Hydrophobic Monomer Copolymerization AM can be copolymerized with hydrophobic monomers to prepare hydrophobic associated polyacrylamide (HAPAM), which involves grafting a certain proportion of hydrophobic groups onto the main chain of PAM molecules, making them amphiphilic molecules. The hydrophobic groups on the HAPAM molecular chain form intramolecular or intermolecular association structures due to repulsive interactions with water molecules. Hydrophobic association is an endothermic process, and heating is beneficial for intermolecular association. Hydrophobic association enhances the thermal stability of HAPAM molecules. The commonly used PAM hydrophobic modified monomers are AM derivatives, acrylic esters, styrene and its derivatives, and ionic hydrophobic monomers.   AM derivatives The hydrophobic groups in this type of HAPAM molecular chain are connected to the main chain through amido groups, and hydrogen bonds can be formed between amido groups, amido groups, and water molecules. The copolymer has good water solubility and is easy to form intramolecular associations. The drag reduction effect may decrease due to the curling of the molecular chain. The most commonly used AM derivative hydrophobic monomer is N-alkylacrylamide. McCormick et al. found that the viscosity of N-n-decylacrylamide/AM copolymer solution increases with increasing temperature. But the amide group undergoes hydrolysis around 80 ℃, causing the hydrophobic group to fall off and the hydrophobic association structure to be destroyed.   Acrylates The hydrophobic groups in this type of HAPAM molecular chain are connected to the main chain through ester groups, and commonly used acrylic ester based hydrophobic monomers are alkyl acrylate, fluorinated acrylate, etc. The hydrolysis temperature of ester groups in alkyl acrylate is lower than that of amide groups; Fluorinated acrylate has good thermal stability, but it can also undergo hydrolysis at high temperatures, causing the detachment of hydrophobic groups, and the cost is high.   Styrene and its derivatives To solve the problem of high-temperature hydrolysis of AM derivatives and acrylic esters, styrene and its derivatives are selected as hydrophobic monomers to copolymerize with AM. The hydrophobic groups of the copolymer are directly connected to the main chain of HAPAM molecules, which is easy to form intermolecular associations and has good thermal stability.   Ionic hydrophobic monomers Ionic hydrophobic monomers contain both hydrophobic and ionic groups (anions and cations), which not only enhance the solubility of HAPAM in water, but also inhibit intramolecular association through electrostatic repulsion, leading to molecular chain extension and increasing fluid mechanical volume. The commonly used ionic hydrophobic monomers are long-chain quaternary ammonium salt monomers (such as octadecyldimethylallyl ammonium chloride). HAPAM generally contains multiple hydrophobic groups, and its association performance improves with the increase of hydrophobic group content and chain length, but its water solubility also decreases. Therefore, the content of hydrophobic groups is usually less than 2% (x), and the carbon number of hydrophobic chains is usually between 6 and 8; Intramolecular association is not conducive to molecular chain extension, resulting in a decrease in drag reduction effect. Hydrophobic monomers that can inhibit intramolecular association are preferred; As the temperature continues to rise, the thermal motion of molecular chains and hydrophobic groups intensifies, and the hydrophobic association weakens. The research results indicate that hydrophobic modification has limited effect on the temperature resistance of PAM drag reducing agents.   Hydrophobic association PAM drag reducing agent can also improve the sand carrying capacity of smooth hydraulic fracturing fluid. The closure stress of deep reservoir strata is high, and higher concentrations of proppant need to be injected to achieve effective support for the fracture network. By adjusting the concentration of drag reducing agent to regulate the viscosity of the fracturing fluid, variable viscosity drag reducing agent technology has gradually formed. This technology not only retains good drag reducing effect, but also solves the problem of poor sand carrying capacity of smooth hydraulic fracturing fluid, meeting the construction requirements of forced sand fracturing and reducing the application cost of fracturing fluid.   Inorganic Nano/PAM Composite Drag Reducing Agent Inorganic nanomaterials and PAM can form high-performance inorganic nano/PAM composites through physical adsorption, hydrogen bonding, and covalent bonding, which have improved temperature resistance, shear resistance, salt resistance, and other properties. They have been widely used in drilling, fracturing, and enhanced oil recovery. The synthesis methods of inorganic nano/PAM composite drag reducing agents mainly include: . Blending method. Inorganic nanoparticles are uniformly dispersed in PAM through mechanical stirring and other methods. The inorganic nanoparticles are combined with PAM molecular chains through physical adsorption, hydrogen bonding, etc. The operation is simple, but the nanoparticles are prone to aggregation; . Copolymerization method. By surface modification of inorganic nanoparticles and grafting copolymerization with monomers such as AM, covalent and ionic bonds can be formed between inorganic nanoparticles and PAM molecular chains, further enhancing the thermal stability of PAM compared to the blending method.   