A Novel Nanodrag Reducer for Low Permeability Reservoir Water Flooding : Long-Chain Alkylamines Modified Graphene Oxide

1State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China 2Research Institute of Natural Gas Technology, Petro China, Chengdu 610213, China 3Sichuan Chuangang Group Corporation, Limited, Chengdu 610017, China 4Sichuan Province Key Laboratory of Oil and Gas Reservoir of Applied Chemistry, Southwest Petroleum University, Chengdu 610500, China


Introduction
Graphene, an allotrope of carbon, has attracted extensive attention from both experimental and theoretical scientific communities due to its extraordinary properties, such as high structural strength, high surface-area to volume ratio, and excellent electrical properties [1][2][3][4][5][6].Therefore, many exciting applications have been envisioned and demonstrated [7][8][9][10][11].Graphene could be produced through several methods, such as mechanical exfoliation, chemical vapor deposition, electric arc discharge, and oxidation-dispersion-reduction.Among these methods, the oxidation-dispersion-reduction technique is widely used for it possesses great potential for larger-scale synthesis of graphene.
Various graphene-based materials could be conveniently synthesized from graphene oxide (GO) which is obtained from natural graphite flakes by the chemical oxidation [12][13][14][15].GO is a layered material featuring a variety of oxygencontaining functionalities with epoxy and hydroxyl groups on the basal plane and carbonyl and carboxyl groups along the edge [16].On the one hand, these oxygen-containing functionalities could provide a platform for rich chemistry to occur both within the intersheet gallery and along sheet edge.On the other hand, it could render the graphene oxide layers hydrophilic and well dispersed in polar solvents to form colloidal suspensions [17,18].Recently, more investigations have been focused on the surface property and wettability of graphene films [19,20].Understanding the wettability of GO is important for the in-depth study of graphene-based functional materials.In general, the wettability of a solid surface is strongly influenced by both chemical composition and geometric structure (or surface roughness) [21][22][23].
To date, the reported organic functionalized methods of graphene include the reaction of GO with isocyanate groups, amide coupling between carboxylic acid and alkylamine, aryl diazonium functionalization, and the functionalization through the 1,3-dipolar cycloaddition of azomethine ylide [24].Rafiee et al. reported that the surface roughness could be tuned by changing the solvent composition, which produced graphene films with the surface property ranging from superhydrophobic to superhydrophilic [25].Zhang et al. indicated that the surface wettability of graphene film was greatly affected by the surface morphologies and achieved reversible transition from hydrophobicity to hydrophilicity via periodic alternation of ultraviolet irradiation and air storage [26].Many significant studies have been done on the effect of amine modified GO on the surface wettability properties.Lin et al. prepared GO films with a contact angle of 53.8 ∘ , and it increased to 132.4 ∘ after chemically grafting octadecylamine, which reduced the surface energy and enhanced surface roughness of the GO sheets [27].
The wettability of GO was transformed from hydrophilicity to hydrophobicity after surface modification.The pressure drop mechanism of hydrophobic nanometer materials is through hydrophobic nanometer adsorbing on the surface of porous walls and nanoparticles layer forming instead of hydrated layer.Consequently, wettability of pore wall turned from hydrophilicity to hydrophobicity, and slip effects on nanoparticles layer happened, and this decreased the flow resistance obviously and made fluid velocity obviously faster.
With the quick development of oil and gas exploitation, many middle and high permeability reservoirs have been in medium or high water cut period development stage, having a problem of lower water flooding recovery, and the low permeability reservoirs reserve account for a larger proportion.As a result, the low permeability reservoirs will be the major developing resources in the next development stage.Unfortunately, the development of low permeability reservoirs was limited by their poor properties, such as low permeability, low abundance, small porosity, low single well production, serious heterogeneity, and high residual oil saturation of water flooding, as well as the poor injectionproduction connectivity, which caused water injection pressure to increase rapidly and water intake capacity to decrease significantly during the water flooding process.In order to reduce water injection pressure and increase water injection volume, we should reduce the water flow resistance during water flooding.
In this paper, we wondrously found that the hydrophobic M-GO could be used as nanodrag reducer in the oilfield water flooding.We demonstrated a facile way for the synthesis of hydrophobic M-GO as nanodrag reducer through grafting octylamine (OA), dodecylamine (DDA), tetradecylamine (TDA), hexadecylamine (HDA), and octadecylamine (ODA) to GO surface.Also, we evaluated the performance of the M-GO as drag reducer.With regard to the properties of drag reduction, the hexadecylamine-and octadecylaminemodified GO would find potential applications in low permeability reservoirs.

