Synthesis and properties of a high-performance environment-friendly micro–nano filtration reducer

In this research study, we modified hydroxyethyl cellulose to obtain hydrophobically associating hydroxyethyl cellulose, and grafted it onto the surface of nano-calcium carbonate to obtain a graft copolymer. The intramolecular or intermolecular associations between the macromolecular chains of the graft copolymers form different forms of supramolecular network structures, and they interact with nanoparticles to form stable structures to enhance their related properties. The structure of the obtained graft copolymer was characterized by Fourier transform infrared spectroscopy (FT-IR) and laser particle size analysis. Thermogravimetric analysis (TGA) showed the thermal stability of the graft copolymer, and the results showed that the graft copolymer obtained thermally decomposed after 370.86 °C, indicating that it has good thermal stability. Scanning electron microscopy (SEM) revealed the mechanism of the graft copolymers in drilling fluids. The fluid loss control performance and rheology of the filtration reducer were evaluated before and after hot rolling at 180 °C for 16 hours. The results showed that the graft copolymer has excellent fluid loss reduction performance, and it has good fluid loss reduction performance in fresh water, brine and saturated brine. The API fluid loss was only 6.4 mL after hot rolling at 180 °C for 16 h in the brine base slurry. Moreover, the obtained graft copolymer is easily biodegradable, has EC50 ≥ 30 000 and good environmental performance, and can be used in high temperature and high salt reservoir with high environmental protection requirements.


Introduction
With the continuous development of oil and gas eld exploration in deep reservoirs, there are more deep and ultra-deep wells, and the demand for high-performance environmentally friendly drilling uid treatment agents is more urgent. In recent years, people have paid more attention to the environmental protection performance of drilling uids. 1 Although ltration reducers for sulfonated and lignite drilling uids have good temperature resistance, they cause irreversible damage to the reservoir due to their poor environmental protection. Under such drilling conditions, the coordination of the lterability and environmental protection of the drilling uid has become more important. Thus, the research and application of highperformance environmentally friendly lter-reducing agents have received extensive attention. At present, many countries have made some progress in drilling uid technology research, but they oen fail to take into account environmental protection and drilling uid performance. In addition, there are problems such as high cost and difficulty in eld promotion. [2][3][4] The high-performance environmentally friendly ltration reducer can take into account both environmental protection and drilling uid performance.
Natural plants such as starch, xanthan gum, tannin, lignin, vegetable gum, and cellulose have the advantages of nonpolluting and biodegradable properties. In addition, high molecular weight polymers based on natural plant synthesis have been widely used in petroleum exploration. 5 In the past few decades, natural high molecular weight polymer ltration reducers have developed rapidly. According to previous studies, some modied starches have API uid loss between 10 and 20 mL aer aging at 150 C. 6 Saudi Arabia uses a ltrate reducer produced by the local plant date palm, which is suitable for freshwater and saltwater drilling uids. It also has good environmental performance, and its HTHP lter loss is 30 mL aer the ltrate reducer is added to the clay-free freshwater drilling uid. Furthermore, its HTHP lter loss is 20 mL aer the ltrate reducer is added to the clay-free seawater drilling uid. 7 A ltrate reducer synthesized from a low-viscosity polyanionic cellulose polymer is suitable for freshwater, saltwater and seawater conditions, can effectively control uid loss, and has little effect on rheology. 8 Although various cellulose-based gra copolymers have been synthesized and applied to water-based drilling uids, the application of nanocomposite polymers synthesized by the introduction of nano-materials into cellulose-based polymers in water-based drilling uids has been reported less. 9 Cellulose has the advantages of wide distribution, high yield and easy biodegradation. 10,11 Cellulose is alkali-treated from rened cotton and reacted with ethylene oxide as an etherifying agent in the presence of acetone to produce hydroxyethyl cellulose (HEC), which has strong water absorption and is a kind of nonionic cellulose. 6,12,13 HEC can be modied by attaching a hydrophobic group to a hydroxyethyl group to obtain a hydrophobically modied HEC (HMHEC). 14,15 The hydrophobic associating hydroxyethyl cellulose has a high aspect ratio, high specic surface area, good mechanical properties and biocompatibility, 16-20 and is composed of a long-chain hydrophilic main chain, but its molecular structure contains a small amount of hydrophobic side chains or end groups. 21,22 In addition, their skeleton structure will make the polymer dissolve in the aqueous medium. The intermolecular or intramolecular association of the hydrophobic groups also forms a reversible three-dimensional network structure. Furthermore, the intermolecular hydrophobic association region has high affinity, and has a signicant impact on its performance. 15, [23][24][25] With the emergence of new nanomaterials, the application of polymer nanocomposites is rapidly developing. Due to the size effect, specic surface effect, selective adsorption and other properties of the nanoparticles, 26 the introduction of nanoparticles into a single polymer matrix can enhance the relevant properties of the polymer and give it other excellent properties. This can reduce the application of other materials and save costs, and the biopolymer nanocomposites have better performance and environmental protection characteristics. 27 Nanomaterials are used in water-based drilling uids to improve the lubricity of drilling uids, and to improve the rheology and uid loss of drilling uids. 11 In this study, we proposed the branching of the short link with hydrophobic groups to the main chain of hydroxyethyl cellulose to obtain hydrophobic polymerized hydroxyethyl cellulose by carrying out a chain transfer reaction to the polymer. The hydrophobically associating hydroxyethyl cellulose is adsorbed on the nano calcium carbonate to form a micro-nano environmentally friendly ltration reducer. 28,29

