Investigation of Different Ionic Liquids in Improving Oil Recovery Factor

In order to improve oil recovery, Enhanced Oil Recovery (EOR) techniques have been applied to several light and medium oil reservoirs. This research was directed towards the development of chemical flooding methods for such reservoirs. The main objective of this experimental work was to investigate the efficiency of introducing various types of Ionic Liquids (ILs), 1-Ethyl-3-methylimidazolium Chloride [EMIM][Cl], 1-Benzyl-3-methylimidazolium Chloride [BenzMIM][Cl], and Trihexyltetradecylphosphonium Chloride [THTDPh][Cl] on the Recovery Factor (RF) of medium oil (Weyburn oil, 30.25 API ̊) at room temperature. The series of flooding experiments were carried out by introducing a slug of IL mixtures. Results demonstrated that maximum oil recovery factor was obtained when [EMIM][Cl] was added in the displacing fluid. Further investigations have been conducted to examine the effect of ILs concentrations on the recovery mechanisms by measuring Surface Tension (SFT), pH, and viscosity of the displacing phases. Finally, the effect of theses ILs in wettability alteration was examined.


Introduction
Given the depletion of oil reservoir energy and the limited discovery of new reservoirs, petroleum researchers have begun seeking more efficient techniques; one of the most promising methods is Chemical Enhanced Oil Recovery (CEOR), which has been used over the last three decades.The entrapped oil can be recovered by introducing chemical fluids to the porous media to reduce the A. Alarbah  system's Interfacial Tension (IFT), increase the capillary number, maintain mobility control, decrease oil viscosity, and change reservoir wettability [1].In recent times, chemicals called Ionic Liquids have been used to enhance the oil recovery factor.
Many studies have measured the properties of ionic liquids at different concentrations mixed either with water or other solvents.These investigations discovered that some ionic liquid types are capable of increasing mixture viscosity [2].Kelkar and Maginn (2007) found that the effect of the ionic liquid on viscosity depends on its concentration in the mixture [3].It has been concluded from Lafo et al. (2013) studies that the brine viscosity increases by adding Trihexyl phosphonium chloride ionic liquid, which reduces the mobility ratio between heavy oil and brine, and results in a higher oil recovery [4].The effect of ILs on some surface behaviors such as surface tension has not yet been thoroughly studied as an aspect of enhanced oil recovery.Surface tension values are used as indications of surface interactions at the liquid interfacial microstructure [5].To enhance the oil recovery process, it was observed that IFT increases as the concentration of salt in the aqueous phase increases.A small amount of chemical such as surfactant in the aqueous solution has the effect of decreasing the interfacial tension with salinity [6].The Critical Micelle Concentration (CMC) is one of the critical surface activity parameters and at this concentration, the surfactant solution cannot reduce the IFT and the SFT any further [7] [8].The CMC value can be determined either from measuring the IFT [9] or SFT [10].
Wettability is one of the major mechanisms that affect fluids flow in the reservoirs [11].Limited studies have been carried out to demonstrate the effects of wettability alteration by ionic liquids.Bin-Dahbag et al. (2014) have done a series of flooding experiments on Berea sandstone samples by using different ionic liquids concentrations to investigate the wettability alteration [12].It was concluded that ILs have the ability to shift the rock wettability from oil wet towards water wet as a result of the interaction between oil, rock, and IL [12].A similar study was conducted by Mohammed and Babadagli (2016) to investigate the effect of several imidazolium ionic liquids to modify the wettability of oil-wet limestone and sandstone.Another study showed that ionic liquids were more efficient in changing the rock wettability than other surfacants [13].
Few studies have demonstrated the results of extracting more oil from consolidated and unconsolidated samples by using several ILs mixed with synthetic water or chemicals as tertiary recovery.Regarding core flooding experiments,

Experimental Work 2.1 Materials
The oil sample used for the studies was taken from Weyburn crude oil with (30.25 API˚ and 15.35 cP).The properties of the oil and the Saturates, Aromatics, Resins and Asphaltenes (SARA) fractions are presented in Table 1.Weyburn brine (56,000 ppm) was used for the initial saturation of the sand-pack samples  2 shows the properties of aqueous solutions.Ottawa sand, 40 -80 mesh, was used to prepare the unconsolidated sand-pack.

Experimental Apparatus
The apparatus used in this study consists of the following components: Physical model: The physical model consisted of 18.75 cm steel core holder with 4 cm inner diameter.
Injection and pressure monitoring system: The conventional core flooding system was used with a 500 D syringe pump to inject the fluids into the sand pack.The pump was connected to three transfer cylinders each with a floating piston with a capacity of 900 cm 3 containing brine, or oil, or chemical mixtures.
The pressure data for each experiment was measured with a Heise Pressure   Pores media and Displacing Fluids: By using sieve analysis, the average size of Ottawa sand was controlled at 40 -80 mesh.The displaced fluids' properties were measured by the subsequent apparatus.A digital weight meters quantified the digital masses of specific chemicals, which were added to the brine to prepare the displacing phases.Then, the solutions were stirred for 30 -45 minutes at a speed of 120 rpm to allow the IL solutions to mix adequately.The polymer solutions were stirred for 24 hrs at 60 rpm.The viscosities of displacing fluids were measured by DV-II + Pro viscometer (supplied by Brookfield) with a temperature controller water-path.In order to measure the densities, the Anton Paar DSA 500 M instrument was used.A KRUSS K100 device was utilized to measure the SFT of the displacing mixtures by using the Wilhelmy plate method; a Cole-Parmer water path was implemented to control the temperature).The pH and conductivity values of the displacing phases were measured with a pH/COND Meter (supplied by HORIBA).