Preparation of GO and M-GO.
GO was synthesized by the improved Hummer's method [28].
M-GO, including OA-modified GO (GO-OA), DDAmodified GO (GO-DDA), TDA-modified GO (GO-TDA), HAD-modified GO (GO-HDA), and ODA-modified GO (GO-ODA), were conveniently prepared through the following steps.The as-prepared GO was dispersed in DI water (1 mg/mL) by ultrasonication for 2 h and centrifuging at 8000 rpm for 30 min to remove aggregates.Ratio of reactants was GO (1.0 g) : alkylamine (0.01 mol).For example, 269 mg of ODA was dissolved in 200 mL ethanol, and then the ODA solution was added in 100 mL GO dispersion at 60 ∘ C and stirred slowly for 12 h.The mixture was filtered and washed with ethanol and DI water to remove unreacted alkylamine.Finally, the GO-ODA was dried at 60 ∘ C in a vacuum oven for 24 hours.Similar synthesis steps were followed in preparing GO-OA, GO-DDA, GO-TDA, and GO-HDA.

Characterization.
The FTIR characterizations of GO and M-GO were performed using ThermoFisher Scientific Nicolet 6700 at ambient temperature.XRD measurements were performed directly on the powder sample using PANalytical XPERT-PRO diffractometer (40 kV, 40 mA) with Cu ( = 1.54 Å) irradiation.TGA was carried out with NETZSCH STA 449F3.The heating rate was 10 ∘ C/min.Each time ∼5 mg sample was measured in an aluminum crucible under inert gas atmosphere in 40∼800 ∘ C. Contact angle was determined using KRUSS DSA30.The slices of core were soaked into tetrahydrofuran solution which contain M-GO (20 mg/L).The solvent was heated and volatilized, depositing M-GO on the slices of core.Scanning electron microcopy (SEM) measurements were carried out on a field emission scanning electron microanalyzer (FEI Quanta 450) at an accelerating voltage of 20 kV.Raman spectroscopy was performed at ambient temperature using a HR800 Raman spectrometer at 532.2 nm wavelength incident laser light.sure during the total injection process was recorded.The parameters of homemade cores were listed in Table 2.The fixed injection rate was 1 mL/min.The detailed process is as follows:

Evaluation of
(1) Injection of DI water in the low permeability cores and determination of the initial water-phase permeability.
(2) Displacement of cores with 0 # diesel fuel to set bound water.
(3) Displacement of cores with DI water again to set bound oil (enabling determination of the stable water flooding pressure  1 ).
(5) Injection of subsequent water flooding until there was no M-GO displaced out (enabling determination of the stable water flooding pressure  2 ).