Materials
When preparing a high-performance environmentally friendly ltration reducer, the reagents and materials used are: hydroxyethyl cellulose, which can be obtained from Feicheng Yutian Chemical Co., Ltd.; and 1-bromododecane, which was purchased from Macklin. The remaining chemical reagents were purchased from the Guo Yao Group Chemical Reagent Co., Ltd. All reagents and materials were stored at room temperature and under anhydrous conditions. The calcium-based bentonite used for the preparation of the drilling uid base slurry was purchased from Weifang Huawei Bentonite Group Co., Ltd. China.
Preparation of base slurry: 0.8 g sodium carbonate was added to 400 mL water. Then, 16 g of bentonite was added on the high-speed mixer while stirring. The mixture was stirred continuously for 20 minutes, and attention was paid to scrape off the bentonite adhering to the wall of the cup during the stirring. The mixture then underwent static hydration for 24 h at room temperature.

Synthesis of silicone copolymer
Solid sodium hydroxide (2.5 g) and absolute ethanol were put into a three-necked ask and magnetically stirred for 20 minutes to dissolve the mixture evenly. Nano-calcium carbonate (10 g) and 1-bromododecane were dissolved in a trace amount of absolute ethanol and added to the solution, magnetically stirred for 20 minutes, and then ultrasonically shaken for 30 minutes. The above suspension was heated to 65 C, while magnetically stirring. Aer the temperature became constant, hydroxyethyl cellulose (10 g) was added and the mixture was stirred for 5 h. The liquid obtained above was then centrifuged at a high speed of 3000 rpm for 10 min to get the lower solid. The obtained solid was put into a thermostat at 80 C for drying and grinding to obtain a hydroxyethyl cellulose gra copolymer ( Fig. 1).

Characterization of ltration reducer
The particle size distribution measurement of the micro-nano particles was obtained by the Zetasizer Nano instrument, which can determine the particle size in the range of 15 nm to 500 mm by laser diffraction method. Before testing, the test product was put into a 0.1% aqueous solution, and then it was congured. The aqueous solution was then placed in the sample cell for laser particle size scanning (Fig. 2). A laser particle size analyzer was used to characterize the infrared spectroscopy. A small amount of ltrate reducer monomer was placed in a mortar, then uniformly ground with potassium bromide, and compressed with a tablet press. Scanning the ltrate reducer monomer in the wavelength range of 500 cm À1 to 4000 cm À1 to obtain its infrared spectrum (Fig. 3). A thermal analyzer (TGA) was used to measure the thermal stability of the ltrate reducer in a nitrogen atmosphere, with a heating rate of 10 C min À1 and a temperature range of 35 C to 950 C (Fig. 4). A Microtox type biological toxicity tester was used to measure the biological toxicity of the uid loss control agent. Table 1 shows the environmental evaluation standards of environmentally friendly drilling uid treatment agents.