The most studied inorganic nanomaterial is nano SiO2. Cao et al. conducted a comparative study on the thermal stability of AM/AMPS copolymer, AM/AMPS/nano SiO2 copolymer, and AM/AMPS/aminated nano SiO2 copolymer. They found that the hydrolysis degrees of the three copolymers were about 50%, 40%, and 25% after aging in an aqueous solution at 140℃ for 12 hours, and the hydrodynamic radius decreased by about 50%, 25%, and 5%, respectively. The experimental results indicate that the thermal stability of the AM/AMPS/aminated nano SiO2 copolymer molecular chain is the best; The aminated nano SiO2 and PAM molecular chains in AM/AMPS/aminated nano SiO2 copolymers are simultaneously connected in three forms: hydrogen bond, ion bond, and covalent bond, which is more stable than the structure of AM/AMPS/nano SiO2 copolymers. However, many studies on inorganic nano/PAM composites have shown that the specific surface area and strength effects of inorganic nanoparticles can easily lead to particle aggregation. This problem needs to be addressed during the preparation process in order to obtain stable and dispersed composite materials for industrial applications.   Application Status of 3 Temperature Resistant PAM Drag Reducing Agents                                     PAM drag reducer can be divided into powder type, water in oil lotion type and water in water lotion type according to its appearance. See Table 1 for main characteristics. The powder drag reducer has the advantages of convenient long-distance transportation and long-term storage, and is more stable than lotion drag reducer; Lotion friction reducer has faster dissolution than powder friction reducer, which is more suitable for continuous mixed fracturing operations, and even can realize direct mixing without configuration, simplifying the fracturing pumping process and reducing the construction cost. The AM/SSS copolymer drag reducing agent synthesized by Wang Wenzhe et al. achieved a viscosity of 2.48 MPa · s after shearing at a shear rate of 170 s-1 for 1 hour in high salinity water at 140 ℃. The AM/SSS drag reducing rate of 0.1% (w) can reach 67.2%. Yao Yiming et al. prepared a high-temperature resistant drag reducing agent using AM, acrylic acid (AA), cationic monomers, and P-type rigid group unsaturated monomers, and applied it in key lateral drilling horizontal wells of Dingshan Structure B. The vertical depth of the formation was 4095.46 meters, the horizontal section length was 1234.00 meters, the formation temperature was 143℃, and the on-site drag reducing rate reached 78.0%. Gao Qingchun et al. synthesized AM/NVP/ACMO/AMPS copolymer drag reducing agent, with a drag reduction rate of 71.9% at 140℃ using 0.1% (w) AM/NVP/ACMO/AMPS. The agent was applied on-site in the THXX well of the Yingshan Formation, with a depth of 6156.05-6249.00 m and a formation temperature of 140℃. The on-site implementation effect was good. Variable viscosity drag reducing agents have been widely used in the fracturing of tight shale oil and gas reservoirs. Ibrahim et al. synthesized a variable viscosity drag reducing agent FR1 using AM, AA, and hydrophobic monomer Z as monomers. The drag reducing rate was higher than 70% in both clean water and saline water, and the viscosity of FR1 at 0.3%（φ） could reach 15 MPa·s. Jia Jinya et al. prepared a hydrophobically associating viscosity reducing agent AM/DMC/MEDDDAB using AM, methacryloyloxyethyltrimethylammonium chloride (DMC), and methacryloyloxyethyldodecyldimethylammonium bromide (MEDDAB) as monomers. The viscosity of 0.1% （w）AM/DMC/MEDDDAB reached 10 MPa·s and the drag reduction rate was 65.74%. Zhao et al. proposed a high viscosity polymer drag reducing agent HVFR, which has been applied on-site in Montney, Canada. Compared with conventional slippery water, viscous slippery water not only has good drag reduction effect, but also has good sand carrying capacity.   The research and application of inorganic nano/PAM composite drag reducing agents are less than those of AM temperature resistant copolymerization drag reducing agents. Yu Weichu et al. developed JHFR-2 type green and clean nanocomposite high-efficiency liquid drag reducer, with a drag reduction rate of 70.2% after aging at 130℃ for 8 hours. There are many synthesis and applications of temperature resistant PAM drag reducing agents, mainly by introducing large side groups, rigid side groups, hydrophobic groups, and combining them with nanocomposites. By fully utilizing the advantages of each method, the contradiction between temperature resistance and drag reducing effect can be solved, which can meet the construction needs of deep shale gas sliding water fracturing.   Conclusion PAM drag reducing agent is the core additive of smooth hydraulic fracturing fluid. PAM molecular chains are prone to thermal oxidative degradation and fracture under high temperature conditions, resulting in a significant decrease in drag reducing effect. The temperature resistance of PAM drag reducing agents can be improved through three methods: copolymerization with temperature resistant monomers, copolymerization with hydrophobic monomers, and synthesis of inorganic nano/PAM composite drag reducing agents. However, the temperature resistance during on-site application is only one of the important properties of drag reducing agents, especially in the development of deep shale gas and other reservoirs, the following research needs to be carried out.   During the smooth water fracturing process, a large amount of fracturing fluid is injected into the formation. PAM drag reducing agent is a high molecular weight polymer that can easily adsorb on the rock surface and block microcracks and pore throats, leading to a decrease in single well productivity. A biodegradable drag reducing agent should be developed, which is synthesized by copolymerizing AM with degradable monomers under certain conditions (time, temperature, etc.) to obtain a molecular chain composed of degradable and non degradable parts, achieving self degradation of the drag reducing agent in the formation and reducing the degree of damage caused by the drag reducing agent to the formation.   As the temperature resistance of PAM type drag reducing agents increases, the flexibility of the drag reducing agent molecules also decreases, resulting in a decrease in drag reducing effect. As the reservoir temperature increases, this contradiction becomes more prominent. How to solve the contradiction between temperature resistance and drag reduction performance will be one of the important directions in the research of PAM drag reducing agents.   In the process of oil and gas exploration and development, scientific and effective environmental protection measures must be taken. During the research and development of drag reducing agents, non-toxic and environmentally friendly synthetic materials must be selected, and green and economical synthesis routes and processes must be optimized to minimize the adverse effects of drag reducing agents on the environment.