Results and Discussion
GO is a nonstoichiometric material prepared by improved Hummer's method, decorating with lots of oxygencontaining functional groups such as hydroxyl, carboxyl, carbonyl group and epoxy group.A previous study showed that hydrophobic long-chain alkyl groups in alkylamines and hydrophilic amine groups chemically reacted with the GO surface mainly via two types of reaction, namely, (1) amidation reaction between amine groups and carboxylic acid sites of GO and (2) nucleophilic substitution reactions between amine and epoxy groups on GO surface.FTIR spectra of GO and M-GO were presented in Figure 2.For the GO spectrum, the major IR stretching vibrations at 3365 cm −1 , 1729 cm −1 , 1623 cm −1 , 1223 cm −1 , 1047 cm −1 , 874 cm −1 , and 826 cm −1 proved that GO contains hydroxyl, carboxyl, carbonyl, and epoxy groups.The existing of these four kinds of oxygen-containing functional groups indicated that GO was hydrophilic, so it could be well dispersed in water to form stable colloidal solution.After alkylamines modification, the peak at 3200 cm −1 ∼3700 cm −1 which was attributed to water molecules weakened significantly; for long alkylamine chains (GO-TDA, GO-HAD, and GO-ODA), the peak disappeared completely.Meanwhile, the peak at ∼1729 cm −1 was related to stretching of the C=O bond of carbonyl or carboxyl groups, which disappeared completely, and the peaks at 874 cm −1 and 826 cm −1 which were related to C-O-C bond intensity decreased.The changing indicated a strong reaction between GO and alkylamines.In addition, the amidation of carboxyl along the GO sheet edges with amino of alkyl amine dominated the reaction.The GO-DDA, GO-TDA, GO-HAD, and GO-ODA showed peaks at 1465 cm −1 and 1646 cm −1 attributed to N-H and amide groups, respectively.The most significant change in FTIR spectrum of M-GO was the large intensity increase for peaks at 2920 cm −1 and 2850 cm −1 , corresponding to the stretching of C-H bond, which indicated the successful introduction of long hydrocarbon chains into GO sheets.
The XRD pattern of M-GO was shown in Figure 3.For the GO, the (001) peak appeared at 2 = 10.6 ∘ (Figure 3(a)), indicating an interlayer spacing of 0.83 nm.After modification, the GO (001) diffraction peak shifted forward (Figure 3(b)); for instance, GO-OA, GO-DDA, GO-TDA, GO-HAD, and GO-ODA showed a broad peak at 2 = 8.5 ∘ , 4.6 ∘ , 4.1 ∘ , 3.9 ∘ , and 3.6 ∘ , respectively.According to Bragg equation, the corresponding distance between two adjacent M-GO sheets was 1.04 nm, 1.92 nm, 2.15 nm, 2.26 nm, and 2.42 nm, which were all larger than that of original GO 0.83 nm due to the successful grafting long hydrocarbon chain onto GO sheet.At the same time, it could be found that the XRD pattern of M-GO showed a new peak around 20∼21 ∘ closing to pristine graphite (002) diffraction peak (around 26 ∘ , Figure 3(a)), which resulted from the disordered stacking of some reduced oxide-containing groups of GO sheets.
Figure 4 showed the TGA curves of GO and GO-ODA.Below 100 ∘ C, GO exhibited a ∼11% weight loss, which could be attributed to the evaporation of water trapped between hydrophilic GO sheets, while the M-GO showed a significant decrease of weight loss, GO-OA only lost 5.5%, and the other four kinds of M-GO kept at a lower level around 2%∼3%, indicating an enhanced hydrophobicity that minimized the amount of adsorbed water compared to pristine GO.Increasing the chain length of the grafted amines resulted in the decrease of weight loss below 100 ∘ C. In addition, the GO showed nearly 39% weight loss from 100 ∘ C∼300 ∘ C; however, the M-GO only lost 20.1%, 21.8%, 19.35, 17.2%, and 13.9%, which was assigned to the decomposition of some oxide-containing groups.So accompanying modification procedure, a part of oxide-containing groups was reduced, which was in accord with the result of XRD.The thermal stability of M-GO increased with chain length of alkylamines below 400 ∘ C (GO-OA<GO-DDA<GO-TDA<GO-HDA<GO-ODA).However, when the thermal decomposition temperature exceeded 450 ∘ C, the thermal stability of M-GO became opposite to that of below 400 ∘ C. M-GO showed a higher weight loss rate than GO except GO-OA, because the C-N bond began to thermally decompose, and the long hydrocarbon chain fell off from the basal or edge of GO sheets.
The surface morphologies of GO and M-GO were characterized using SEM.According to SEM images (Figure 5), compared with the GO, the M-GO exhibited an enhanced surface roughness.And the roughness was in the following order: GO<GO-OA<GO-DDA<GO-TDA<GO-HAD<GO-ODA, suggesting a rise in surface roughness with increasing alkyl chain length grafted on GO surface.For the microinterface, the rougher the interface, the more the hydrophobicity.Thus, significant surface roughness could be produced, which was critical for fabricating hydrophobic surface.It offered the possibility of drag reduction in low permeability reservoirs.
Raman spectroscopy was a powerful approach to characterize the graphite degree of carbon-based material.Figure 6 showed the Raman spectra of graphite, GO, and M-GO.After oxidizing, the D band of GO located at 1347 cm −1 increased obviously, which resulted from the decrease in the size of the in-phase sp 2 domains due to the extensive oxidation.The G band of GO became much broader and shifted to 1596 cm −1 owing to the present of isolated double bonds that resonated at the higher frequencies than the G band of graphite.The G band positions of M-GO were red shifted relative to pristine GO, owing to the reduction of oxygen-containing groups after grafting long-chain alkyl on the GO surface.The results coincided with the results of XRD and TGA.The  D / G ratio (where  D and  G were the D-peak and G-peak Raman intensity, resp.) was widely used to evaluate the quality of graphene and graphene-based materials.The  D / G ratio values of graphite, GO, and M-GO were shown in Table 1.
There was no significant variation in the  D / G ratio for M-GO when compared to pristine GO, revealing that grafting alkyl on the GO surface did not affect the graphitic structure.
The  D / G ratio of M-GO increased slightly indicating that the regularity decreased due to the long-chain alkyl increasing the interplanar spacing.The wetting properties of M-GO were analyzed by measuring the water droplet contact angle on the surface of cores, shown in Figure 7.As for the core sample untreated with M-GO, the water droplet spread rapidly as soon as it contacted  the surface of core due to the capillarity phenomenon; the CA of core could be considered as 0 ∘ .So the cores were hydrophilic.After M-GO deposited on the surface of core, the contact angles of cores were 107.53 ∘ , 111.92 ∘ , 114.35 ∘ , 116.09 ∘ , and 118.97 ∘ , respectively.The grafting of alkylamines increased the water contact angle that may be due to the increase of hydrophobicity as well as surface roughness of the GO.
The M-GO would produce two important effects after being injected into the cores.In the first one, it would adsorb on the pore throats' surface of the rock, the wettability of the surface changed from hydrophilicity to hydrophobicity, then the slip effect appeared, and the flow resistance reduced.In the other one, the M-GO would reduce the diameter of pore throats after adsorbing on the surface, causing the water injection pressure to increase.Therefore, whether M-GO could reduce the water injection pressure or not depended on both the overall effects.If the nano slip effect was dominant, the flow resistance and injection pressure would decrease and vice versa.
The core flooding experiments were carried out to evaluate the ability of drag reduction of M-GO for low permeability.The GO-OA was unable to disperse in diesel oil completely to form homogeneous dispersion, so we only analyzed the drag reduction effect of GO-DDA, GO-TDA, GO-HAD, and GO-ODA.The cores and experiment parameters were listed in Table 2 and the pressure curve of flooding was shown in Figure 8.According to the pressure of the four kinds of nanodrag reducer, the permeability was calculated with Darcy equation (Table 3).With the increase of alkyl chain length on the GO surface, the decompression and augmented injection effect of nanodrag reducer was better.After M-GO injected into cores, the subsequent water flooding pressure of GO-DDA and GO-TDA became higher than before, which could not lower water injection pressure, while the subsequent water flooding pressure of GO-HAD and GO-ODA decreased 15.32% and 45.55%, respectively, and the permeability was 1.18 and 1.84 times higher than before, so GO-DDA and GO-TDA could be used as water flooding nanodrag reducer agent for low permeability reservoirs.