Characterization of the microscopic morphology of the lter cake
The medium pressure lter loss instrument (Kence Instrument (Shanghai) Co., Ltd.) was used to measure the uid loss volume of the base pulp before and aer aging at 160 C/16 h. The obtained lter cake was dried naturally, and then gold was sprayed on the lter cake. Then, an electron microscope, JSM 7900F, (Japan Electronics Co., Ltd.) was used to scan the lter cake.

Rheological and uid loss properties of drilling uid with ltrate reducer
The rheology of the drilling uid added with the ltration reducer was tested using a MOD ZNN-D6 six-speed rotary viscometer (Qingdao Haitongda Special Instrument Co., Ltd.). Its apparent viscosity (AV), plastic viscosity (PV) and yield point (YP) were measured according to API standards. Under the conditions of temperature of 25.0 C and pressure of 0.69 AE 0.03 MPa, a SD3B triple medium pressure lter loss instrument (Kense Instrument (Shanghai) Co., Ltd.) was used to measure the static lter loss (FL API ). The GGS42 type high temperature and high-pressure lter device (Qingdao Jiaonan Tongchun Machinery Petroleum Instrument, China) was used to measure the ltration loss (HT HPAPI ) of HTHP at a pressure of 4.2 AE 0.03 MPa and a temperature of 180.0 C. The measurement was performed within 30.0 minutes, according to API standards. The rheological parameters were calculated as follows: 30,31 In order to study the rheology and uid loss properties of the ltrate reducers, different mass fractions (0%, 0.5%, 1%, 2%) of ltrate reducers were added to the 4% fresh water base slurry and stirred at high speed for 20 minutes. The samples were then heated at 180 C for 16 hours. Then, its medium pressure lter loss, high temperature and high-pressure lter loss and various rheological parameters, and the results are shown in Fig. 5.   Biodegradability refers to the possibility of environmental pollutants being degraded by microorganisms. It is an important indicator for evaluating the acceptance of organic matter by the environment. At present, the commonly used evaluation method at home and abroad is the BOD 5 /COD cr ratio evaluation method. The biodegradability of organic matter can be evaluated by the BOD 5 /COD cr ratio. The higher the ratio, the easier it is to be biodegraded.
The biodegradability of the uid loss control agent was analyzed by measuring the biochemical oxygen demand, the BOD 5 and chemical oxygen demand COD cr of the decomposed uid loss control agent, and the BOD 5 /COD cr ratio was then calculated. The chemical oxygen demand COD cr is the amount of oxidant consumed to oxidize the reducing substances in 1 L of water sample. 20 mL of 1% ltrate reducer aqueous solution was oxidized by adding 10 mL of potassium dichromate with a concentration of 0.25 mol L À1 , and then titrated with (NH 4 )Fe(SO 4 ) 2 to calculate the chemical oxygen demand of the uid loss agent.
The biochemical oxygen demand BOD 5 was collected and domesticated by surrounding bacteria aer adding a certain amount of ltrate reducer solution was added, incubated for 5 days and measured by BOD 5 tester.
2.6.2. Biodegradability. Testing the EC 50 value of the biological toxicity of the drilling uid with the uid loss control agent, this experiment used the luminescent bacteria method to determine the biological toxicity of the drilling uid, and used EC 50 (when the relative luminescence rate is 50%) to characterize the biological toxicity of the test object. The larger the EC 50 value, the lower the biological toxicity of the test substance; the smaller the EC 50 value, the greater the biological toxicity of the test substance.