Abstract                                                         Slick water fracturing is one of the main technical means of economic development for shale gas reservoirs，and polyacrylamide(PAM) drag reducer is the primary additive of slick water fracturing fluid. China has an abundance of deep shale gas resources，and the reservoirs are generally high temperature. PAM is susceptible to thermal degradation which leads to performance degradation. The development history of polymer drag reducers and the mechanism of temperature influence on the molecular stability of PAM drag reducers are reviewed. Then，three molecular design methods of temperature resistance PAM drag reducers were reviewed. Finally，the synthesis method of temperature resistance PAM drag reducer and its application in the fracturing deep shale gas wells are described. This paper aims to provide theoretical basis for the research of high temperature resistant fracturing fluids suitable for the development of deep oil and gas reservoirs. China has abundant shale gas resources, with deep shale gas (buried at depths of 3500 to 4500 meters) accounting for over 65% of the total shale gas resources. It will undoubtedly become the main force in China’s oil and gas exploration and development to increase reserves and production. Slick water fracturing is the main technical means for efficient development of shale reservoirs, and polymer drag reducing agents are the core additives of slick water fracturing fluids. Deep shale gas has the characteristics of large burial depth and high temperature (up to 155℃). Smooth hydraulic fracturing fluid generates a large amount of turbulence during the injection process, losing a large amount of kinetic energy that should have reached the reservoir, causing the fracturing fluid to fail to achieve its expected goals. The addition of polymer drag reducing agents can suppress the generation of turbulence and significantly reduce the friction generated by long wellbore during hydraulic fracturing. Polymer drag reducing agents mainly consist of polyacrylamide (PAM), which has good thermal stability under anaerobic conditions and is less prone to breakage of molecular main chains at deep shale gas reservoir temperatures. However, when PAM is applied, it is mostly in an aqueous solution state, and the entry of dissolved oxygen in the air is inevitable during solution preparation, leading to the oxidation and degradation of PAM. High temperature can exacerbate this process, causing damage and fracture of PAM molecular chain structure, and significantly reducing the application performance of PAM. Therefore, the development and application prospects of temperature resistant PAM drag reducing agents are broad, and researchers mainly enhance the thermal stability of PAM by enhancing the rigidity of molecular chains.   This article reviews the research progress of polymer drag reducing agents, discusses the influence of temperature on the drag reducing effect of PAM, summarizes the research and application progress of temperature resistant PAM drag reducing agents, and introduces the technical status of PAM drag reducing agents suitable for high temperature reservoir slick water fracturing, in order to provide reference for the research and selection of fracturing fluids.   Overview of PAM Drag Reducing Agents Development History of Polymer Drag Reducing Agents Polymer drag reducing agents can be divided into natural plant gum drag reducing agents and synthetic polymer drag reducing agents based on their sources. Natural plant gum drag reducing agents mainly include guanidine gum, xanthan gum, coumarin gum, etc. In the 1860s to 1980s, guanidine gum and its derivatives (hydroxypropylation, carboxymethylation, cation, etc.) were mostly used in fracturing fluids. However, the drag reduction rate of natural plant gum is generally lower than 65%, which is not an ideal drag reducing agent. In order to solve the problem of poor drag reduction effect of natural plant gum, researchers have conducted a large amount of research on synthetic polymer drag reducing agents. Synthetic polymer drag reducing agents mainly include polyethylene oxide, polyisobutylene, and PAM, among which PAM has the best drag reducing effect, with a drag reduction rate of over 70%. With the continuous deepening of research, low-cost and good drag reduction PAM has become the main research object.   Effect of Temperature on PAM Drag Reducing Agents The high temperature of deep shale gas reservoirs puts forward higher temperature resistance requirements for fracturing fluids. The increase in reservoir temperature causes a synchronous increase in temperature in the wellbore, and the temperature resistance of drag reducing agents has become an important performance indicator of fracturing fluids. Thermogravimetric analysis found that PAM molecules began to degrade under anaerobic conditions at 326℃, indicating that pure PAM had good thermal stability. The thermal oxidative degradation process of PAM is shown in Figure 1. As shown in Figure 1, PAM undergoes oxidative degradation under aerobic conditions, resulting in the breakage of the main chain of molecular carbon and a decrease in molecular weight. High temperature can accelerate the oxidative degradation of PAM. Zhu Linyong et al. found that PAM in aqueous solutions under a 90℃ air atmosphere undergoes significant degradation after 20 hours, resulting in a viscosity loss of over 80% of the PAM solution. Therefore, when using slick water for fracturing in high-temperature reservoirs, PAM drag reducing agents need to have good temperature