Synthesis and evaluation of properties of N,N-bis(perfluorooctyl)imine acetate sodium as a gas-wetting alteration agent

The wettability of a rock’s surface is a vital factor for gas flow and fracturing fluid backflow. As a result of this, novel and effective gas wetting alteration agents are required. In this work, a gas-wetting alteration agent, N,N-bis(perfluorooctyl)imine acetate sodium, was synthesized and characterized by different methods. The wettability of a rock’s surface was evaluated by contact angle and imbibition measurements, the Owens two-liquid method and glass capillary tube rise testing. The results showed that after treatment with 0.5 wt% N,N-bis(perfluorooctyl)imine acetate sodium the contact angles of water and n-hexadecane on the surface of the rock increased from 36° and 0° to 140° and 119°, respectively. The surface free energy rapidly reduced from primeval 72 mN m−1 to 3.4 mN m−1 after treatment with 0.3 wt% N,N-bis(perfluorooctyl)imine acetate sodium. These values agreed with the imbibition measurements and the results of the glass capillary tube rise testing. Moreover, analysis by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) showed that the roughness of the rock surface significantly increased. The above results fully proved that the wettability of the rock surface is altered from its original water-wetting or oil-wetting to gas-wetting. Furthermore, thermal analysis demonstrated that the gas-wetting alteration agent has good thermal stability, which indicates its great potential to be used as a gas-wetting alteration agent for unconventional gas reservoirs under high temperature conditions.


Introduction
With the change in energy patterns, shale gas has recently emerged as a relatively important energy source that provides an opportunity for a lot of countries around the world to reduce their dependence on energy imports and their struggle to achieve energy independence. 1 It may also be a potential transition fuel that will permit the transformation from non-renewable to renewable energy resources as it is conducive to reducing emissions of carbon dioxide and other pollutants. 2 However, shale gas is generally extracted from low-permeability or tight porous media. It cannot be captured from original source rock without an extra driving force. 3 Hydraulic fracturing is the technology classically used to increase the production of shale gas, 4 but this method is characterized by large displacement of liquid volume. Thus, fracturing uid owback plays a very important role in hydraulic fracturing operations. Its purpose is to promote the outow of extra liquid which maintains the crevice with strong gas ow and to reduce damage to the formation.
Reservoir wettability is a crucial factor that dominates the distribution and ow of gas in shale media. It also has a remarkable inuence on both the re-ow of fracturing uid and the relative permeability of the reservoir. 5,6 Thus, the investigation of reservoir wettability is of profound signicance for improving shale gas recovery. Li et al. 7 put forward the gaswetting concept in which the wettability of rock was altered from liquid-wetting to neutral using a uorocarbon surfactant. Tang and Firoozabadi 8 demonstrated the altering of the wettability of sandstone from strong liquid-wetting to neutral gaswetting using polymers. Feng et al. 9 achieved the gas wetting alteration of a rock's surface with organosilicon-acrylic and signicantly improved gas production. Jin and Wang 10 investigated the concept that the wettability of low-permeability rock could be permanently altered from strong water-wetting to intermediate by a novel gas-wetting alteration agent. Furthermore, Al-Yaseri et al. 11 veried that the density of gas in the gasliquid-solid system has a signicant effect on the wettability. Iglauer et al. 12,13 reported that gas-wetting increases with increasing pressure, salinity and dissolved ion valencies, while the effect of temperature is not well understood. Moreover, the inuence of various subsurface minerals and rock types on the wettability and the mechanism of interaction were also investigated.
However, the hydrophobic and oleophobic groups of the gaswetting alternation agents used in the previous studies were single-chain structures that cannot effectively alter the wettability of rock surfaces from liquid-wetting to gas-wetting. The objective of this study is to synthesize a gas-wetting alteration agent with a double-chain hydrophobic and oleophobic structure. The product can more effectively alter the reservoir wettability and improve the owback rate of the fracturing uid.

Materials
Peruorooctanoyl chloride was provided by Aladdin Industrial Corporation. Triethylamine was offered by Shandong Chemical Reagents Co., Ltd. Dichloromethane and sodium chloroacetate were supplied by Sinopharm Chemical Reagent Co. Ltd. Anhydrous ethanol and acetone were purchased from Shanghai Han Guang Chemical Reagent Co., Ltd. All of the above chemicals are analytical reagents. The rock samples used in this study were obtained from the Shengli Oileld, China, and the basic parameters are shown in Table 1. During the experiment, the rock samples were polished using sandpaper to get a relatively smooth surface, then washed with 1.0 wt% NaCl solution and dried at 80 C for 12 h. 14 Deionized water was prepared in the laboratory.

