Preparation and Performance of an Adsorption Type Gel Plugging Agent as Enhanced Oil Recovery Chemical

A novel adsorption type gel plugging agent (ATGPA) was prepared using acrylamide (AM), acrylic acid (AA), diallyl dimethyl ammonium chloride (DMDAAC), 2-acrylamido-2-methylpropanesulfonate (AMPS), formaldehyde (HCHO), resorcinol (C 6 H 6 O 2 ), and thiocarbamide (CH 4 N 2 S) as raw materials under mild conditions. ATGPA was characterized by infrared (IR) spectroscopy, elemental analysis, and scanning electron microscope (SEM). It was found that ATGPA exhibited higher elastic modulus (G) and viscous modulus (G) than AM/AA gel plugging agent (AAGPA) under the same scanning frequency. It was also found that ATGPA had moderate temperature resistance and salt tolerance. Core plugging tests results indicated that ATGPA could achieve up to higher plugging rate (P R ) than AAGPA (97.2% versus 95.7%) at 65C. In addition, ATGPA possessed stronger antiscouring ability by core plugging experiments at 65C.


Introduction
Gel plugging agent plays an important role in the field of enhanced oil recovery (EOR) for high water cut oilfield [1,2].However, the current widely used gel plugging agent, prepared by polyacrylamide (PAM) or partially hydrolyzed polyacrylamide (HPAM), cannot completely meet the requirements due to the poor antiscouring ability in porous medium and the degradation under high temperature or high salinity [3][4][5][6][7].
In general, while increasing the concentration of gel plugging agent is beneficial to enhance the sealing strength, the antiscouring ability of the gel plugging agent is not helpful.And this method will lead to a substantial increase in the cost of gel plugging agent.Recently, many studies have demonstrated that the adsorption property of composite material could be significantly improved by copolymerization with a cationic functional monomer with high charge density [8][9][10].The composite material containing cationic functional monomer, such as polymeric adsorbent, chelating resin, and polymer supported catalyst, may exhibit more satisfactory adsorption stability owing to the effect of quaternary ammonium basic ion [10][11][12][13][14].However, there are no papers about the application of cationic functional monomer with high charge density in EOR to develop antiscouring ability of gel plugging agent.
Keeping in mind these fundamental conditions, herein, diallyl dimethyl ammonium chloride and AMPS were introduced into gel plugging agent aiming to obtain satisfying antiscouring ability, temperature resistance, and salt tolerance.

Characterization.
ATGPA and AAGPA solutions (0.6 wt%) were prepared with distilled water.These solutions were placed in a constant temperature box, and the crosslinking reaction took 72 h.All characterization and evaluation were carried out until all of the plugging agent was crosslinked unless otherwise indicated.Infrared (IR) spectra of ATGPA and AAGPA were measured with potassium bromide (KBr) pellets using a PerkinElmer RX-1 spectrophotometer.The elementary analysis of ATGPA and AAGPA was carried out with a Vario EL-III elemental analyzer.The microstructures of ATGPA and AAGPA were observed by a scanning electron microscope (SEM).
2.4.Temperature Resistance and Salt Tolerance.ATGPA and AAGPA solutions (0.6 wt%) were prepared with distilled water.These plugging agents were placed at different temperature, and the time required for the formation of the gel was 72 h.Then the apparent viscosity of these plugging agents was tested using Brookfield DV-III viscometer at different temperatures.
The plugging agents solutions were prepared by using brine with different salt concentration (NaCl or CaCl 2 ).These plugging agents solutions needed to be aged for 72 h at 65 ∘ C. The apparent viscosity of these plugging agents was measured via Brookfield DV-III viscometer at 65 ∘ C.

Viscoelasticity.
Viscoelasticity measurements of ATGPA and AAGPA were conducted on a HAAKE RS 600 Rotational Rheometer (Germany).The test system was coneplate system and the rotor was P35TiL in viscoelasticity measurements.The stress was 1.0 Pa, and the scanning range of frequency () was 0.1-10.0Hz. 2.6.Core Plugging Tests.Two sandstone cores were used for core plugging experiments.The cores were dried at 65 ∘ C, and then their diameter, length, gas permeability, and porosity were measured by using a SCMS-B2 core multiparameter measurement system.A Hassler core holder was used with 3.5 MPa confining pressure and 1.5 MPa backpressure [18].The core was saturated with injecting water at 0.1 mL/min, and then the plugging agent solution (0.6 wt%, non-crosslinked) was injected at 0.1 mL/min until injection mass reached 0.6 pore volume (PV).In 3 days, the core was flooding with injecting water at 0.1 mL/min to obtain stable injection pressure.All the core plugging procedures were conducted at 65 ∘ C. Chemical composition and total dissolved solids (TDS) of the injecting water are listed in Table 1.The plugging rate is calculated with the following equation: where   is plugging rate, %,  wb is aqueous phase permeability before plugging, mD, and  wa is aqueous phase permeability after plugging, mD.Flow chart of the core plugging experiments is shown in Figure 1.