resistance to maintain good drag reducing effects. Research on Temperature Resistant PAM Drag Reducing Agent The drag reduction performance of polymers in aqueous solutions is mainly related to their molecular weight, flexibility, chain structure, and solubility. The higher the molecular weight of the polymer, the stronger its molecular flexibility, the smaller the proportion of side groups, the stronger its water solubility, and the higher its degree of extension in water, the better its drag reduction effect. With the increasing development scale of deep shale gas and other deep oil and gas reservoirs, drag reducing agents need to maintain good performance under high temperature conditions. The methods to improve the molecular thermal stability of PAM drag reducing agents include: . Synthesis of ultra-high molecular weight PAM; . Copolymerization with temperature resistant monomers to improve molecular chain rigidity; . Hydrophobic modification, utilizing intermolecular and intramolecular hydrophobic associations to enhance thermal stability; . Nano material composite modification enhances molecular structural strength.   With the increase of molecular weight, the temperature resistance of PAM has been improved to a certain extent. However, the industrial production of ultra-high molecular weight PAM is difficult, and the molecular weight cannot be infinitely increased. The water solubility and shear resistance of PAM will also deteriorate with the increase of molecular weight, and the cost of raw materials will increase. Therefore, the synthesis of ultra-high molecular weight PAM is not an ideal method to improve the temperature resistance of drag reducing agents. The molecular design of temperature resistant PAM drag reducing agents includes temperature resistant monomer copolymerization, hydrophobic monomer copolymerization, and inorganic nano/PAM composites.   Temperature Resistant Monomer Copolymerization By copolymerizing acrylamide (AM) with temperature resistant monomers, large or rigid side groups can be introduced into the PAM molecular chain, increasing the motion resistance of the PAM molecular segment, weakening the thermal motion such as rotation and vibration inside the PAM molecule, increasing the rigidity of the molecular chain, and improving the thermal stability of PAM. The commonly used temperature resistant comonomers are monomers containing sulfonic acid groups and monomers containing cyclic structures.   Monomers containing sulfonic acid groups – SO3- is a strongly polar group with good thermal stability and is not sensitive to high temperatures, which can improve the rigidity and water solubility of PAM molecular chains. The commonly used sulfonic acid containing monomers are 2-acrylamide-2-methylpropane sulfonic acid (AMPS), sodium styrene sulfonate (SSS), sodium vinyl sulfonate, sodium propylene sulfonate, etc. Currently, most research reports are about AMPS. The AM/AMPS copolymer molecular chain contains large side groups, which increases the steric hindrance and further enhances the rigidity of the PAM molecular chain, making the AM/AMPS copolymer have good temperature resistance. However, the improvement of temperature resistance of AMPS on PAM is limited, as the amide group on AMPS undergoes hydrolysis with increasing temperature, leading to the detachment of sulfonic acid groups and reducing the stability of AM/AMPS copolymer molecular chains. Some researchers have proposed using AMPS to enhance the temperature resistance of PAM at temperatures below 93℃. Compared with AM/AMPS copolymers, the side groups of AM/SSS copolymers contain benzene rings, while the stability of the benzene sulfonic acid side group is better but the water solubility is poor. Borai et al. prepared AM/SSS copolymers through radiation induced template polymerization, proposing that hydrogen bonds can be formed between sulfonic groups and carbonyl and amine groups in AM, thereby forming association structures within and between PAM molecules, enhancing the intermolecular forces of the polymer and enhancing the stability of PAM.   Monomers with circular structure The commonly used monomers with cyclic structures are N-vinylpyrrolidone (NVP), chitosan, and vinyl β- Cyclodextrin and acryloyl morpholine (ACMO), etc. The introduction of a circular structure into the side groups of PAM molecules increases the steric hindrance of molecular chain segment movement, reduces the number of conformations generated by the internal rotation of C-C single bonds, and locally becomes rigid, improving the thermal stability of PAM molecular chains. The most commonly used monomer with circular structure is NVP. Xu et al. used thermogravimetric and infrared spectroscopy techniques to study the thermal stability of AM/NVP copolymers. When the temperature is below 300 ℃, NVP can effectively inhibit the hydrolysis of amide groups, and hydrogen bonds are formed between the carbonyl and amide groups in NVP, enhancing the stability of PAM molecular chains. Moradi Araghi et al. compared the thermal stability of 2-AM-2-methylpropylsulfonic acid sodium (AM/NaAMPS) copolymer and AM/NVP copolymer at 121 ℃ and found that the AM/NaAMPS copolymer fully hydrolyzed after 30 days of aging, while the hydrolysis degree of the AM/NVP copolymer was about 80% after 100 days of aging. The experimental results indicate that the thermal stability of AM/NVP copolymer is better. The copolymerization of AM with temperature resistant monomers increases the resistance of molecular chain segment motion, weakens the thermal motion within and between molecules, improves the rigidity of molecular chains, and enhances the temperature resistance of PAM. When designing the molecular structure of temperature resistant monomers, attention should be paid to: . The first choice for industrial production and application is water-soluble and temperature resistant monomers; . The drag reduction effect of PAM is positively correlated with the flexibility of the molecular chain. While improving the rigidity of the molecular chain, consideration should be also given to the flexibility of the molecular chain; . A single temperature resistant monomer may not be able to meet the needs of temperature resistance and drag reduction at the same time, and multiple temperature resistant monomers can be considered for multicomponent copolymerization.   We (Dico Energy) has a series of pipeline drag reducing agent products, and the detailed information of each product can be found on our company’s website www.dicoenergy.com in Drag Reducer.  

