Synthesis of a Novel Filtrate Reducer and Its Application in Water-Based Drilling Fluid for Ultra-High- Temperature Reservoirs

The high-temperature stability and filtration property controlling of ultra-high-temperature water-based drilling fluids is a worldwide problem. To resolve this problem, a high-temperature-resistant quaternary copolymer (HTRTP) was synthesized based on molecular structure optimization design and monomer optimization. The physical and chemical properties were characterized by infrared spectroscopy, thermal weight, and spectrophotometry, and their temperature and salt resistance was evaluated in different drilling fluids, combined with adsorption, particle size analysis, and stability test. The results show that the thermal stability of HTRTP is very strong, and the initial temperature of thermal decomposition is above 320C. The salt resistance of HTRTP is more than 162 g/L, and the calcium resistance is more than 5000mg/L, which is equivalent to the foreign temperature-resistant polymer DCL-a, and is superior to the domestic metal ion viscosity increasing fluid loss agent PMHA-II for drilling fluids. It has excellent high-temperature resistance (245C) and fluid loss reduction effect in fresh water base mud, fresh water weighted base mud, saturated brine base mud, and composite salt water base mud, which is better than foreign DCL-a (245C) and domestic PMHA (220C). The adsorption capacity of HTRTP on clay particles is large and firm, and the adsorption capacity changes little under the change of chemical environment and temperature. Both before and after HTRTP aging (245C/16 h), the permeability of filter cake can be significantly reduced and its compressibility can be improved. By optimizing the particle size gradation of the drilling fluid and enhancing the colloid stability of the system, HTRTP can improve the filtration building capacity of the drilling fluid and reduce the filtration volume. The development of antithermal polymer provides a key treatment agent for the study of anti-high-temperature-resistant saline-based drilling fluid.


Introduction
The fluid loss additive, also known as the fluid loss control preparation and water loss additive, is an important drilling fluid treatment agent to ensure the stability of drilling fluid performance, reduce the loss of harmful fluids to the formation, stabilize the well wall, and ensure the well diameter rule [1,2]. At present, the following categories of fluid loss agents are used at home and abroad: (1) modified lignite [3]; (2) modified starch [4]; (3) modified cellulose [5]; (4) modified resin [6]; and (5) olefin monomer polymers [7]. With the development of the depth of petroleum exploration and development, the drilling depth keeps deepening and the bottom hole temperature is getting higher and higher, which puts forward higher requirements for the control of drilling fluid filtration and rheological properties [8,9]. Commonly used natural and natural modified fluid loss reducers could not meet the needs of deep and ultradeep well drilling. Polymer fluid loss additive not only reduces fluid loss but also regulates rheological properties [10,11]. It is an indispensable key treatment agent for high-temperature and ultrahigh-temperature water-based drilling fluids [12,13]. Since the 1970s, the new synthetic polymer drilling fluid treatment agent which is resistant to high temperature and electrolyte pollution has been studied at home and abroad and has been applied in deep and ultradeep well drilling [14,15]. With excellent performance of resistance to high temperature and salt resistance, the treatment agent has the following several kinds: Oseh et al. [16] on synthesizing a polypropylene-silica nanocomposite (PP-SiO 2 NC) using hot-emulsion sol-gel method as a viscosity modifier and filtration control agent for WBMs. PP-SiO 2 NC is spherical, with a particle size distribution between 80 and 390 nm, finely dispersed; a stronger resistance from 430°C to 485°C; and better filtration control and rheological properties, and plastic viscosity is reduced by 22.7%. Chang et al. [17] have developed a new environmentally friendly fluid loss agent nano-LS (nano-LS) with good thermal stability, salt tolerance, and calcium resistance. The water-based drilling fluid prepared with it maintains its stability at 200°C and can control the water loss at 7.5 mL. The high-temperature-and saltresistant polymer DCL-a, developed by Philips and Chevron, can effectively reduce the high-temperature and highpressure filtration of drilling fluid. The temperature resistance is more than 230°C, and it has strong salt and calcium resistance. At present, the thermal resistance of polymer filtrate reduction agents at home and abroad still cannot meet the requirements of ultra-high-temperature filtration control in deep and ultradeep well drilling [18,19]. Therefore, this paper carried out the synthesis and characterization of high-temperature-and salt-resistant quaternary polymer, evaluation of filtration loss performance and stability in drilling fluid, and analysis of its action mechanism [20,21].
IRAffinity (1) Competitive Rate of Polymer. The reactivity rate is the ratio of the rate constants of homopolymer (self) growth and copolymer (cross) growth. The reactivity ratios of the designed high-temperature-resistant quaternary copolymers can be estimated using the -e equation proposed by Alfrey and Price. The calculated values of the reactivity ratios of the four monomers (Table 1) show that the product of the reactivity ratios between the monomers is less than 1, indicating that the nonideal copolymerization occurs in this synthesis [24].
(2) Structural Formula. According to the principle of polymerization reaction, the possible structural formula of HTRTP is shown in Figure 1.

