Research on graphene oxide modified polymer as multifunctional filtration reducer for oil-well cementing

Conventional filtration reducers have limited utility in cementing at deep oil and gas fields because of their poor temperature resistance and single function. As a novel multifunctional filtration reducer, graphene oxide-modified polymer GO-PADI was developed by radically copolymerizing 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N, N-dimethyl acrylamide (DMAM) and itaconic acid (IA) between graphene oxide sheets. The microstructure of GO-PADI was confirmed through the utilization of different methods such as fourier transform infrared spectroscopy (FT-IR). The performance evaluation results showed that GO-PADI had the high thermal stability, and could control filtration of cement slurry as conducted by the American Petroleum Institute (API) less than 50 mL, which is 17 % lower than that of the traditional filtration reducer PADI. GO-PADI could increase compressive strength of cement stone by 30.3 %, and decrease elastic modulus by 10.9 % in three-day curing period. GO-PADI has the multifunctional functions of reducing fluid loss, strengthening and toughening, and can effectively improve the engineering performances for high-temperature cement slurry.


Introduction
Cement slurry has great effect on the guarantee of cementing quality, and it has great significance to petroleum and gas exploitation.In oil and gas well cementing, a multitude of additives are employed in order to enhance the general effectiveness of the cement slurry, e.g., filtration reducers, retarders and dispersants, so as to meet a variety of cementing needs [1][2][3] .Filtration reducers are indispensable additives utilized to reduce the filtration and guarantee the general performance of cement slurry.These additives encompass a wide range of options, such as natural polymers, chemical polymers and certain solid materials.Among these, the chemical polymers derived from AMPS have been most extensively investigated and employed [4,5] .
To improve the effectiveness of AMPS polymer filtration reducers, several traditional approaches can be implemented.One is copolymerized by temperature-resistant and salt-tolerant monomers with optimal molecular structure and weight [6][7][8] , another is introduced functional inorganic substances or organic molecules to graft and modify the filtration reducers [9,10] .And formulating filtration reducers and other additives is for the synergism can also be available [11] .Cadix et al. [12] developed a new block copolymer of DMAM or AM and AMPS as fluid loss additive with a short strong adsorption first block and a long second block.The research results showed that the diblock copolymer with lower molecular weight than the conventional polymer can control fluid loss without negative impacts.Xia et al. [9] synthesized a new type of hydrophobically associative polymer/nano-silica composite material with micro-crosslinked structure to improve the ultra-high temperature performance of traditional filtration reducers.The composite demonstrates exceptional thermal stability and possesses the ability to strengthen cement.This is attributed to its robust multidimensional staggered space network structure, which is a result of its pozzolanic activity and nucleation.Cao et al. [13] incorporated sodium metasilicate pentahydrate (SMP) in an AMPS-based polymer to prepare filtration reducer which contributes to early-strength for cement stone.Graphene oxide has been widely recognized as a novel nano-material, which is featured with excellent mechanical strength, barrier ability and specific surface area [14][15][16] .The potential application in cement for graphene oxide lies in the ability for to manipulate hydration, minimize adverse porosity and improve interfacial adhesion [17][18][19] .
In this paper, the AMPS copolymer was radically synthesized between graphene oxide sheets to prepare a graphene oxide-modified polymer GO-PADI as a multifunctional filtration reducer for cement slurry.Introducing graphene oxide into the AMPS copolymer can not only enhance the capability of reducing fluid loss in high-temperature environment, but also promote the cement strength development through exerting a significant influence on the hydration [20][21][22] .

Synthesis of GO-PADI
Using AIBA as initiator, the filtration reducer was synthesized by polymerizing.Adding AMPS, DMAM and IA to a beaker with distilled water, and then completely dissolved by magnetic agitation.After adjustment of the pH with NaOH, GO (1mg/ml) was added.The mixtures were then subjected to ultrasonic irradiation for 30 minutes in a water-bath, and then transferred into the three-tube flask with agitator.The initiator (AIBA) is dropped into the system after being heated up to 60 °C, and the reaction is conducted for 2 h.Finally, the black viscous filtration reducer was obtained after naturally cooled to room temperature.

Fourier transforms infrared spectroscopy (FT-IR) measurement
After GO-PADI was cleaned, dried and crushed, the mixture of 1 mg GO-PADI and 100 mg Kr was prepared.Then, FT-IR spectra was used to measure the specimen's composition between 4000 cm -1 and 500 cm -1 .

Thermogravimetric analysis (TGA)
The experiment was carried out in an atmosphere of high purity nitrogen.The temperature of the heatup speed and the airflow were respectively set as 10 ℃/min and 50 mL/min.

Transmission electron microscope (TEM) measurement
Dissolved appropriate amount of sample in solution separated by ultrasound in water bath.Then dropped an appropriate amount onto the copper mesh, and made it dry naturally.At last, the morphology of the specimen was examined by TEM.

X-Ray Photoelectron Spectroscopy (XPS) measurement
XPS analysis was performed using a monochromatic X-ray source of Al Kα (hv = 1486.6eV).