Results and discussion
3.1 Characterization of ltrate reducer 3.1.1. Infrared spectroscopy characterization. As shown in Fig. 2, a comparative analysis of the FTIR spectrum of the uid loss agent and hydroxyethyl cellulose shows that the tensile vibration of the -OH group in the cellulose molecule is represented by a peak at 3419.91 cm À1 . The peak at 1599 cm À1 represents the C]C tensile vibration in the cellulose molecule. There is no peak position between the peaks of 1449 cm À1 and 1058 cm À1 in the FTIR spectrum of hydroxyethyl cellulose. Compared with the FTIR spectrum of the uid loss agent, the peak position has decreased to 1327.45 cm À1 for the reactive -OH bending vibration. The analysis shows that the more reactive hydroxyl group on the hydroxyethyl cellulose molecular chain undergoes a Williamson etherication reaction with halogenated alkanes, 32 and the graing reaction introduces long-chain alkanes into the hydrophilic molecular chain of hydroxyethyl cellulose. In addition, the infrared spectrum of the ltrate reducer increased the peak at 878.68 cm À1 . The bending vibration at 878.68 cm À1 is formed by the long chain of the introduced alkane. It can be seen from the infrared spectrum of the copolymer that the copolymer contains all of the molecular groups originally designed, which indicates that the hydroxyethyl ber bundle and 1-bromododecane has formed a gra copolymer.
3.1.2. Particle size analysis. Fig. 3 shows the particle size distribution curve of the hydrophobically modied hydroxyethyl cellulose and ltrate reducer. The average particle size of the ltrate reducer is slightly larger than that of the hydrophobically modied hydroxyethyl cellulose, indicating that the modied hydroxyethyl cellulose is successfully adsorbed on the nano calcium carbonate particles. It can be seen from Fig. 3 that the particle size of the gra copolymer is still in the nanometer domain, and still good for plugging and reducing uid loss. 33, 34 3.1.3. Thermal stability of the ltrate reducer. As shown in Fig. 4, when the temperature range is 35 C to 132.86 C, the weight loss rate at this stage is relatively fast, and the weight loss rate is basically unchanged for a long period of temperature range thereaer. This is due to the presence of a certain number of hydrophilic groups in the molecular structure of the gra copolymer, which makes the measured sample absorb part of the free water in the air, and the faster rate of weight loss is caused by the volatilization of this part of the water. In the temperature range of 132.86 C to 370.86 C, the weight loss rate at this stage slows down. This is because the stronger molecular groups in the molecule interact with the water molecules in the environment and form strongly adsorbed bound water on the groups. The volatilization of bound water is slower than that of free water before, so the rate of weight loss is also slower. In the temperature range of 370.86 C to 530.86 C, the thermal weight loss rate is very fast at this stage, and groups such as hydroxyl and ether groups begin to decompose rapidly at this stage. When the temperature is between 530.86 C and 949.3 C, the main chain and side chains of the copolymer begin to break. With the endothermic process, the quality of the copolymer begins to continue to decline. In addition, nano-calcium carbonate begins to thermally decompose at about 600 C, and nano-calcium carbonate is converted into CaO and CO 2 . The nano-calcium carbonate begins to decompose rapidly at about 800 C. Most of the mass residues at 949 C are CaO solids and a very small amount of nano-calcium carbonate that has This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 43204-43212 | 43207 not been completely decomposed. When the temperature reaches 949.3 C, the mass retention rate is 26.29%. In summary, the thermogravimetric curve shows that the ltrate reducer has good thermal stability at high temperatures. Fig. 5 shows the effect of the ltrate reducer addition on the rheology and uid loss control properties of freshwater-based drilling uids. As the temperature rises, AV (a), PV (b) and YP (c) all increase. This is formed by several factors. In the rst aspect, in the drilling uid, the hydrophobic groups of the polymer are associated with each other. The intramolecular or intermolecular association occurs between the macromolecular chains, forming different forms of the micellar nanostructuresupramolecular network structure. In a dilute solution, the gra copolymer occurs mainly via intramolecular association, in which the macromolecular chain curls, the hydrodynamic volume decreases, and the intrinsic viscosity decreases. When the gra copolymer concentration exceeds a certain value, the macromolecular chain will form a dynamic physical crosslinked network structure dominated by intermolecular association, and the solution viscosity will be greatly increased. The second aspect is different from the irreversible shear degradation of general high molecular weight polymers. Under the action of a higher shear force, the cross-linked structure formed by the hydrophobic association of the gra copolymer is destroyed and the solution viscosity decreases. When the shearing effect is eliminated, the cross-linked network structure formed by the association between the macromolecules is reformed and the viscosity is restored. On the other hand, in addition to the van der Waals force, hydrogen bonding force and electrostatic force between HMHEC and nano calcium carbonate particles, hydrophobic association also plays a major role. HMHEC is adsorbed on the surface of nano-calcium carbonate and forms a larger spatial network structure with it using the bridging mechanism, which improves the degree of cross-linking of the entire inorganic/organic system. Moreover, the hydrophobic association is an endothermic effect. 35 In summary, the ltrate reducer has a certain temperature resistance and viscosity increase.