2.2.2.
Synthesis. The reaction monomers (AMPS : DEAM : DMDAAC : NVP = 6 : 3 : 3 : 1) were added in the four-point flask, and then, the reaction monomers were added in turn. The pH value of the system was adjusted to 7.0 with 30% NaOH solution, and the temperature was raised to 60°C. The initiator of 0.1% of the total monomer was added, and the viscous liquid product was obtained by stirring for 4 h. The product was dried and crushed at 105°C to obtain HTRTP in powder form.

Composition and Molecular
Weight. Acetone was used as precipitant, and HTRTP was purified and dried by solution precipitation. The structure of HTRTP was analyzed by Fourier transform infrared spectroscopy (FT-IR) with KBr.
The "one-point method" was used to determine the intrinsic viscosity of the polymer (fluid loss additive), and the viscosity average molecular weight of the polymer was estimated by the Mark-Houwink empirical formula [25]. The calculation is as follows: where C is solute concentration, g/mL; M is the molecular weight of the solute; and K and α are constants related to the determination conditions and polymer structure, and the value of K is 3:684 × 10 −2 , and the value of α is 0.646.

2.3.2.
Resistance to Temperature. In N 2 atmosphere, the purified HTRTP was subjected to thermogravimetric analysis on a thermogravimetric analyzer at a temperature of 29°C-800°C and a heating rate of 5°C/min [26]. With 0.2 mol/L NaCl solution as solvent, 0.2% HTRTP and DCL-a solutions were prepared, and their intrinsic viscosities after aging at different temperatures for 8 hours were measured by an ohmmeter.
2.3.3. Ability to Antisalt. The salting-out resistance of hightemperature-resistant quaternary copolymer was tested and evaluated by UV-visible photometry [27]. In making polymer aqueous solution (PMHA, DCL-a, and HTRTP), the concentration is 0.5%. Add different amounts of distilled water and 180 g/L sodium chloride solution to a test tube 2 Geofluids containing 1.0 mL sample solution, and shake well. Distilled water was used as blank experiment, and the transmissivity of each solution was measured with a 722 spectrophotometer. After salting out, the solution becomes turbid, which causes the change of transmission ratio of the solution. Therefore, when the salt in the solution reaches or exceeds the turbidity point, the transmission ratio of the solution is significantly reduced, and the turbidity point salinity of the copolymer is determined.

Evaluation of Fluid Loss Reduction Effect in Fresh
Water Base Slurry. The experimental base slurry is composed of 400 mL tap water +0.8 g anhydrous sodium carbonate +4% secondary bentonite +3% evaluation soil, respectively, to the early hydration good base paste to add 1% of the sample evaluation (PMHA, DCL-a, and HTRTP), stirring evenly, measuring the rheological property and filtration [28]. The experimental slurry was then put into a high-temperature aging tank and rolled at 220°C and 245°C for 16 h, respectively. The rheological properties and filtration loss of the slurry were measured by the same method.

Effect of Fluid Loss Reduction in Fresh Water Weighted
Base Slurry. The experimental base slurry is composed of400 mL tap water + 1:2 g anhydrous sodium carbonate + 6 %secondary bentonite + 5%evaluation soil + 30%barite, respectively, to the early hydration good base paste to add 1% of the sample evaluation (PMHA, DCL-a, HTRTP), stirring evenly, into the high-temperature aging tank, respectively, under 220°C and 245°C hot rolled out after 16 h, measuring the rheological property and filtration.