Conclusions
In summary, modified graphene oxide (M-GO) was synthesized by grafting long alkylamines chain with varying lengths, which could be used as nanodrag reducer in oilfield water flooding.We utilized FTIR, XRD, TGA, SEM, and Raman spectra measurements to characterize the successful modification of M-GO.SEM revealed that the long hydrocarbon chain in alkylamines increased the GO surface roughness.
Water contact angle measurements results indicated that the chain length of the grafted alkylamine was crucial for increasing the hydrophobicity of the GO, which maybe corroborated the decrease in surface energy values and increase in surface roughness.Flooding experiments showed that the GO-HDA and GO-ODA had an ability to reduce water injection pressure and increase water injection volume in the low permeability reservoirs.Our work may lay foundation for further industrial applications of modified GO in oilfield exploitation.

Figure 1 :
Figure 1: Schematic diagram of M-GO flooding experiment.

Figure 2 :
Figure 2: FTIR spectra of GO and M-GO.

Figure 4 :
Figure 4: TGA curves of GO and M-GO.

Table 1 :
D / G of graphite, GO, and M-GO in Raman spectra.Samples D / G Graphite 0

Figure 8 :
Figure 8: Pressure of water flooding before and after nanodrag reducer injection.

Table 2 :
Parameter of cores and experiment.

Table 3 :
Pressure drop and permeability ratio before and after nanodrag reducer injection.