Synthesis of the gas-wetting alteration agent
The gas-wetting alteration agent was synthesized according to the following procedures: Firstly, peruorooctanoyl chloride and calcium oxide were added to a ask in a molar ratio of 2 : 1 under a nitrogen atmosphere, and then anhydrous ethanol was added as solvent. Then, ammonia (a 3-fold molar excess of peruorooctanoyl chloride) was injected and stirred for 4 h. The mixture was ltered under reduced pressure and dried in a universal oven at 78 C to obtain peruorooctanoyl ammonia. This rst reaction is described in Scheme 1(a).
Secondly, 0.1 mol of peruorooctanoyl ammonia was placed in a three-necked ask with anhydrous ethanol and triethylamine as the solvent and catalyst, respectively. Then, 0.1 mol of peruorooctanoyl chloride was added dropwise under stirring and the temperature was kept below 5 C for 6 h. N,N-bis(per-uorooctyl)imine was produced aer removing the solvent and impurities. This procedure is shown in Scheme 1(b).
Finally, N,N-bis(peruorooctyl)imine was added into a solution of acetone in a three-necked ask and sufficiently dissolved at 60 C. Next, a solution of sodium chloroacetate was added dropwise to the mixture and vigorously stirred at room temperature for 6 h. The reaction mixture was ltered and concentrated under reduced pressure. Then, the residue was puried by recrystallization aer removing the solvent. The puried material was steamed under reduced pressure to give the nal product N,N-bis(peruorooctyl)imine acetate sodium. This reaction is shown in Scheme 1(c).

Characterization
An FT-IR spectrum of the sample was obtained using a Bruker VERTEX 70 series Fourier transform infrared spectrometer in the wavenumber range of 4000-500 cm À1 ; 1 H NMR spectra were recorded in the stated solutions, on a Bruker DRX-400 spectrometer, operating at 400 MHz for H; the thermal stability of the compound was analyzed using a TGA/SDTA851 thermal gravimetric analyzer with a heating rate of 10 C min À1 . The morphology and elements of the rock surface before and aer treatment were examined by a JEOL JSM-6390A scanning electron microscope and energy dispersive spectrometer (EDAX Ltd., USA).

Contact angle measurement.
In order to measure the contact angle of water or n-hexadecane drops on original and treated rock surfaces, we used a micropipette to place a liquid drop on the surface of air-saturated rock at room temperature. The morphology of a sessile droplet on the rock surface was enlarged on a monitor screen. Pictures of the droplets were taken using a digital camera under proper illumination of a light source. The contact angle of the gas-liquid-rock threephase system was measured using the soware's goniometry tool. 15 2.4.2 The Owens two-liquid method. There is a close relationship between the surface free energy and the wettability of a rock surface. In order to investigate the wettability alteration of the rock, its surface free energy was studied using the Owens two-liquid method. 16 The surface free energy consists of the dispersion force and the polar force and the specic calculation equation is as follows: 17 In eqn (1) and (2), g s is the surface free energy of the solid; g D s and g P s are the dispersion force and polar force of the solid, respectively; g L is the surface free energy of liquid; q is the contact angle; g D L and g P L are the dispersion force and polar force of the liquid, respectively.
Variables g D s and g P s can be calculated using eqn (3) and (4). 18 In eqn (3) and (4), g L1 is the surface free energy of water; q 1 is the contact angle of water on the rock surface; g D L1 and g P L1 are the dispersion force and polar force of water, respectively; g L2 is the surface free energy of n-hexadecane; q 2 is the contact angle of n- hexadecane on the rock surface; g D L2 and g P L2 are the dispersion force and polar force of n-hexadecane, respectively.

Imbibition.
A spontaneous imbibition test can be conducted to evaluate the effect of wettability on uid imbibed into the rock. The amount of liquid imbibed versus time was recorded using a balance when the bottom of rock touched the liquid surface. 19,20 The readability of the balance was 0.001 g. A schematic showing the spontaneous imbibition test is exhibited in Fig. 1. 2.4.4 The glass capillary tube rise test. The glass capillary tube rise test was performed to verify the effect of the gaswetting alteration agent on wettability alteration. The capillary radius used in this study was about 0.5 mm. Firstly, the glass capillary tube was washed with distilled water and dried at high temperature. Then, the cleaned glass capillaries were treated in a gas-wetting alteration agent solution for 24 h and dried at a certain temperature to remove excess solution from the tube. Finally, the treated glass capillaries were inserted into the solution. During the process, the liquid was sucked into the tube by capillary force. A schematic diagram is shown in Fig. 2. If the liquid was positive in the capillary (h > 0), the contact angle of the liquid on the rock surface was less than 90 ; when the liquid was neutral in the tube (h ¼ 0), the contact angle was equal to 90 ; and if the liquid was negative (h < 0), then the contact angle of liquid was greater than 90 . The specic contact angle in the gas-liquid-rock system could be computed using eqn (5), where s and r represent the surface tension and density of the liquid, respectively; r is the radius of the glass capillary; q is the contact angle; h is the height of the liquid in the glass capillary and g is the gravitational acceleration. 21,22 q ¼ arc cos rghr 2s