Effect of Fluid Loss Reduction in Saturated
Salt-Water-Based Slurry. The experimental base slurry is composed of-400 mL tap water + 1:2 g anhydrous sodium carbonate + 6% secondary bentonite + 10%evaluation soil + 30%sodium chloride, respectively, to the early hydration good base paste to add 1.5% of the sample evaluation (PMHA, DCL-a, HTRTP), stirring evenly, into the high-temperature aging tank, respectively, under 220°C and 245°C hot rolled out after 16 h, measuring the rheological property and filtration.
2.5. Research on Mechanism of Action 2.5.1. Adsorption Characteristics on Clay Particle Surface. A known amount of polymer solution was added to 4 wt % calcium montmorillonite (purified with 15 wt % H 2 O 2 ) suspension and then diluted with 50 mL distilled water. The initial concentration of polymer was recorded as c 0 . The suspension was agitated vigorously and then left to stand for 20 min to allow the adsorption process of the quadripolymer to complete at given temperature. The suspension was subsequently centrifuged for 30 min at a speed of 1000 rpm using a KUBOTA 3500 centrifuge. The concentration of unadsorbed quadripolymer, c 1 , located in the clear centrifuged supernatant was measured via absorbance at wavelengths of 390 nm and 330 nm for polymer solution using an ultraviolet spectrophotometer (722, Shanghai Yuanxi Instruments Co., Ltd., China). Applying material balance of the polymer, the adsorption of polymer on clay particles can be calculated via

Geofluids
where τ is the mass of polymers absorbed onto specific weight calcium montmorillonite, mg/g; v is volume of solution, L; c 0 and c 1 are polymer concentration before and after polymer absorbed onto calcium montmorillonite, mg/L; and G is the mass of calcium montmorillonite, g.

Influence on Filter Cake
Quality. The effect of its addition in fresh water base slurry on the permeability of filter cake was investigated. After measuring the API filtration loss, carefully remove the substrate slurry on the surface of the filter cake [29], replace it with distilled water, measure the volume of water for 30 min, measure the thickness of the filter cake with a steel plate ruler, and calculate the permeability K of the filter cake according to the following formula: where K is the permeability of mud cake, μm 2 ; v is the effluent volume, mL; h is the thickness of mud cake, mm; and 0.175 is the conversion factor (0.07 for high temperature and high pressure). The effect of several treatment agents on the compressibility of base mud was investigated by using the "twostage water loss method" which is simple, feasible, and reliable. If other factors are set to be constant, the compressibility of filter cake can be expressed by a simple proportional relationship of filtration rate at 500 psi and 100 psi: in which R is the compressibility coefficient of the filter cake and V 500 and V 100 are static filtration loss under pressure difference of 500 psi and 100 psi, respectively.

Particle Size and Specific Area
Measurements of Calcium Clay. The suspensions were prepared by mixing deionized water, 4% (w/v) calcium montmorillonite, and 0.3% (w/v) viscosity breaker (XY-28, sulfonated tannin and quadripolymer separately), stirring for 20 minutes at a high speed of 10,000 rpm and aging for 24 hours at room temperature. Aging experiments of suspensions were carried out in a XGRL-4A-type rolling oven through hot rolling at 220°C (XY-28) or 245°C (sulfonated tannin and quadripolymer) for 16 hours. Concentrated NaOH (10 mol/L) and HCl (6 mol/L) solutions were used to change pH so as not to cause excessive dilution of samples. The size distribution and specific area of clay particles in suspension were determined using a Bettersize2000 laser particle analyzer.

Influence on Stability of Fresh Water Base Slurry
Colloid. Turbiscan, an infrared scanning dynamic stability tester, can quantitatively evaluate the stability of dispersion systems such as colloid, foam, and suspension. The effects of 0.1%, 0.2%, and 0.3% HTRTP on the colloidal stability of 3% bentonite slurry before and after aging (245°C/16 h) were investigated.       5 Geofluids the molecular chain of the product has the predicted group of molecular structure design, which verifies the correctness of the molecular structure design and the correctness of the HTRTP synthesis reaction scheme.
It can be seen from the test results in Table 2 that the molecular weight of HTRTP is about 1.04 million, which is smaller than that of high-temperature-resistant polymer DCL-a, and its influence on drilling fluid viscosity will be smaller than that of DCL-a.