2.2 Process Simulation of Polymer Systems 2.2.1 Process Simulation within Polymer Systems PAM hydrogel is widely used as profile control agent in oil and water wells for profile control and water plugging. Among them, cross-linking agent has an important impact on the gelling performance of PAM hydrogel. The cross-linked PAM represented by water-soluble phenolic forms a cross-linked network structure through covalent bonds. There are many active sites and condensation reaction types in PAM/water soluble phenolic system. The study of crosslinking reaction mechanism is helpful to further develop phenolic crosslinked PAM hydrogels. Ni et al. used density functional theory, MM and MD methods to study the reaction mechanism of water-soluble phenolic aldehyde and PAM crosslinking from the thermodynamic and kinetic aspects of the crosslinking reaction. They further explained the mechanism by combining XPS characterization results and established a network structure of the crosslinked polymer (Figure 3). The average density and number of crosslinking points of the crosslinked polymer were calculated and simulated. The simulation results indicate that the density of the cross-linked product decreases, which is due to the network supported by trimethylolphenol (THMP) between PAM molecules after cross-linking, and the loose arrangement of amorphous polymer molecular chains. In addition, it can be seen from the type of cross-linking points that the number of cross-linking points of PAM/THMP is greater than that of THMP/THMP, and the amide group involved in the cross-linking reaction accounts for about 60% of the total amide group. Further structural characterization of the cross-linked product using XPS shows that the total number of amide groups involved in the reaction accounts for 61% of the total amide group, which is consistent with the calculation and simulation results. It is confirmed that the condensation reaction of PAM and THMP in the system is the main dynamic reaction. Understanding this mechanism will help to develop a new type of PAM gel. In addition to organic cross-linked polymer system, inorganic cross-linked polymer gel system is also widely used in low-temperature oil and gas wells. Hamza et al. crosslinked aluminum acetate (AlAc) with PAM, and added bentonite as an additive to improve the stability of the gel system. In this study, the effects of AlAc particle size and the amount of bentonite added on the PAM/AlAc gel process were investigated. The experimental results show that the low content of〔0.5%~1.0% (w)〕bentonite can be used as an additive to delay the crosslinking process of small particle size AlAc, and will not have a significant impact on the strength of gel. Simulate and calculate the relevant parameters of Al3+ adsorption on the surface of bentonite. The adsorption energy of Al3+ is 11.854 eV. When another Al3+ is added to the simulation system, due to the repulsive effect of charges between Al3+, the adsorption energy decreases to 9.25 eV, and the Al-O bond length increases to 0.185 nm. The adsorption of Al3+ on the bentonite surface hindered the interaction between AlAc and PAM, thus prolonging the gel time.   The combination of polymers and surfactants can produce synergistic effects, resulting in excellent performance of the composite system. In recent years, research on the interaction between polymers and surfactant systems has become a topic of interest for researchers, and with the introduction of molecular simulation technology, the study of polymer surfactant systems has gradually deepened from macroscopic properties to microscopic structures. Various ordered aggregates can be formed in a mixed system of polymers/surfactants, such as spherical aggregates, rod-shaped aggregates, vesicles, etc. Hu et al. investigated the self-assembly behavior of a mixture of HPAM and cationic surfactant dodecyltrimethylammonium bromide (DTAB) using CGMD method and Martini force field simulation. The Martini force field was parameterized systematically and is a coarse-grained force field suitable for molecular dynamics simulation of biomolecular systems. This study investigated the effects of HPAM hydrolysis degree and DTAB concentration on self-assembly morphology and formation process at the micro level (Figure 4). Under different hydrolysis conditions, with the increase of DTAB concentration, the aggregates transform from spherical to rod-shaped, and under high hydrolysis conditions, the rod-shaped aggregation structure tends to bend (Figure 5). The effect of DTAB concentration on the transformation of ball/rod morphology was quantitatively analyzed by calculating relative shape anisotropy and Rg. Simulation results showed that the spatial hindrance effect caused by the increase of DTAB concentration had an important impact on the self-assembly morphology of aggregates. Exploring the dynamic information of the three-dimensional structure and self-assembly process of polymer/surfactant mixtures at the molecular level is beneficial for expanding their applications. 