **Figure 6**. The maximum solubility of CO2 in brine was observed in the

### **Figure 3.**

*The effect of salinity during hybrid LSWAG injection on recovery of low-viscosity oil (green curve is for secondary water flooding, red curve is for LSWAG, and blue curve is the total recovery) [7].*

*Hybrid EOR Methods Utilizing Low-Salinity Water DOI: http://dx.doi.org/10.5772/intechopen.88056*

*Enhanced Oil Recovery Processes - New Technologies*

is not recommended.

in **Figure 4**.

Another factor that should be considered for an effective hybrid LSW/gas method is the performance of CGI. Wherever the recovery by stand-alone gas injection is high, application of the hybrid method does not provide noticeable incremental oil. For example, Al-Shalabi et al. modeled the hybrid injection of LSW and miscible CO2 into carbonates using UTCOMP simulator. In their studies, the recovery factor by CGI was 98.9%, and shifting to the hybrid method just changed it to 99.7% by viscous fingering control [9]. Hence, in similar conditions, where the dominant mechanism is the miscibility of the gas, application of the hybrid method

An important parameter that affects the success of the hybrid LSW/gas method is the initial wettability of the rock. Wettability alteration from oil wet to water wet is considered as one of the main reasons for the positive performance of LSW, especially in sandstones [10]. Hence, if the initial wettability of the rock is water wet, LSW and consequently the hybrid method do not work. In this condition, the salting out effect controls the recovery mechanism during hybrid injection. Ramanathan et al. experimentally studied seawater alternating gas (SeaWAG) and LSWAG injection to recover oil from water-wet sandstone [11]. The recovery factor in LSWAG was lower than SeaWAG as the rocks were initially strongly water wet. Conversely, in an aged oil wet core, recovery by WAG changed from 76 to more than 97% when the water utilized changed from seawater to low-salinity brine, as shown

The importance of the initial wettability was also confirmed by Teklu et al. [1]. They did several contact angle measurements for carbonate, sandstone, and shale samples at different initial wettability conditions, as shown in **Figure 5**, in which, case A shows the initial contact angle of an oil-wet disk, while case D shows the original water-wet condition of rock before aging by the oil. B and C cases show alteration in the contact angle by sample aging in seawater and CO2 and in lowsalinity brine and CO2, respectively. LSW/CO2 is more effective in the presence of CO2 because the wettability of the rock changes toward a more hydrophilic surface. As shown, this mechanism is more effective when the initial wettability is oil wet. In [13], Al-Abri et al. experimentally studied the hybrid immiscible CO2 and smart water injection into sandstone core samples. They investigated the synergistic effects between gas injection and different ions in the water samples. Three synthetic brines were used in their work which contained 5000 ppm NaCl, MgCl2, and KCl, respectively. Considerable improvement in oil recovery was observed, as shown in **Figure 6**. The maximum solubility of CO2 in brine was observed in the

*The effect of salinity during hybrid LSWAG injection on recovery of low-viscosity oil (green curve is for* 

*secondary water flooding, red curve is for LSWAG, and blue curve is the total recovery) [7].*

**4**

**Figure 3.**

**Figure 4.** *Oil recovery and pressure drop across an oil-wet core during LSW alternating CO2 [12].*

**Figure 5.**

*Contact angle alteration by sample aging in high-salinity brine/CO2 and low-salinity brine/CO2 [1].*

water containing MgCl2, which also showed the lowest oil recovery among the three tests. In their work, smart water samples were effective, altering the wettability of the rock to more water wet without gas injection due to multicomponent ion exchange.

Hence, generally, wherever the LSW alone is effective, the hybrid method shows good performance and provides higher oil recovery than CGI and high-salinity water alternating gas. This point was also observed and confirmed by [14, 15]. AlQuraishi et al. [14] showed that low-salinity alternating miscible CO2 method was useless for clay-free sandstones, but wherever clays were present, the recovery value was 35.1% of the OOIP for LSWAG.

A study by Yang et al. [16] showed that at a constant pressure and temperature conditions, the presence of CO2 reduces the oil/brine IFT. Hence, this may also be considered as one of the mechanisms for incremental oil recovery by the hybrid method. References [1, 12] observed a reduction in IFT of less than 10 dynes/cm. On the other hand, in [8], higher IFT in the presence of CO2 was reported. It should be noted that the alteration in IFT is not significant and cannot be considered as a dominant mechanism during the hybrid LSW/gas approach. In [17], Bennion

### **Figure 6.**

*Oil recovery and pressure drop for hybrid smart water alternating CO2 flooding. (a) 5000 ppm MgCl2 (top left), (b) 5000 ppm NaCl (top right), and (c) 5000 ppm KCl (bottom) [13].*

et al. experimentally concluded that the increase in CO2 solubility in low-salinity brine reduces the IFT of CO2 and brine, which may lead to wettability alteration. Therefore, it can be considered that wettability alteration and mobility control are the dominant mechanisms for hybrid LSW/gas methods. In cases in which the initial wettability is water wet, the LSW stand-alone is not effective. In these situations, mobility control is the mechanism that shows the most important effect in improving oil recovery.