Experimental Procedure
A preparatory stage during which all apparatus were cleaned and calibrated was undertaken prior to research implementation.The following paragraphs describe in detail the preparatory stages: Packing Procedure: A vertically-oriented core holder was packed utilizing the dry-packing method, using (40 -80 mesh) Ottawa sand with consistent sand size parts.The core sample was vibrated during the packing process to distribute, therefore making the sand grains uniform in the core holder.The sand-pack sample was 100% vacuumed by using vacuum pump for approximately 10 hrs (until no bubble appeared); this step was preceded by packing the sand and fixing the cap after which the weight was measured.Subsequently, the model was connected to the pump and fully saturated with Weyburn brine.Then the porosity and the absolute permeability were calculated.The average petrophysical properties of the sand-pack samples are presented in Table 3.
Saturation Process: After finishing the routine core analysis, both the core holder and the core flooding system were connected vertically to determine the irreducible water saturation and the initial oil saturation, at a rate of 1 cm 3 /min.The sand-pack flooded with medium oil until no free water seeped out from the core holder outlet.The total displaced brine represents the initial oil saturation (So i ) while the remaining brine represents the irreducible water saturation (Sw i ).
The moment that the core sample was thoroughly saturated with oil, determined that the sand pack was ready for the flooding experiments.
Flooding Procedure: In this stage, the cores were positioned horizontally and flooded with either ILs, or polymer mixtures alone or ILs, and polymer mixture.
For one of the scenarios the core samples were initially flooded with 1 pore volume of chemicals mixture then flushed by two pore volumes of brine.The optimum flow rate (2 cm 3 /min) was applied as injection rate for flooding experiments that were done to investigate the effect of chemical concentration, the chemical solution slug size, the initiation time for injecting the chemical mixture as well as the flow rate that was obtained from previous study [16].
The produced oil and water samples from the sand pack were collected in graduated tubes and then placed in a separator to determine the exact amount of recovered oil.The total oil recovery factor was expressed as the ratio of produced oil to the original oil in place.
Subsequently, all the above-mentioned procedures were carried out at room temperature (21.5˚C ± 1˚C), and they all have been repeated with unused sand to ensure the consistency.

Results and Discussion
Effect of IL Type on RF: To test the effectiveness of ILs on the enhanced oil Table 3.The average petrophysical properties of the sand-pack samples.[Ac] concentration in the mixtures lead to an increase in the PH value of the solutions whereas the effect of temperature was minimal [17].As anticipated, the increase in water content led to declining the PH values of extraction mixtures.This is presented in the previous Table 4, and Table 5).).These differences in RF values could be due to the differential impact of ionic liquids on wettability.The K ro and K rw during these runs were measured by using the relative permeability graphical technique.This method was elaborated by Jones and Roszelle [18].Relative permeability curves, as presented in Figure 8, clearly show that the maximum shift in K ro and K rw intersection (S w = 0.82, K ro and K rw = 0.08) was obtained after flooding

Effect of Wettability
and flooding experiments.([EMIM][Cl], 98 wt%), ([BenzMIM][Cl], 97 wt%), and ([THTDPh][Cl], 95 wt%) were used to investigate their efficiency on improving oil recovery factor).These chemicals were purchased from Sigma-Aldrich and used without any additional purification.The chemical structures of the employed ILs are shown in Figure 1.Also, different displacing fluids were made in the laboratory by mixing brine with the above mentioned chemicals that were later used in sand-pack flooding experiments, Table

Figure 1 .
Figure 1.The Chemical Structures of Ionic Liquids.

Figure 2 .
Figure 2. Ionic liquid type effect on the recovery factor.

Figure 6 .
Figure 6.Effect of CMC of Ionic liquids on RF.

Figure 7 .
Figure 7. Effect of CMC Ionic liquids on RF in Continuous Secondary Recovery Mode.
et al.

Table 1 .
Properties of the Weyburn at 21.5˚C ± 1.

Table 2 .
Properties of Aqueous Solutions.

Table 4 .
Properties of IL Mixtures at 1000 ppm.To investigate the effect of the ILs' CMC on extracting more oil, the CMC for each of these ILs was mixed with brine and injected into the sand pack.The flooding procedure was similar to the optimum scenario obtained from the previous experiments.As presented in Figure6, the results showed that the effectiveness of adding the CMC of the ILs [EMIM][Cl], [BenzMIM][Cl], and [THTDPh][Cl]) to brine and flooding the mixture into the sand packs, more oil was able to be extracted than when 1000 ppm of the same ILs mixtures were injected.The additional RF values of injecting the CMC of [EMIM][Cl], [Benz-MIM][Cl] and [THTDPh][Cl] were 1.74 [% OOIP], 2.50 [% OOIP], and 1.41 [% OOIP], respectively.This enhancement of oil recovery could be due to the reduction in SFT and slight increase in viscosity as presented in Table5.Overall, the results indicate that using the CMC of ILs and when mixed with brine was efficient in improving the RF.

Table 5 .
Properties of ILs Mixtures at CMC.