2.2.2 Simulation of Polymer Flooding Process Polymer solutions have both viscosity and elasticity, and as displacement agents, they must have sufficient ability to overcome the interaction between oil and rock walls. PAM is the most widely used oil displacement polymer, and a deep understanding of the displacement mechanism of polymer oil displacement is of great significance for improving oil recovery. At present, physical simulation experiments such as sand filled pipe models and artificial rock cores have become the main means of studying the mechanism of polymer flooding, but these experiments are still limited to the level of macroscopic oil recovery. More and more researchers are exploring the mechanism of polymer flooding from the perspective of molecular theory. Song Kaoping proposed to describe the molecular forces involved in the polymer flooding process based on the relevant theories of molecular dynamics. By comparing the molecular effects of water flooding, he proved that the viscoelasticity of polymer solutions is a macroscopic reflection of the friction and impact forces between polymer molecules and crude oil molecules, thereby confirming that polymer flooding can significantly improve oil recovery efficiency.   Based on molecular simulation, the study of the micro flow mechanism of polymer flooding at the atomic scale can provide a theoretical basis for improving oil recovery technology in complex oil reservoirs through polymer flooding. Fan et al. investigated the molecular mechanism of using polymer viscoelasticity for oil recovery. A model of viscoelastic polymer displacement of residual oil in nanochannels was established using MD simulation method, and the displacement process of trapped oil droplets in nanochannels was studied (Figure 6). A comparison was made between the oil displacement effects of water flooding and polymer flooding with different chain lengths, and it was found that there were significant differences in the oil displacement effects of polymers with different chain lengths. According to Figure 7, as the chain length increases, using a smaller amount of polymer can displace more oil, resulting in higher micro oil recovery efficiency. The analysis results of polymer viscoelasticity show that with the increase of polymer chain length, the storage modulus increases, the relaxation time prolongs, and the time required for adjusting the configuration of polymer long chains is extended, resulting in an enhanced polymer elastic effect. This enhancement of elasticity allows the polymer to stretch more in the pores and apply stronger pulling force to the oil droplets trapped in dead corners, which is beneficial for improving oil recovery efficiency. During the oil recovery process, in addition to the influence of viscosity, the wettability of rocks can reflect the affinity or hydrophobicity of the rock surface towards oil and water. If the rock is hydrophilic, water can easily drive oil away from the surface of the rock; If the rock is lipophilic, it is not easy to drive oil away from the surface of the rock. Exploring the effect of polymers on rock wettability can lay the foundation for further developing polymers as wettability regulators in enhanced oil recovery technologies. Ahsani et al. used MD simulation method to conduct molecular dynamic simulation of the adsorption process of HPAM on rock surfaces, and observed the changes in wettability of carbonate reservoirs by HPAM at the microscopic scale. They calculated the non bonding energy, bonding energy, and surface tension of the polymer, water molecules, and rock three-phase system. Under the conditions of environmental temperature and reservoir temperature, when the polymer adsorbs on the surface of carbonate, the adsorption contact angle of the polymer decreases to 130°and 140°, respectively, from high lipophilicity to moderate lipophilicity, that is, the surface wettability tends to shift towards water wettability. This simulation result is consistent with the experimental results. The effect of temperature on the contact angle of HPAM adsorption on rock surface was investigated, and the simulation results were opposite to the experimental results. The simulation results showed that in the Lennard Jones model, as the temperature increased, the molecular kinetic energy increased and the polymer desorbed on the rock surface; Under experimental conditions, due to the occurrence of ion exchange and thermal decarboxylation phenomena, the adsorption of polymers on the rock surface increases with the increase of temperature, and the polymer surface tends to be in a water wet state.   3.Conclusion PAM has a wide range of applications in the field of oil fields, and molecular simulation technology can be used to deeply study the micro mechanisms of polymer system structure and size changes. At present, the simulation research system mainly studies polymer viscoelasticity, intermolecular interactions, and interactions between molecules and rock walls at the molecular level of PAM, in order to analyze the performance of polymer systems and explain the oil recovery process. With the continuous progress of oilfield development technology, the use of molecular simulation technology to guide experimental research is an inevitable trend in the future, such as the micro mechanism of functional polymers, rational design of polymer molecules, and effect prediction. With the continuous development of computer technology, the system that can be simulated in the future will become more complex, and the simulation results will also be more accurate. Molecular simulation technology will play a more powerful role in the field of oil fields.

Abstract Polyacrylamide has a wide range of applications in petroleum production, and it is of great significance to study the properties of polyacrylamide solution and the mechanism of its application at the microscopic level. In recent years, computer molecular simulation technology has been continuously developing, which has high flexibility and fills the gap between macroscopic experiments and theory. It is increasingly being applied to the study of molecular behavior in polymer systems. This article reviews the research progress of computer molecular simulation technology in the application of polyacrylamide in oil fields, elaborates on the main molecular simulation technologies and application types, and reveals the beneficial role of this technology in the field of petrochemicals.   