Rheology and uid loss reduction of the ltrate reducer
It can be seen from Fig. 6 and 7 that aer 180 C aging and increased added salt concentration the viscosity rst decreases and then increases to a certain extent, and the uid loss reduction increases. The viscosity is lower aer adding salt than the condition without the added salt. This is because the added salt compresses the diffusion electric double layer of the clay particles. This makes the hydration lm on the surface of the clay particles thinner and reduces the zeta potential between the clay particles, thereby reducing the repulsive force between the particles. This makes it easier for the clay particles to coalesce together, resulting in a decrease in the viscosity of the drilling uid system. Later, as the amount of salt increased, the viscosity increased slightly. The reason for the analysis was that the addition of the small-molecule salt electrolytes would increase the polarity of the solution and increase the hydrophobic association between macromolecules. In addition, the macromolecular long chain of the gra copolymer can prevent the collision and coalescence between the clay particles to a certain degree, weaken the inuence of the salt on the clay particles, and reduce the coalescence phenomenon of the clay particles. At the same time, as shown in Fig. 8, the hydrated group on the long chain of the macromolecule will increase the thickness of the clay particle diffusion electric double layer, and help to form a thinner and lower permeability lter cake. 36,37 In addition, the electron pair on the carbonyl group on the gra copolymer long chain easily adsorbs calcium ions, reducing the inuence of the calcium ions on the clay particles, thereby enhancing the stability of the drilling uid system. However, as the amount of salt increased, the uid loss of the drilling uid did not increase signicantly aer aging at 180 C. This is because the copolymer macromolecular chain can effectively shield the inuence of salt on the clay particles, protect the clay particles from aggregation, help improve the stability of the drilling uid system, and reduce the uid loss under high salt conditions. In summary, the ltrate reducer has obvious salt and calcium resistance.

Microscopic morphology of drilling uid lter cake
As shown in Fig. 9(a) and (b), the mud cake formed by high temperature aging at 180 C for 16 hours without ltration reducer has a rough surface and large clay particle aggregate. In addition, the clay particles have very irregular shapes. There are some tiny pores and cracks on the surface of the mud cake. The clay cannot be fully hydrated to form a hydration lm, which makes it easier for water to lter through the mud cake, resulting in excessive uid loss. As shown in Fig. 9(c) and (d), the surface of the mud cake formed with the ltration reducer is smooth and compact. In addition, there is no large unevenness and no large clay particles are formed. This indicates that the uid loss agent inhibits the coalescence of the clay particles and maintains the clay particles. The dispersibility makes the mud cake have excellent performance, and it is difficult for water to enter the formation, which improves the performance of reducing uid loss. The active hydroxyl group in the modied hydroxyethyl cellulose molecule of the ltrate reducer can form hydrogen bonds with the Si-OH and Al-OH in the bentonite, further blocking the coalescence of clay particles. The ltration reducer has a small volume, large specic surface area, and many surface-active hydroxyl groups. It can form a spatial network structure connected by hydrogen bonds and van der Waals forces. The strength of this kind of space structure is limited, and it is destroyed under shearing action, but the rate of its reformation is also very fast. With the increase of the shear rate, the destruction and formation of the space grid structure become a dynamic balance. This reects the excellent shear thinning. It can effectively block the lter cake and micro-nano voids, and play a role in reducing ltration. In addition, ltration reducers are micro-nano particles, and their higher specic surface area contributes more to heat. [38][39][40] On the other hand, the uid loss additive is a macromolecular copolymer, which has a certain viscosity increase and dynamic shearing effect in the drilling uid base slurry. It can reduce the uid loss to a certain extent, and the macromolecular polymer ltration reducer can also block the smaller pores in the drilling uid, which can effectively reduce the uid loss. 41-43