X-ray diffraction (XRD) measurement
Using Cu as the object, and Kα ray at 40 kV pipe voltage, 40 mA pipe current and scanning velocity of 2 °/min, the specimen was measured by X ray.

Cement slurry preparation
The preparation of cement slurries complied with API Standard "Recommended Practice for Testing Well Cement, API Recommended Practice 10B" as described.

Performance evaluation of filtration reducer
The rheological property of cement slurry has been determined with a viscosimeter (ZNN-D6, Tong Chun Oil Apparatus Company, Qingdao, China).The flowability was measured by the expansion diameter of the slurry in a truncated cone on a specially designed platform.The API filtration loss was introduced a type of high-temperature-high-pressure (HTHP) Stainless Steel Filtration Unit (4300, Chandler, USA) to be examined.The Compressive Strength Tester (YJ-2001, Taige Oil Instrument Company, Shenyang, PRC) was applied to study on compression strength of cement.The elastic modulus was determined with a three-axis tester (TAW-1000, Changchun Chaoyang Test Instrument Company, China).The sample was solidified in a mould (5.08 cm x 5.08cm x 5.08cm) under a special temperature-pressure-curing chamber.The morphology of cement stone was investigated using the SEM (S-4800, HITACHI, Japan) of which accelerating voltage was 3.0 kV.The specimens were all secured with a carbon black label on the specimen stand, followed by an ion spray (E-1045, HITACHI, Japan) for 90 seconds to ensure adequate electric conduction.

FT-IR measurement
The infrared spectral images of GO and GO-PADI were showed in Figure 1.The characteristic peaks of graphene oxide were located at 1730 cm -1 (stretching vibration peak of C=O), 1620 cm -1 (vibrational absorption peak of C=C), 1399 cm -1 (bending vibration of carboxyl's O-H), 1240 cm -1 (stretching vibration of C-OH) and 1052 cm -1 (stretching vibration of C-O) [23,24] .In the infrared spectrum of GO-PADI, the stretching vibration absorption peaks of 1189 cm -1 and 1042 cm -1 were the symmetry and asymmetry of -SO3 in GO-PADI, respectively.And compared with GO, the stretching vibration peak of C=O of GO-PADI was blue-shifted to 1720cm -1 , and the vibrational absorption peak of C=C was blue-shifted to 1580 cm -1 , indicating that the polymer was polymerized successfully between graphene oxide [25] .In addition, the -OH stretching vibration peak was further broadened and continues to move to the low wavenumber direction to 3396 cm -1 .This observation suggested the formation of a robust hydrogen bond between graphene oxide (GO) and the polymer [26,27] .
Figure 1.FT-IR spectrum of GO, PADI and GO-PADI.

Thermogravimetry analysis
The TGA results for GO, PADI and GO-PADI were presented in Figure 2. The mass loss of GO approximately 7.4% from 30 °C to 100 °C was probably caused by the mass loss of water molecules physically absorbed by the hydrophilic surface.The loss of weight was above 30% in the 100-300 ℃, which was attributed to the degradation of -COOH and -OH of GO [28] .There were different decomposition processes of GO-PADI: the decrease in mass below 100 ℃ could be caused by the evaporation of bound water within GO-PADI.And the main mass loss from 100 °C to 345 °C was due to the thermal decomposition of polymer's branch chains and oxygen-containing functional groups of graphene oxide.From 345 ℃ to 375 ℃, the scission of polymer main chain may cause 30 % weight loss of the specimen.And the carbonization process continued from 375 ℃ to 412 ℃ with the 43 % residual weight which showed that the significant molecular breakage occurs only when the temperature is above 345 ℃.These results suggested that the molecular structure of GO-PADI had excellent thermal resistance.It is pertinent to acknowledge that the TGA curve of GO-PADI didn't be in line with that of GO and PADI, indicating that the monomers was polymerized between the layers of GO.

TEM measurement
The TEM results of GO-PADI and GO were shown in Figure 3, respectively.The Figure 3. (a) showed that the GO sheet was wrinkled and folded with the flexible and ultra-thin characteristics structure of two-dimensional sheet [29,30] .The modified GO sheets were shown in Figure 3. (b).With compared, the lamellar structure of GO-PADI was thicker and the transparency of the lamella was weaker than GO.

X-ray photoelectron spectroscopy (XPS) measurement
The specimens' chemical structures were characterized using XPS method.The X-ray diffraction spectra of GO and GO-PADI were illustrated in Figure 4.The C1 binding energy of GO was 284.8eV (C-C), 286.8 eV (C-O) and 287.9 eV (C=O) in XPS spectroscopy.The peak of GO-PADI at 286.3 eV (C-O) was markedly lower than that of GO, which was attributed to the heat-reducing effect of partially oxygenic functional groups from graphene oxide.In the XPS spectrum of GO-PADI, the binding energy of N1s in the copolymer segment was 399.6 eV, and the binding energy of O1s in the graphene oxide and polymer was 531.6 eV [31,32] .The above results indicated that the polymer chains were polymerized between GO layers.