Elementary Analysis of ATGPA and AAGPA.
The elementary analysis of ATGPA and AAGPA was carried out using a Vario EL-III elemental analyzer.The content of different element in these plugging agents can be obtained by detecting the gases, which are the decomposition products of the samples at high temperature.The test results of ATGPA and AAGPA are shown in Table 2.

Microscopic Structure.
The microscopic structures of ATGPA and AAGPA were observed through SEM at room temperature.ATGPA and AAGPA solution samples (0.6 wt%) were prepared with distilled water and cooled with liquid nitrogen, and then these samples were evacuated to keep original appearance as far as possible.As shown in Figure 3, the form of molecular coils was obviously changed when DMDAAC and AMPS were introduced into the AM/AA copolymer.Compared with the images of AAGPA, the network structure of molecular coils of ATGPA was closer than that of AAGPA, and the molecular coils of ATGPA possessed uneven surfaces due to the introduction of DMDAAC and AMPS.

Temperature Resistance.
ATGPA and AAGPA solutions were prepared with distilled water.These plugging agents were placed at different temperatures for 72 h.The apparent viscosity of these plugging agents was tested using Brookfield DV-III viscometer at different temperatures.The apparent viscosity versus temperature curves of ATGPA and AAGPA solutions are shown in Figure 4. Comparing with AAGPA, we could easily find that ATGPA owned better temperature resistance.The data indicated that ATGPA could withstand higher temperature and exhibited higher apparent viscosity at the same temperature.This phenomenon might be explained by the five-membered ring structure of ATGPA polymeric chains which could enhance the temperature resistance of this plugging agent.In addition, it might be attributed to  the -SO 3 − groups, which usually could improve the temperature resistance.

Salt Tolerance.
The ability against Na + of ATGPA was shown in Figure 5(a).The salt tolerance of ATGPA was much more excellent than AAGPA.The tests about Ca 2+ (0.6 wt%, 65 ∘ C) gave similar results (Figure 5(b)).These phenomena revealed that ATGPA could withstand higher salinity.And this may be related to the -CONHC(CH 3 ) 2 CH 2 SO 3 − units on the chain of ATGPA.Comparing with -CONH 2 − groups the hydration layer formed by -CONHC(CH 3 ) 2 CH 2 SO 3 − groups was more difficult to damage when neutralized with counter ions.Therefore, the salt resistance of ATGPA was better than AAGPA.
3.6.Viscoelasticity Measurements.ATGPA and AAGPA solutions (0.6 wt%) were prepared with distilled water.These plugging agents were placed at 65 ∘ C, and the time required for the formation of the gel was 72 h.The viscoelasticity curves of ATGPA and AAGPA were shown in Figure 6.In the linear viscoelastic region, the elastic modulus (  ) of ATGPA and AAGPA was higher than the viscous modulus (  ).This meant that the ATGPA and AAGPA are mainly based on elasticity.Compared with AAGPA, ATGPA exhibited higher   and   under the same scanning frequency.This phenomenon might support that the adsorption type functional monomer could enhance the acting force between ATGPA molecular coils.3, the plugging rate of ATGPA (0.6 wt%) was 97.2% at 65 ∘ C.However, the plugging rate of AAGPA (0.6 wt%) was 95.7% under the same conditions.The core plugging experiments results showed that ATGPA revealed more superior ability of core plugging.As shown in Figure 7, compared with AAGPA, ATGPA exhibited stronger antiscouring ability.In the subsequent water flooding process, the fluctuation of the injection pressure of ATGPA was relatively large, but there was no trend of decline.This phenomenon might support that the antiscouring ability was obviously improved in porous media due to the introduction of DMDAAC.

Conclusions
ATGPA was prepared using AM, AA, DMDAAC, AMPS, HCHO, C 6 H 6 O 2 , and CH 4 N 2 S as raw materials.ATGPA was characterized by IR spectrum, elemental analysis, and scanning electron microscope.The solution properties, such as viscoelasticity, temperature resistance, salt tolerance, and plugging ability of ATGPA, were investigated under different conditions.The results indicated that ATGPA possessed moderate or good viscoelasticity, temperature resistance, salt tolerance, plugging ability, and antiscouring ability as EOR chemical.

Table 1 :
Composition and TDS of injecting water.
∘ C under N 2 atmosphere for 6 h.After being cooled to room temperature (25 ∘ C), the products were separated by precipitation using ethanol and dried in vacuum oven at 40 ∘ C for 24 h to give the powdered copolymer samples.6.08 g HCHO (37 wt% aqueous), 0.60 g C 6 H 6 O 2 , and 0.15 g CH 4 N 2 S

Table 2 :
The test results of elementary analysis of ATGPA and AAGPA.

Table 3 :
The parameter of core and the results of core plugging experiments.