Different injection schemes have been applied to study the benefits of hybrid methods. Besides WAG and SWAG, the injection of CO2 after LSW flooding provides extra oil recovery and can be considered as a novel approach for application in oil fields. Teklu et al. observed more than 20% incremental oil recovery by this approach [1].

Hybrid methods are also beneficial in terms of decreasing the operational costs of enhanced oil recovery processes. Previous research [9] has shown that the application of simultaneous LSW alternating gas leads to faster production of oil, as shown in **Figure 7**. This occurs due to the alteration of the reservoir rock wettability that increases the oil relative permeability. Also, the application of hybrid gas/ LSW methods reduces the gas utilization factor. Kulkarni et al. observed a lower gas utilization factor during LSW alternating miscible CO2 flooding [18]. However, this issue should be considered carefully during immiscible flooding, because the higher solubility of CO2 in LSW requires more gas to make contact with the oil, which increases the gas utilization factor. Thus, more experimental and modeling studies are required in this area.

Most of the previous research in this area has focused on the performance of hybrid methods in sandstones. Analysis of the special interaction of LSW and gas with carbonate rock needs more investigation. For example, rock dissolution is considered as an effective mechanism during LSW injection in carbonates. Hence, geochemical analysis is essential to study the hybrid LSW/gas injection in rocks with high calcite and dolomite content. An earlier work [19] has performed simulations to compare the geochemical analysis results of LSW, CGI, and hybrid methods for different types of carbonates. Results indicated that hybrid methods can accelerate the dissolution process especially for high dolomite concentrations. Consequently, comprehensive experimental studies are required to investigate the *Hybrid EOR Methods Utilizing Low-Salinity Water DOI: http://dx.doi.org/10.5772/intechopen.88056*

**Figure 7.**

*Enhanced Oil Recovery Processes - New Technologies*

improving oil recovery.

are required in this area.

approach [1].

**Figure 6.**

et al. experimentally concluded that the increase in CO2 solubility in low-salinity brine reduces the IFT of CO2 and brine, which may lead to wettability alteration. Therefore, it can be considered that wettability alteration and mobility control are the dominant mechanisms for hybrid LSW/gas methods. In cases in which the initial wettability is water wet, the LSW stand-alone is not effective. In these situations, mobility control is the mechanism that shows the most important effect in

*Oil recovery and pressure drop for hybrid smart water alternating CO2 flooding. (a) 5000 ppm MgCl2 (top* 

*left), (b) 5000 ppm NaCl (top right), and (c) 5000 ppm KCl (bottom) [13].*

Different injection schemes have been applied to study the benefits of hybrid methods. Besides WAG and SWAG, the injection of CO2 after LSW flooding provides extra oil recovery and can be considered as a novel approach for application in oil fields. Teklu et al. observed more than 20% incremental oil recovery by this

Hybrid methods are also beneficial in terms of decreasing the operational costs of enhanced oil recovery processes. Previous research [9] has shown that the application of simultaneous LSW alternating gas leads to faster production of oil, as shown in **Figure 7**. This occurs due to the alteration of the reservoir rock wettability that increases the oil relative permeability. Also, the application of hybrid gas/ LSW methods reduces the gas utilization factor. Kulkarni et al. observed a lower gas utilization factor during LSW alternating miscible CO2 flooding [18]. However, this issue should be considered carefully during immiscible flooding, because the higher solubility of CO2 in LSW requires more gas to make contact with the oil, which increases the gas utilization factor. Thus, more experimental and modeling studies

Most of the previous research in this area has focused on the performance of hybrid methods in sandstones. Analysis of the special interaction of LSW and gas with carbonate rock needs more investigation. For example, rock dissolution is considered as an effective mechanism during LSW injection in carbonates. Hence, geochemical analysis is essential to study the hybrid LSW/gas injection in rocks with high calcite and dolomite content. An earlier work [19] has performed simulations to compare the geochemical analysis results of LSW, CGI, and hybrid methods for different types of carbonates. Results indicated that hybrid methods can accelerate the dissolution process especially for high dolomite concentrations. Consequently, comprehensive experimental studies are required to investigate the

**6**

*Faster production of oil by LSW alternating gas injection than CGI [9].*

geochemical parameters during hybrid LSW/gas injection methods to explain the oil recovery mechanisms of these processes.

Up to now, most of the work conducted in this area has been experimental research. There are few simulation studies in this field. Dang et al. [20] simulated a hybrid LSW alternating miscible CO2 flooding injection in a 1D heterogeneous core and then upscaled the model to simulate the process at field scale. Their study showed that the hybrid approach overcomes the WAG late production problem. We recommend a comprehensive field-scale simulation study based on experimental work to analyze the practical benefit of hybrid approaches.