Temperature
Resistance. It can be seen from the thermal difference curve in Figure 3 that the thermal decomposition process of HTRTP can be divided into many stages, of which there are two main pyrolysis stages. The first stage occurs between 320 and 360°C with a weight loss rate of 20%-25%, which is mainly the decomposition of carboxylic acid group 9 and sulfonic acid group on the side chain. The second stage, which occurs above 400°C, is mainly the decomposition of the backbone chain of polymerization. The total weight loss rate of these two stages is 52.41%. It can be seen that the high-temperature-resistant quaternary copolymer HTRTP has strong thermal stability.
As shown in Figure 4, with the increase of aging temperature, the intrinsic viscosity of HTRTP decreased more slowly than that of DCL-a, indicating that HTRTP had better temperature resistance than DCL-a in aqueous solution. Table 3 that when the Clconcentration of PMHA increases from 126 g/L to 144 g/L, the transmittance of PMHA solution changes suddenly, indicating that the solution begins to become cloudy; that is, the antisalting out ability of PMHA (calculated by Cl -) is between 126 g/L and 144 g/L. At the Clconcentration of 162 g/L, the transmittance of DCL-a solution and HTRTP solution still has no obvious change, indicating that the solutions have not become turbid; that is, the salting-out resistance of DCL-a and HTRTP (calculated by Cl -) is greater than 162 g/L.

Resistance to Salting Out. It can be seen from
It can be seen from Table 4 that when the concentration of CaCl 2 increases from 1500 mg/L to 2500 mg/L, the transmittance of PMHA solution changes suddenly, indicating that the solution begins to become cloudy; that is, the calcium resistance of PMHA (calculated by CaCl 2 ) is between 1500 mg/L and 2500 mg/L; with the increase of CaCl 2 concentration, the transmittance of DCL-a solution and HTRTP solution has no significant change. When CaCl 2 concentration reached 5000 mg/L, the transmittance did not change  6 Geofluids obviously, which indicated that the solutions did not become turbid; that is, the calcium resistance of DCL-a and HTRTP (calculated by CaCl 2 ) was greater than 5000 mg/L.

Evaluation of Fluid Loss Reduction in Fresh Water Base
Slurry. As shown in Table 5, the API filtration loss of HTRTP was the lowest before and after aging, and the viscosity changed the least before and after aging. The filtration loss of fresh water base slurry treated with 1% HTRTP decreased from 41 mL to 11 mL after aging at 245°C/16 h at high temperature. 1% PMHA was processed in fresh water slurry in 220°C/16 h after high-temperature aging filtration quantity from 34 mL of 12.4 mL, but after aging experiment, paste viscosity decreased; treatment may have serious degradation. When the fresh water base slurry treated with 1% DCL-a was aged at 245°C/16 h, the filtrate loss reduction effect was not as good as that of HTRTP. The results indicated that HTRTP not only had a good filtration loss reduction effect but also had a better temperature resistance in fresh water base slurry.

Effect of Fluid Loss Reduction in Fresh Water Weighted
Base Slurry. As can be seen from Table 6, API filtration loss of experimental slurry after aging is low, among which the experimental slurry treated by 1% HTRTP has the lowest filtration loss, which is 6.0 mL. It was shown that in fresh water increased base slurry, HTRTP fluid loss effect is better than PMHA and DCL-a.

Effect of Fluid Loss Reduction in Saturated
Salt-Water-Based Slurry. It can be seen from Table 7 that, compared with the base slurry, the API filtration loss of the aging experimental slurry treated with filtration reduction agent was significantly reduced, among which the experimental slurry treated with 1.5% HTRTP had the lowest filtration loss, which was 18.0 mL. It is shown that the filtration reduction effect of HTRTP is better than that of PMHA and DCLa in saturated salt-water-based slurry.