XRD measurement
The diffraction angle of GO undergoes variation as the layer distance experiences alteration.In Figure 5, graphene oxide had a characteristic diffraction peak at 2θ = 11.6 ° [33] .According to the Bragg equation 2dsinθ = nλ, it is calculated to be 0.77 nm of the interlayer spacing of GO.At the same time, it could be seen that GO-PADI showed a strong crystalline diffraction peak at 2θ = 19.9°, and there was no characteristic diffraction peak of GO at 2θ = 11.6 °, indicating that GO could be well dispersed in the form of sheets and there is no aggregation of GO.

Rheological properties of Cement Slurry
The Herschel-Bulkley (H-B) model commonly utilized to evaluate the rheological properties of cement slurry [34,35] .It allows for the calculation of shear stress and velocity, providing valuable insights into the rheological behavior of the system.Considering the n and k values could be estimated the rheology of fluid through the model formula = 0 + k n .In case of n < 1, the fluid is a kind of pseudo-elastic fluid with shear-thinning performance.And k can be used to estimate the viscosity of fluid.Typically, an augmentation in the k value corresponds to an increase in viscosity.From Table 2 it could be found that n value was from 0.8 to 1.0 and k was no more than 0.4 when the flurries mixed with GO-PADI at various temperatures.The comparison between GO-PADI and conventional filtration reducer PADI reveals minimal disparities which indicated that GO-PADI had a limited impact on the rheology of cement slurry.

Fluid loss control of Cement Slurry
The GO-PADI was added into the cement slurry (a density of 1.90 g/cm 3 , water-cement ratio 0.44), and the effects of incorporating amount of filtration reducer on API filtration was assessed evaluated.Experimental tests were conducted at 90 ℃ and 6.9 MPa, and the results were presented in Figure 6.
Obviously, the addition of GO-PADI resulted in a decreased loss of fluid in the cement slurry.When the dosage exceeded 3 %, GO-PADI could control the fluid loss in 50 mL, and GO-PADI has better performance in controlling fluid loss compared to PADI.

Mechanical properties of cement stone
Illustrations of the cement stones' compression strength under different curing durations were in Table 3. Cement stones with GO-PADI had a significant increase in strength compared with those with PADI.
The compression strength of GO-PADI at concentrations of 4 %, 5 % and 6% exhibited significant enhancements of 25.9%, 26.5% and 30.3% respectively after a three-day curing time.Similarly, the compressive strength growth rates were 29.9%, 18.3%, and 24.2% separately after a seven-day curing time.Thus, GO-PADI demonstrated its efficacy in developing the strength of cement.Furthermore, the cement stones' elastic modulus after three days of solidification was determined.The results showed that GO-PADI in cement stone resulted in a decrease in the elastic modulus.With the added GO-PADI from 4% to 6%, the elastic modulus of GO-PADI was also decreased from 9.01 GPa to 8.16 GPa.This reduction was observed to be lower compared to cement stone with PADI.When the addition amount was 6 %, the elastic modulus was declined by 10.9 % compared to that of the cement mixed with PADI.The reason for the strengthening mechanism of stone could be the existence of GO in GO-PADI, which provided growth sites for cement hydration products and compensated for defects of cement stones' microstructures [36] .Consequently, the compressive strength improved.The Figure 7 showed the states of different cement stones after compressive strength tests.Obviously, the cement stones with GO-PADI had better mechanical properties than with PADI.
To investigate the impacts of GO-PADI on the mechanic performance, SEM method was used to determine the micromorphology of the solidified cement.The results were presented in Figure 8.The crystal hydration products of cement mixed with PADI mainly consisted of needle-like and rod-like crystals with disordered distribution.These crystals were mainly distributed in the loose structures, such as cracks and holes, as demonstrated in Figure 8. (a)-(c).However, the structure of cement stone with GO-PADI instead of PADI was more compact and regular.And the incorporation of GO-PADI led to a significant reduction in the pore structure of cement stone, as shown in Figure 8. (d)-(f), suggesting that the addition of GO-PADI facilitates the consistent formation of microstructures for cement stones, thereby enhancing its mechanical properties effectively.
Table 3.The results of mechanical strength and elastic modulus of cement stone with different fluid loss control additives.

Conclusion
In this research, GO-PADI modified with graphene oxide, has been successfully developed as an additive of filtration reducer in cement slurry.The results have been verified by IR, XRD and XPS techniques.Moreover, it can be used to strengthen the cement stones by enhancing its compact and regular structure.GO-PADI is superior to PADI in thermal stability that achieve a reduction of 17.0%

Figure 2 .
Figure 2. (a) TGA curves of GO, PADI and GO-PADI and (b) TG curves of GO, PADI and GO-PADI.

Figure 5 .
Figure 5. XRD spectrum of GO and GO-PADI.

Figure 6 .
Figure 6.API filtration of cement slurry including (a) GO and (b) GO-PADI.

Figure 7 .
Figure 7.The fracturing situation of cement stone with (a) GO-PADI and (b) PADI of 4 %.

Table 1 .
The chemical and mineral compositions of G-grade high-sulphur-resistant oil-well cement.

Table 2 .
Rheology parameters of cement flurries with PADI and GO-PADI fitted by H-B model.