FTIR spectrum
The infrared spectrum of the gas-wetting alteration agent is shown in Fig. 3. The absorption peak of C-H appears at 2957 cm À1 . The peaks at 1252 cm À1 and 1149 cm À1 correspond to the characteristic absorption of -CF 3 and -CF 2 , respectively.
Scheme 1 Synthesis of the gas-wetting alteration agent. Fig. 1 The simple device for spontaneous imbibition testing.
The stretching vibration absorption of the C]O-N group occurs at 1695 cm À1 . Moreover, the characteristic peak of the C]O-NH group at 3300-3100 cm À1 disappears, indicating that the monomers have reacted adequately. Furthermore, the carboxylate stretching peaks are shown at 1603 cm À1 and 1398 cm À1 .

1 H NMR spectrum
The gas-wetting alteration agent was characterized by 1 H NMR and the spectrum is presented in Fig. 4. The peaks at d 2.508 and d 3.331 correspond to the solvent (DMSO) and deionized water, respectively. The single peak at d 5.601 for H is assigned to the -CH 2 group. This observation can be explained by the chemical shi of hydrogen in the methylene group moving upeld due to the strong electron-withdrawing amide group and the role of the carboxylate. The result is in good agreement with the infrared analysis, indicating that the product has been successfully synthesized.

Thermal stability
The thermal analysis revealed the stability of the gas-wetting alteration agent when the temperature is increased. As seen in Fig. 5 there are four stages of weight loss in the TGA curve.
During the rst stage, before 170 C, there is almost no weight loss. In the second stage, from 170 C to 230 C, the weight loss is about 9%, which may be attributed to the evaporation of the residual solvent during the purication process. There was a signicant increase in weight loss between 230 C and 400 C, with the maximum weight loss rate peaking at about 1.267% C À1 at 270 C with a relatively narrow reaction interval. The acceleration of weight loss at this stage indicates that thermal    Paper decomposition has occurred. Above 400 C, the remaining weight ratio is less than 10%, which can be assigned to the carbon residue. These results show that the synthesized gaswetting alteration agent decomposes at higher temperatures compared to literature data (120-140 C). 23,24 This demonstrates that it has a great potential in terms of temperature tolerance and can be applied to unconventional reservoirs.

Contact angle measurements
The contact angles of water and n-hexadecane on the rock surface before and aer treatment were measured and the results are shown in Fig. 6. Initially, the rock surface showed strong liquid-wetting with contact angles of water and n-hexadecane of 36 and 0 , respectively. However, aer treatment with the gas-wetting alteration agent, the wettability of the rock was changed to preferential gas-wetting. When the concentration of the gas-wetting alteration agent was 0.1 wt%, the contact angle of water changed from 36 to 60 , while that of n-hexadecane increased from 0 to 43 . With the increase in concentration of the agent, the contact angles of water and n-hexadecane increased continuously. When the concentration was 0.5 wt%, the contact angles reached maximums of 140 and 119 , respectively. The results were better than the literature data. [25][26][27] The respective contact angles of water and n-hexadecane remained at about 140 and 119 with further increases in concentration. Pictures of the drops on the rock surface before and aer treatment with different concentrations are shown in Fig. 7. This observation may be explained as follows: at low concentrations, a small amount of molecules are adsorbed on the surface of the rock through van der Waals forces and these adsorbed molecules are easily desorbed or replaced by molecules in other solutions. At higher concentrations, the adsorption capacity is greater than that of desorption, resulting in an increase of the amount of gas-wetting alteration agent adsorbed on the rock and the formation of a gas-wetting molecular layer on its surface. As a result of this, the wettability of the rock is altered from strong liquid-wetting to gas-wetting. With a further increase of concentration, the adsorption capacity of the rock surface tends to be saturated and so the wettability becomes stable. Therefore, the contact angle of water or n-hexadecane remains essentially unchanged. 28,29