Adsorption Properties on the Surface of Clay Particles
(1) Influence of Temperature. As can be seen from Figure 5, at the same temperature, with the increase of HTRTP and DCL-a concentration, their adsorption capacity on clay particles increased. With the increase of temperature, the adsorption amount decreased. It can also be seen from the figure that both HTRTP and DCL-a have different adsorption isotherms at different temperatures. The reason is that adsorption is an exothermic reaction, and increasing temperature is conducive to equilibrium in the direction of desorption. In addition, when the temperature rises, the thermal movement of clay particles intensifies, which is not conducive to adsorption.
(2) Influence of pH. As can be seen from Figure 6, when pH is 8, the adsorption capacity of the two polymers on the surface of clay particles reaches the maximum. When pH is higher or lower than this value, the adsorption capacity decreases. Within the test range, the higher the pH value, the lower the adsorption capacity. The reason is that when the pH is 8, the clay particles are fully dispersed and the negative charge on the surface of the particles is moderate. When pH becomes lower, it is not conducive to the full dispersion of clay particles, which can be used for adsorption of small surface area; when the pH value is higher than this value, more OHis adsorbed on the surface of clay particles, which further enhances the electronegativity of the surface. The electrostatic action is not conducive to the adsorption of polymer molecules. As can be seen from the figure, not only is the adsorption capacity of HTRTP greater than that of DCL-a, but with the increase of pH, the adsorption capacity of HTRTP on the surface of clay particles decreases more slowly than that of DCL-a. This is because certain cationic groups are introduced into HTRTP molecules, which can weaken the influence of the increase of electronegativity on the surface of clay particles on adsorption.

Geofluids
(3) Effect of Sodium Chloride Addition on Adsorption Capacity. It can be seen from Figure 7 that with the increase of sodium chloride dosage, the adsorption amount of HTRTP and DCL-a on clay particles decreases. This is because, after adding sodium chloride, a large amount of Na + accumulates near clay particles, compressing their diffusion electric double layer; the ζ potential decreases; the repulsive force weakens; the agglomeration of clay particles increases; and the surface area decreases. In addition, the addition of sodium chloride affects the extension of polymer molecular chain, which is not conducive to adsorption. Therefore, in sodium chloride brine drilling fluids, the contribution of polymer plugging and improving filtrate viscosity to fluid loss reduction performance is increased.

3.3.2.
Influence on the Quality of Filter Cake. It can be seen from Figure 8 that there is a good linear relationship between filter cake permeability and API filtration amount. With the increase of HTRTP addition, API filtration amount decreases before and after experimental slurry aging, and the permeability of filter cake also decreases.
It can be seen from Table 8 that the compressibility coefficient of the experimental slurry before aging is lower than that after aging, indicating that high temperature will cause the compressibility reduction of the drilling fluid filter cake. The compression coefficient of experimental slurry with filtrate loss additive before and after aging is lower than that of base slurry, which indicates that improving the compressibility of filter cake is also one of the action mechanisms of filtrate loss additive. The filter cake added with HTRTP slurry has the lowest compressibility coefficient, which is consistent with the best filtration loss reduction effect.

Effect on Particle Size Distribution of Fresh Water Base
Slurry. As can be seen from Figure 9, after aging, the proportion of particles with smaller particle size in the experimental slurry decreases, while the content of particles with larger particle size increases; both particle size and distribution become larger, which is one of the reasons for the increase of filtration loss after aging. The mean particle size of different suspensions was 1.7 μm, 21.5 μm, 7.5 μm, and 17.6 μm, respectively (tagged in Figure 9). After adding HTRTP, both before and after aging, the particle size distribution of exper-imental slurry is wider and the grading is more reasonable, so the filtration loss of experimental slurry is reduced.

