Enhancement of the Wettability and Lubrication of Shale Rock via Nanoemulsions

Nanoemulsions have been widely used as additives for drilling fluids in recent years. With the development of nanotechnology, multifunctional nanomaterials have been added to nanoemulsions. The improvement of wettability of the surfaces, alteration of oil-wet on shale rock surfaces, and environmentally friendly conditions are considered as the future development directions of nanoemulsions. In this work, a novel nanoemulsion was prepared by using hydrocarbon-based polyoxyethylene ether, oil (hydrocarbon), distilled water, and formation crude oil as the main raw materials. The shale rocks before and after immersion with as-prepared nanoemulsion were characterized by contact angle measurement, atomic force microscope (AFM), and Fourier transform infrared spectroscopy (FTIR). It is clearly observed that the nanoemulsion greatly improved the wettability of the sandstone and rock surface by forming a layer of active agent film on the surface of the rock. The as-prepared nanoemulsion had good ability to curb the anticollapse and lubricate and protect the oil and gas layer.


Introduction
Nanoemulsion is a transparent/translucent (50-200 nm) or milky white emulsion (about 500 nm).It also has developed as small emulsion, ultrafine emulsion, latex liquid, unstable emulsion, and submicron emulsion.The introduction of nanomaterials in the drilling completion fluid can solve urgent issues, such as stability performance and environmentally friendly composites [1].When the nanoemulsion was applied to drilling, solid particles and the filtrate from the drilling and completion were prevented from the deep formation to avoid permanent blockage in the oil and gas flow channel [2].This played very important role in the protection of oil and gas layer.The current pilot exploration research showed that in the field of oil and drilling fluid nanoemulsion had a very strong attraction.
The freezing point was 24.84 ∘ C, which belonged to high temperature and high freezing point.The basic information about Jimusaer dense oil reservoir was listed in Table 1.
In this work, the surface of the rock in Jimusaer oil field was modified by nanoemulsion due to the improved wettability of the rock surface and formed active agent film.This provided a new idea for the optimization of sandstone drilling fluid and the improvement of the stability of sandstone gas shaft [8][9][10].

Experiment.
The experiments were processed as follows: (1) Diluted crude oil: a diluted crude oil with a viscosity of 10.83 mPa⋅s was obtained by mixing crude oil with kerosene at a ratio of 1 : 1.25.(2) Nanoemulsion mother liquor configuration: a certain amount of the surfactant (hydrocarbyl polyoxyethylene ether), alcohol, and oil (hydrocarbons) were mixed.Subsequently, distilled water was added to generate microemulsion mother liquor.(3) Nanoemulsion configuration: nanoemulsion was obtained by mixing microemulsion mother liquor and 2% KCl solution at the ratio of 0.1 : 100.(4) Rock soaking: the core was cut into 1 cm thick and 2.5 cm diameter with 1000 mesh sandpaper polished smooth; then, it was soaked with diluted oil for 10 days.(5) Remove the rock and immerse it into nanoemulsion.(6) The soaked rock was analyzed by FTIR, atomic force microscopy (AFM) and contact angle.It is clearly seen that a strong peak at 3300 cm −1 occurred after nanoemulsion soaking [11][12][13][14][15].The longer the soaking time, the stronger the relative intensity of the peak.This peak corresponded to a group with strong polarity, indicating that the nanoemulsion can greatly improve the wetting of the sandstone surface [15][16][17][18].  2 gives the adhesion forces between probes and rock surfaces under different nanoemulsion soaking time.All the tests were performed at a temperature of 23 ∘ C with humidity 23%.The same positions on the rocks were tested (experimental conditions: ramp size 500 nm, reserved velocity 1000 nm/s, preload time 0 s, and temperature 19 ∘ C) [19].Figures 2(a) and 2(b) are a typical adhesive force curve and tip probe detected under AFM.It can be seen from Figure 2(a) that the adhesion force between particle and substrate increased with the increase of soaking time.The adhesive force shown in Figure 2(c) also increased as the loading force increased from 500 to 2000 nN, indicating that the surfactant in the nanoemulsion was effectively spread on the shale rock surface and formed a layer of active agent film with high stability performance.

Contact Angle.
Wettability refers to the tendency of another fluid to expand or adhere to the solid surface when there is a mixed fluid [20].It is the interaction between oil and water and reservoir rocks under reservoir conditions.According to the affinity between the reservoir rock surface and water and oil, the reservoir wetting property is divided into water wetting, oil wetting, neutral wetting, and subwetting.Reservoir wettability is the main factor to control the flow and distribution of fluid in reservoirs [21].Therefore, the study of reservoir wettability is of great significance to the development of oil field and the final recovery rate of crude oil [22].
The contact angle method is mainly used for the determination of wettability of pure fluid and artificial core systems.The wettability of the system is usually defined according to the determination of the angle  of the water on the solid surface.Generally,  < 75 ∘ , 105 ∘ >  > 75 ∘ , and  > 105 ∘ are defined as water wetting, neutral wetting, and oil wetting, respectively.
Figures 3(a)-3(d) are the contact angle of the rock sample immersed in the nanoemulsions for different soaking times.It can be clearly seen that the oil was wetted when the rock was soaked in the nanoemulsion for 1 day (Figure 3(a)) but weakly wetted when immersed for 3, 7, and 9 days (Figures 3(b)-3(d)).The graph inferred that the wetting of the rock surface changed with the immersion time.
3.4.The Schematic of the Reaction.Nanoemulsions are usually thermodynamically stable isotropic dispersions of oil, water, surfactants, and cosurfactants spontaneously formed.Figure 4 is the model of nanoemulsion structure [23].From the microscopic principle analysis, the nanoemulsion is a heterogeneous dispersion, which consists of a continuous aqueous phase and a polymer dispersed in water in a microsphere form.During the emulsion formation process, the water is gradually volatilized or absorbed by the porousness of the surface.Then, the microspheres of the polymer slowly  approach and finally contact each other until the polymer particles fuse with each other to form a continuous uniform coating [24].
Figure 5 is the schematic diagram of shale rock soaked in nanoemulsions.It can be seen that when the nanoemulsions are dispensed onto water surface under gravity, it can mix with rock in two ways; one is spreading on the surface and the other is infiltration into rocks.Since the infiltration speed of water is much higher than that of nanoemulsions, thus nanoemulsions start to get concentrated under surface tension and form a solid thin film on top of the shale rock surfaces.That is the reason why the wettability of the shale rocks was changed [25][26][27][28].

Conclusions
In this work, we designed a new nanoemulsion and proved that the nanoemulsions can greatly improve the wettability of the sandstone surface and form a layer of active agent film on the surface of the rock.The drilling fluid used in the oil field had good ability to curb the anticollapse and lubricate and protect the oil and gas.AFM and FTIR results represent a thin layer of film formed on the surface of the shale rock.Those findings may guide the scientists and engineers regarding designing nanoemulsions with good wettability and stability properties.

Figure 1 :
Figure 1: FTIR spectra of soaked sandstone samples: (a) sandstone soaked under NaCl solution for different immersion times and (b) sandstone soaked with nanoemulsions at different immersion time.

Figure 2 :
Figure 2: (a) A typical force curve data, (b) a typical AFM tip probe, and (c) adhesion force between rocks and probes under different soaking time with nanoemulsions.

Figure 3 :
Figure 3: Contact angle of water droplets in air; rocks soaked in the nanoemulsion for various times, (a) 1 day, (b) 3 days, (c) 7 days, and (d) 9 days.

Table 1 :
The basic information of the Jimusaer dense oil reservoir.