The surface free energy
The surface free energy is a crucial feature when altering the wettability of a rock surface. Table 2 shows the change in the surface free energy of the rock before and aer treatment with different concentrations. It can be seen that the surface free energy was rapidly reduced from 72 mN m À1 to 3.4 mN m À1 aer treatment with a concentration of 0.3 wt%. With the increase in concentration, the surface free energy did not decrease signicantly. This result is consistent with studies by Gindl and Jiang et al. 30,31 However, when the concentration exceeded 0.9 wt%, the surface free energy had a tendency to gradually increase. A reasonable explanation is that when the polar segments of the gas-wetting alteration agent are adsorbed into the rock by electrostatic forces, the non-polar segments with double-repellent chains are exposed to the surface of the rock. Moreover, these further form a strong hydrophobic and oleophobic molecular layer which alters the wettability of the rock surface. Unfortunately, when the concentration exceeds Fig. 6 The contact angles of water and n-hexadecane on rock surfaces. Fig. 7 The pictures of the droplets on the rock suface. the optimum value, the surface free energy has a tendency to gradually increase with an increase in concentration. The reason is that the bimolecular chains may be entwined with each other, leading to a portion of the nonpolar segments being locked. As a result, the surface free energy exhibits a small rebound. 32

Imbibition
The wettability of the rock before and aer treatment was determined by spontaneous imbibition. The dynamic process of water or n-hexadecane imbibed into the rock was conducted by recording the weight of the liquid over time until there was no signicant change. Fig. 8(a) shows that the volume of water imbibed into the rock was 0.83 g without the chemical treatment, while the value sharply decreased to 0.07 g aer treatment with 0.3 wt%. This indicates that the rocks have become strongly hydrophobic. Fig. 8(b) shows the n-hexadecane imbibed into untreated and treated rock. The amount of imbibed rock decreased signicantly from 0.87 g to 0.25 g aer treatment, which demonstrates that the rock has an oleophobic property aer chemical treatment. The substantial reduction in both water and n-hexadecane reveals that the wettability of the rock has been altered from strong liquid-wetting to gas-wetting, which agrees with the results of Giraldo et al. 33 and Lai et al. 34

Capillary tube rise testing
The glass capillary tube rise test was performed to further demonstrate the wettability alteration of the rock. Fig. 9 summarizes the liquid level in the capillary tube as a function of the treatment concentrations. It can be clearly observed that the water level was rapidly reduced from 31 mm to approximately À13 mm as the concentration of the agent increased. Moreover, the n-hexadecane level was conrmed to show a similar trend to the water. Therefore, it was sufficient to indicate that the wettability of the capillary inner wall was changed to gaswetting. The results of the glass capillary rise testing were consistent with literature data. 35 Accordingly, the agent could effectively improve the rate of fracturing uid owback and maintain the blocks with high yields, while promoting the outow of extra liquid to reduce any damage to the formation.  Fig. 10(a) shows that almost no material covered the surface of the original rock and its surface was relatively smooth. This observation could promote the expansion of liquid on the rock's surface, which further illustrates the reason why the initial wettability of the reservoir is liquidwetting. However, it can be clearly seen from Fig. 10(b) that the morphology of the rock surface has been signicantly changed aer treatment with the gas-wetting alteration agent. The surface of the rock adsorbs a large number of agent molecules through van der Waals forces and forms a tight adsorption layer, which effectively improves the roughness of the rock surface. In addition, the surface free energy can be signicantly reduced aer treatment. Thus, the wettability of the rock surface is altered from water-wetting or oil-wetting to gaswetting. This is consistent with what has been reported in the literature. 36,37 Fig. 9 The water level in the capillaries before and after treatment. Fig. 11 illustrates the elements of the rock surface before and aer it was modied by the gas-wetting alteration agent. As shown in Fig. 11(a), there was no uorine element in the original rock sample, which was mainly composed of elements such as calcium, potassium, oxygen and silicon. The presence of Au was due to the spray before the test. It can be seen from Fig. 11(b) that uorine has been found on the treated rock surface. This shows that uorine can be sufficiently adsorbed on the surface of the rock. This is the primary cause of the gaswetting alteration of the reservoir and is consistent with the research of Aminnaji et al. 38

Conclusions
In this work a gas-wetting alteration agent was synthesized and characterized by FTIR and 1 H NMR. Thermal gravimetric analysis exhibited that the synthesized gas-wetting alteration agent can resist temperatures up to 230 C. The contact angle method was used to examine the wettability of the rock before and aer treatment. The results show that the wettability of the rock surface is altered from its original strong liquid-wetting to gaswetting using 0.5 wt% of the agent. The results can be further veried by imbibition measurements and capillary tube rise testing. The wettability alteration of the reservoir surface can be explained by the special chemical structure, compared with the studies of Iglauer et al. 39 and Sharma et al. 40 The polar segments can be adsorbed into the rock by electrostatic forces, while the non-polar segments with double-repellent chains are exposed to its surface and form a strong hydrophobic and oleophobic molecular layer. Furthermore, the analysis by SEM and EDS also suggests that the effect of the agent on gas-wetting alteration can be ascribed to the increase in the roughness of the rock surface and the decrease in the surface free energy.

Conflicts of interest
There is no conict to declare.