Polyacrylamide (PAM) is a linear water-soluble polymer formed by homopolymerization of acrylamide or copolymerization with other monomers. PAM is currently widely used in fields such as oil extraction, sewage treatment, food processing, and medicine, and is known as a “versatile additive”. In recent years, China’s PAM production has reached one million tons, of which the demand for increased oil production accounts for 44.9%, making it the largest downstream market. Especially in the fields of profile control and water plugging, tertiary oil recovery, and drilling, PAM has a wide range of applications. The presence of amide groups on the side chains of PAM molecules gives them excellent water solubility and high reactivity, making them easy to obtain branched or network structures through grafting or crosslinking; The good viscoelasticity of PAM solution mainly comes from intermolecular interactions, molecular entanglement, and hydrogen bonding. Due to the significant impact of factors such as the degree of hydrolysis and modification methods of polymer molecules on their solution properties and application effects, studying the spatial configuration and intermolecular forces of polymer chain like macromolecules on solution properties and exploring the structure-activity relationship is of great significance. The traditional method studies the related properties of polymer systems by exploring the rheological and viscoelastic properties of polymer solutions. Methods such as UV-Vis, SEM, light scattering, and chromatography have been applied to the study of polymer system configuration information. However, the above methods can only obtain the basic properties and configurations of polymer solutions, and cannot accurately describe the behavior and structure-activity relationship of molecules in polymer solutions at the micro scale. Computer simulation technology can qualitatively describe the process by establishing mathematical models and can simulate some quantitative relationships between the structure and performance of the system. Studying the molecular behavior of polymer systems at the micro scale is of great significance for analyzing polymer properties and predicting performance. This article reviews the research progress of the application of computer molecular simulation technology in PAM used in oil fields, and elaborates on the main molecular simulation technologies and application types.   1.Overview of Molecular Simulation According to the different spatial and temporal scales, molecular simulation methods can mainly be divided into electronic scale, molecular (atomic) scale, and mesoscopic scale. The simulation at the electronic scale is mainly based on quantum mechanics methods. Quantum mechanics simulation is based on the study of the non localization of electrons in molecules, mainly including ab initio calculations, semi empirical molecular orbital calculations, and density functional theory. It can calculate most of the properties of molecules, such as conformation, ionization energy, electron density, transition states, and reaction pathways. The molecular (atomic) scale calculation methods mainly include molecular mechanics (MM), molecular dynamics (MD), and Monte Carlo calculation. They do not consider the motion and changes of electrons, and describe the interaction forces within and between molecules using atoms or molecular clusters as the smallest unit. Based on statistical theories such as ensemble averaging, problems can be analyzed. They can simulate the static structure and dynamic behavior of the system, obtain the equilibrium structure and thermodynamic properties of the molecule, but cannot obtain other properties related to electronic structure. The above simulation methods are all focused on microscale simulations, which may have limitations when dealing with multi-scale problems. The model of the mesoscale method is larger in both spatial and temporal scales than traditional methods, with the basic unit being beads composed of several functional groups. Therefore, the application of mesoscale simulation can simulate more complex systems, and can study complex fluids, polymer composite systems, and complex material systems at the nanoscale to micrometer scale. The calculation methods at the mesoscale include coarse-grained molecular dynamics (CGMD), dissipative particle dynamics (DPD), Brownian dynamics simulations, and lattice Boltzmann simulations.   Most studies on PAM systems aim to explore molecular structures or system properties, and relevant simulation methods based on classical mechanics are the best calculation methods. Discuss the structural changes of molecules through typical structural parameters and forces, and characterize bond length, bond angle, dihedral angle changes, and non bonding interactions in the form of mathematical functions. The main molecular simulation methods involved are: 1) MD. MD solves the time evolution state of the system based on Newton’s laws of motion. Set the initial position, initial velocity, and molecular potential energy of each molecule. Through intermolecular forces, the initial position moves towards lower energy states, generating new molecular coordinates and momentum. The obtained result will be used as new data input, and the above steps will be repeated until equilibrium is reached. By integrating Newton’s equations of motion, the dynamic information of the entire system can be obtained, including basic thermodynamic properties, mechanical properties, and radial distribution functions of liquids. MD has fast computing speed and high accuracy, but there are also limitations, such as harsh application conditions and limited time scale to the nanosecond level. 2) CGMD. CGMD ignores the intramolecular information in the system, treating several atoms or atomic clusters as a large coarse particle and defining them as coarse-grained particles, and then applying a force field for calculation. The basic principles and simulation techniques of CGMD are the same as traditional all atom/molecular dynamics, but currently there is no unified standard potential field, which is usually developed based on the specific situation of the studied system. 