Synthesis of a copolymerization and the anti-high temperature property in oil well cement slurry fluid

s. Four monomers including AMPS, acrylic acid (AA), N,N-dimethylacrylamide (DMAA), and sodium allylsulfonate (AS) were used to synthesize quaternary fluid loss reducer (FRW) through copolymerization. The optimal reaction conditions were optimized by orthogonal experiment method, and the water loss reduction performance of FRW was evaluated. The results show that the optimal synthesis conditions for FRW are that the molar ratio of AMPS:AA:DMAA:AS is 4.2:1.8:3.5:0.5, the amount of initiator added is 0.3% of the total monomer mass, the monomer mass percentage is 40%, The reaction temperature was 50°C, and the pH was 7. The performance evaluation results show that FRW has good fluid loss control ability. Under the condition of adding 2%, the fluid loss is 141mL when the temperature is 220℃, which has no negative impact on the comprehensive performance of cement slurry and has strong practicability.


Introduction
The main function of the oil well cement slurry fluid loss additive is to prevent or reduce the rate of the mixing water in the cement slurry from filtering to the formation [1,2]. With the gradual depletion of shallow oil and gas resources, the exploration and development targets have shifted to deeper oil and gas reservoirs [3][4][5]. The cementing process will not only face the problem of high temperature and high pressure, but also may encounter high salinity [6,7]. Stratum of formation water. This poses a new challenge to the performance of oil well fluid loss additives. At present, the fluid loss agent used in my country is mainly a copolymer of 2-acrylamido-2methylpropanesulfonic acid (AMPS) and acrylamide (AM) as the main monomers [8,9,10]. This kind of fluid loss agent has low temperature resistance and salt resistance, and is easy to decompose at high temperature, causing excessive retardation of the slurry, and also reducing its ability to control fluid loss [11,12,13]. In order to solve the above problems, based on the summary of previous research results, four monomers including AMPS, acrylic acid (AA), N,N-dimethylacrylamide (DMAA) and sodium allyl sulfonate (AS) were selected. A quaternary fluid loss agent (FRW) was synthesized through copolymerization, and its structure and performance were tested.

Synthesis method of fluid loss agent
According to a certain molar ratio, weigh 4 kinds of monomers, AMPS, AA, DMAA, AS, etc., and dissolve them in a certain amount of distilled water, then transfer them into a 4-neck flask, and adjust the pH value with 30% sodium hydroxide solution by mass fraction. Blow in nitrogen and heat to the reaction temperature, then add initiator (ammonium persulfate: sodium bisulfite=1:1), keep the temperature constant, stir for a certain period of time, and then obtain the quaternary copolymerization fluid loss agent (FRW). FRW performance test The performance evaluation of fluid loss agent is carried out in accordance with my country's oil and gas industry standard SY/T5504.2-2005 oil well cement admixture evaluation method. The cement slurry formula is: Jiajiang Grade G cement + 35% quartz sand (adding at an experimental temperature greater than or equal to 110°C) + 5% microsilica + 0.75% dispersant (SXY) + x% FRW + x% retarder (BXR-300L) + x% NaCl tap water. The x% in the formula represents the mass percentage of the additive in the cement, and the slurry is formulated according to the water-solid ratio of 0.44.

Effect of monomer ratio on fluid loss reduction performance
FRW performance is mainly determined by the number and ratio of functional groups, so the monomer ratio is one of the key factors that determine FRW performance. According to the literature investigation and preliminary investigation, the following synthesis conditions are used: the initiator is 0.4% of the total monomer mass, the monomer mass fraction is 40%, the experimental temperature is 50°C, the experimental pH value is 7.0, and the reaction time For 4h, 5 groups of different monomer ratio experiments were designed (Table 1). Then test its water loss performance at a FRW addition of 2% and an experimental temperature of 150°C. It can be seen from Table 1 that the FRW samples with the proportions numbered 4 and 5 have low fluid loss and close to each other after adding cement slurry. Although the fluid loss of No. 4 is less than that of No. 5, it is considered that if the content of carboxyl groups in FRW is higher, it will cross-link with Ca 2+ and Al 3+ produced by cement hydration, resulting in increased slurry consistency and thickening. The "wrapped heart" phenomenon will also appear in the experiment [14].
Considering the fluid loss of cement slurry and the properties of the slurry, the optimal monomer ratio relationship is determined to be AMPS: AA: DMAA: AS=4.2: 1.8: 3.5: 0.5.