Effect on Stability of Fresh Water Base Slurry Colloids.
As can be seen from Figure 10, the colloidal stability of the experimental slurry was significantly better than that of the base slurry after the addition of HTRTP, and the colloidal stability of the experimental slurry increased with the increase of the amount of HTRTP. After aging at 245°C/16 h, high temperature promoted the hydration and dispersion of bentonite. In the experiment, it was measured that the thickness of the upper "clear liquid" of the aging base slurry was 0.30 mm after standing for 24 h, 0.1 mm when adding 0.1% HTRTP, 0.06 mm when adding 0.2% HTRTP, and 0.04 mm when adding 0.3% HTRTP. Combined with the previous particle size test results, it can be seen that for the base slurry with low bentonite content, the contribution of adhesive protection of HTRTP to filtration loss reduction is greater than that after aging. After aging, the mechanism  Base slurry before aging Base slurry after aging Basic slurry + 1% HTRTP, before aging Basic slurry + 1% HTRTP, after aging 8 Geofluids of HTRTP reducing filter cake permeability by improving particle size grading in the system and enhancing the compressibility of filter cake is more important. The action mechanism of high-temperature-resistant quaternary copolymer HTRTP is as follows: (1) Temperature resistance mechanism: the molecular structure difference is the main factor that determines the temperature resistance of the fluid loss agent. High-temperature-resistance quaternary copolymer HTRTP molecule main chain is C-C bond and has the internal structure of high temperature resistance; C-S and C-N bonds are used between the main chain and the side chain, and the thermal stability of the main chain and the side chain is strong (2) Mechanism of salt resistance and calcium resistance: the hydrophilic group is the sulfonic acid group (-CH2SO 3 -), which has strong hydration ability and can thicken the hydration film on the surface of clay particles with strong salt resistance. The sulfonic acid group does not react with Ca 2+ to form precipitation, which has a strong anticalcium ability. In addition, the side groups introduced into the HTRTP chain (e.g., -CONHC(CH 3 ) 2 CH 2 SO 3 -) are large and rigid, and the chain is not easy to curl in electrolyte solutions (salt solutions)  Figure 10: Effect of HTRTP on colloid stability of bentonite grouting (after aging).

Geofluids
(3) Adsorption mechanism: in addition to the phthalamine group (-CON(CH 2 CH 3 ) 2 ), the cationic adsorption group (-CH 2 N + (CH 3 ) 2 CH 2 -) is also introduced into the HTRTP molecular chain. -CON(CH 2 CH 3 ) 2 is adsorbed by hydrogen bond, which has strong adsorption stability and is not easy to be desorbed under temperature change. -CH 2 N + (CH 3 ) 2 CH 2 -is chemisorbed through static ion bonds, and the adsorption is stronger (4) Mechanism of filtration loss reduction: firstly, HTRTP is adsorbed on the surface of clay particles to form an adsorption layer, which prevents flocculation of clay particles from becoming larger. At the same time, it stabilizes the fine particles separated by stirring so as not to become larger. So it can improve clay particle grading in drilling fluid. It has excellent protective adhesive effect, forming thin and compact filter cake, reducing filtration loss. Secondly, HTRTP increases the degree of hydration by adsorbing on clay particles, so that the hydration film on the surface of clay particles is thickened to enhance the compressibility of the filter cake and reduce the permeability of the filter cake, thus reducing the filtration loss. Thirdly, the molecular size of HTRTP is just within the range of colloidal particles, and the molecular chain wedges into the pores of the filter cake or curls into a ball to block the micropores of the filter cake, making the filter cake thin and compact, thus reducing the filtration loss. Finally, HTRTP is a high molecular weight polymer that is added to the drilling fluid to increase the viscosity of the filtrate and thus reduce filtration loss

Conclusions
The high-temperature-resistant quaternary copolymer HTRTP was synthesized by molecular structure optimization and monomer optimization. The viscosity average molecular weight of HTRTP was about 1 million, which had less effect on the viscosity of drilling fluid than DCL-a. HTRTP molecular structure has good thermal stability, temperature resistance in drilling fluid up to 245°C, salt resistance and calcium resistance, and good compatibility with fresh water and salt water experimental base slurry. The performance of fluid loss reduction, temperature resistance and salt and calcium resistance in drilling fluid is better than that of foreign temperature resistance polymer DCL-a and domestic similar products. The adsorption capacity of HTRTP on clay particles is higher than that of foreign antitemperature polymer DCL-a. In the case of chemical environment and temperature change, the adsorption amount changed little and was still higher than that of DCL-a, and the adsorption was firm. Before and after aging (245°C/16 h), HTRTP can significantly reduce the permeability and improve the compressibility of filter cake. Improving the particle size grading in drilling fluid system and enhancing the colloidal stability of the system are the important mechanisms of high-temperature-resistant qua-ternary copolymers. The quad-element polymer reducer was suitable for fresh water-based drilling fluid and lowmineralized saline drilling fluid.

Data Availability
All data and research results are listed in the manuscript.

Conflicts of Interest
The authors declare that they have no conflicts of interest.