3) DPD. DPD is based on a collection of multiple atoms or molecules and is defined as “beads”. The DPD model includes three pairs of interaction forces: conservative force, dissipative force, and random force. Through the damping effect of dissipative force on the motion of beads and the random collision effect generated by random force, the entire system maintains momentum balance during the reaction process, which has certain advantages in analyzing the interaction forces between particles in complex fluids.   Application of Molecular Simulation in PAM 2.1 Performance Analysis and Simulation of Polymer Systems PAM has a good viscosity increasing effect, but viscosity loss is more severe in high salinity environments of oil reservoirs. There has been extensive research on the effect of inorganic salts on the viscosity of polymer solutions. In recent years, the use of molecular simulation technology to provide a detailed explanation of the effect of inorganic salts on the viscosity of polymer solutions from a microscopic perspective has attracted the attention of researchers. By studying the structural changes of polymer molecules under different mineralization levels through computer simulation, the macroscopic properties of polymers can be intuitively understood, which has important guiding significance for polymer molecule modification and synthesis.   Jin Yanxin et al. used a combination of experimental and MD methods to study the effect of inorganic salts on the viscosity of hydrolyzed acrylamide (HPAM) solution. The particles in the simulated system were all subjected to a COMPASS force field. The experimental results indicate that the viscosity of polymer solutions is highly sensitive to inorganic salts, and the influence of MgCl2 and CaCl2 on solution viscosity is greater than that of NaCl. The simulation results indicate that there is an electrostatic attraction between Na+, Ca2+, Mg2+and negatively charged carboxylic acid groups. The ion layer formed by cations around the molecular chain can shield the repulsive effect between carboxylic acid groups in the molecular chain, causing the molecular chain to contract, thereby reducing the viscosity of the polymer solution. Polymer solutions are heterogeneous at the microscale, where polymer chains connected by cross-linking bonds expand and form a dense phase, while dispersed polymer molecules form a diluent phase. Yang et al. further studied the HPAM solution by dividing it into micro dense phase, medium phase, and dilution phase. They used MD method to investigate the molecular configuration of HPAM under high mineralization and the changes in viscosity of polymer solution from micro dense phase to dilution phase. The experimental results indicate that HPAM molecules in the micro dense phase are interconnected through electrostatic attraction to form a network structure, and the electrostatic attraction is three orders of magnitude higher than the dilution phase. Under high concentration of calcium ions, the electrostatic energy between HPAM molecules is significantly reduced, and salt ions form a network connection with HPAM molecules, disrupting the network structure between HPAM molecules in the dense phase. HPAM molecules tend to curl and gradually transition from a curved shape to a long spherical shape (Figure 1). The calculated apparent viscosity decreased by 37%, which is similar to the experimental results, further indicating that the network structure formed by electrostatic attraction in the dense phase is the key to solution viscosity. Yao et al. introduced three different monomers into the polymer chain to obtain modified anionic PAM (HM-HPAM). Each of these monomers has a benzene ring on its side chain, and their structural differences lie in the length of the side chain and the number of oxygen atoms in the side chain. MD was used to study the kinetic parameters such as gyratory radius (Rg), hydrodynamic radius (RH), and intrinsic viscosity ([η]) of HPAM and three types of HM-HPAM in salt solutions with different ionic strengths. The simulation results show that compared with HPAM, the Rg, RH, [η] changes of HM-HPAM are not significant, and it has stronger salt resistance, and the salt resistance of HM-HPAM-1 to HM-HPAM-3 gradually increases. This is because the steric hindrance of monomers can reduce the curling degree of macromolecular chains, thereby improving the salt resistance of modified polymers. Therefore, introducing steric hindrance monomers into polymers for modified molecular design can improve the salt resistance of polymers. Deng et al. investigated the rheological properties of hydrophobic associated acrylamide copolymer (HA-PAM) in salt solutions, systematically investigated the effects of polymer concentration, salt solution concentration, temperature, and shear rate on the rheological properties of the solution, and used DPD mesoscopic simulation method to describe the molecular behavior of the polymer in detail. Figure 2a shows the results of DPD simulation of polymer molecules in solution. From Figure 2a, it can be seen that hydrophobic groups aggregate to form hydrophobic water bodies; With the increase of shear rate, the hydrophobic interaction between side chains is enhanced, and the degree of polymer aggregation is enhanced. This indicates that an appropriate shear rate can promote the association of hydrophobic side chains. The influence of salt on the behavior of HA-PAM molecules was investigated at the microscopic level, as shown in Figure 2b. On the one hand, the shielding effect of electrolytes leads to macromolecular curling, reduced RH, weakened intermolecular entanglement, and a decrease in solution viscosity. On the other hand, an increase in salt concentration helps to enhance intermolecular hydrophobic binding. When the sodium chloride content is 0.4%~3.0% (w), HA-PAM will form very tight intramolecular hydrophobic microregions, and the viscosity of the solution will decrease again. Therefore, both reductions in viscosity of the solution are caused by the shielding effect of inorganic salts .