Environmental performance evaluation
Testing the biodegradability of the drilling uid base slurry added with a uid loss agent, BOD 5 /COD cr can indicate the biodegradability of the uid loss agent. If BOD 5 represents the non-biodegradable part of COD cr , the proportion of the nonbiodegradable organic matter in the base pulp can be represented by BOD 5 /COD cr . When the ratio exceeds 0.45, it means that the non-biodegradable organic matter accounts for less than 20% of all organic matter. When the ratio is lower than 0.2, the non-biodegradable organic matter accounts for more than 60% of all organic matter. Using the dichromate method to test the value of COD cr . The determination of BOD 5 adopts the GB7488-87 water quality ve-day biochemical oxygen demand determination method. 44 To test the EC 50 value of the biological toxicity of the drilling uid base slurry with added uid loss agent, this experiment uses the luminescent bacteria method to test the biological toxicity of the uid loss agent. Using EC 50 (when the relative luminescence rate is 50%) to characterize the biological toxicity of the test substance, we found that the biological toxicity of the test substance decreased with larger EC 50 value. In addition, a greater biological toxicity of the test substance resulted in a smaller EC 50 value. 45 Aer the environmental performance test of the ltration reducer, the ltration reducer has Y ¼ BOD 5 /COD cr ¼ 17.42%, which is a type that is easily biodegradable. A Microtox-type biological toxicity tester was used to test the biological toxicity of the ltrate reducer. The EC 50 value of the ltrate reducer was 3.41 Â 10 4 mg L À1 , indicating that the ltrate reducer was nontoxic (Fig. 10).

Comparison with other uid loss additives
We compared the synthetic uid loss agent with sulfonated lignite (SMC) 46 and sulfonated phenolic resin (SMP), 47 which are commonly used in oil elds. It can be seen that the uid loss of the micro-nano environment-friendly uid loss control agent synthesized in this paper before aging at 180 C is 5.6 mL, and the uid loss aer aging at 180 C is 6.4 mL. The micro-nano environment-friendly uid loss reducer synthesized in this paper is better than other similar products.

Conclusion
In this paper, the hydrophobically associating hydroxyethyl cellulose was obtained by modifying the hydroxyethyl cellulose, and the gra copolymer was obtained by graing it onto the surface of the nanometer calcium carbonate. Finally, the micronano environment-friendly ltration reducer was successfully synthesized. The structure of the ltration reducer was characterized by particle size analysis and infrared spectroscopy, and the mud cake was characterized by scanning electron microscope SEM. Its thermal stability was characterized by TGA, and the results showed that the ltration reducer has strong thermal stability. This is because the synthetic ltration reducer belongs to the micro-nano level, has a high specic surface area, and its contribution to contrast heat is higher. Moreover, the ltration reducer is a macromolecular copolymer, which has the effect of increasing the viscosity and dynamic shear force in the drilling uid base slurry, and can reduce the uid loss to a certain extent. In addition, the macromolecular polymer ltration reducer can block the micro-nano pore size in the drilling uid by itself, stabilize the well wall, and can effectively reduce the uid loss. The results showed that when the added amount of ltration reducer was 2.0 wt%, the uid loss could be reduced to 6.4 mL aer aging at 180 C for 16 h. The results show that the ltration reducer has good thermal stability, salt and calcium resistance. In addition, from the scanning electron micrograph analysis of the mud cake, it can be seen that the surface of the mud cake formed by adding the ltration reducer to the base slurry is smooth and compact, which indicates that the ltration reducer inhibits the coalescence of the clay particles and maintains the dispersion of the clay particles. This makes it more difficult for water to enter the formation, and improves the performance of reducing uid loss. Finally, through its environmental performance test, the results show that the ltration reducer has no biological toxicity and is easily biodegradable.

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