Optimization of synthesis conditions
On the basis of determining the best monomer unchanged, a set of orthogonal experiments was designed, taking the fluid loss of the cement slurry as the evaluation standard, and considering its influence on the strength and performance of the cement stone, and optimizing the synthesis conditions. The factors of the orthogonal experiment are shown in Table 2. The 24h strength test condition is 21MPa×150℃ in water bath. Through the analysis of the orthogonal experiment results, the optimal synthesis conditions were determined as follows: the initiator is 0.3% of the total monomer mass, the monomer mass percentage is 40%, the reaction temperature is 50°C, the pH value is 7, and the nitrogen flow time is 30min, the reaction time is 5h.

High temperature resistance
One of the most important performance indicators of FRW is high temperature resistance. Therefore, the above formula will be used to test the high temperature resistance of FRW. The addition amount of FRW is 1.0% and 2.0%, and the addition amount of retarder is 2.0%.
The experimental results are shown in Figure 1. It can be seen from Figure 1 that although the fluid loss of cement slurry gradually increases as the temperature increases, the fluid loss is 152 mL when the temperature is 220°C. As the amount of FRW increases, the fluid loss of cement slurry decreases. For example, when the temperature is 220°C and the FRW increases from 1.0% to 2.0%, the fluid loss decreases by 8%.

Salt resistance
Stratums with high salinity often greatly reduce the effectiveness of fluid loss additives. In order to test the salt resistance of FRW, cement slurries were prepared with different concentrations of NaCl solution to investigate the salt resistance of FRW. The addition amount of FRW is 2%, the addition amount of retarder is 2%, and the curing condition of cement stone is 150℃×21MPa×48h water bath curing. The experimental results are shown in Table 4. It can be seen from Table 3 that with the increase of the salt content in the cement slurry, the fluid loss and water separation of the cement slurry increase slightly, but the increase is small, indicating that FRW has good salt resistance. The morphology of C-S-H gel changes, and the strength of cement stone gradually decreases. But in the case of saturated brine, the strength still reaches 20.5MPa, which is very different from the general low strength of saturated brine cement slurry.

The influence of FRW on cement slurry engineering performance
The fluid loss agent not only achieves the function of reducing fluid loss, but also meets other performance requirements of the cement slurry, such as rheology and mechanical properties. The effect of FRW on the engineering performance of cement slurry is studied here. The curing condition of cement stone is 150℃×21MPa×48h. The results are shown in Table 4. It can be seen from Table 4 that as the amount of FRW increases, the fluidity index (n) of the slurry decreases slightly, and the consistency coefficient (K) increases slightly, but overall the value of n is greater than 0.80 and the value of K is less than 0.40Pa•s, indicating that the cement slurry has excellent rheological properties, which is very beneficial for improving the displacement efficiency of the eccentric annulus. The water separation rate of the cement slurry is 0, and the filtration loss is less than 50mL, indicating that the slurry stability is good. The thickening time of cement slurry and the strength of cement stone did not change much with the increase of FRW, indicating that FRW would not decompose at high temperature.

Conclusions
In the experiment, a quaternary high-temperature and saltresistant FRW sample was synthesized by the solution copolymerization method, and the best synthesis conditions were optimized: the molar ratio of AMPS:AA:DMAA:AS was 4.2:1.8:3.5:0.5, and the initiator was added as a monomer. 0.3% of the total mass, 40% of the monomer mass, reaction temperature of 50°C, and pH value of 7. At the same time, FRW has good fluid loss control ability. Under the condition of 2% addition, the fluid loss is 141mL when the temperature is 220 ℃, and it has no negative impact on the comprehensive performance of the cement slurry and is practical.