Effect of Synthetic Quadripolymer on Rheological and Filtration Properties of Bentonite-Free Drilling Fluid at High Temperature

: High temperature would dramatically worsen rheological behaviors and increase ﬁltration loss volumes of drilling ﬂuids. Synthetic polymers with high temperature stability have attracted more and more attention. In this paper, a novel quadripolymer was synthesized using 2-acrylamido-2-methylpropanesulfonic acid (AMPS), acrylamide (AM), sodium styrene sulfonate (SSS), and dimethyl diallyl ammonium chloride (DMDAAC). Firstly, the molecular structure was studied by Fourier transform–infrared spectroscope (FT-IR) and nuclear magnetic resonance ( 1 H-NMR) analysis. It was shown that the synthetic polymer contained all the designed functional groups. Moreover, the effect of temperature and the quadripolymer concentration on the rheological behavior and ﬁltration loss of the bentonite-free drilling ﬂuid were investigated. It was experimentally established that when the adding amount of the quadripolymer was 0.9 wt%, the prepared drilling ﬂuid systems exhibited relatively stable viscosities, and the ﬁltration losses could be controlled effectively after hot rolling aged within 180 ◦ C. Further, it was conﬁrmed that the bentonite-free drilling ﬂuid containing the synthesized quadripolymer had good reservoir protection performance. In conclusion, the synthetic quadripolymer is a promising rheology modiﬁer and a ﬁltrate reducer for the development of the bentonite-free drilling ﬂuid at high temperature.


Introduction
Drilling fluid is indispensable in oil and gas drilling engineering to maintain wellbore stability, carry and transport drilled cuttings, and reduce water loss. It is divided into water-based drilling fluid system (WBDF), oil-based drilling fluid system (OBDF), and gas-based drilling fluid system (GBDF) [1]. Among them, a water-based working fluid is the most commonly used one because of its environmental friendliness and low cost, and it is mainly composed of bentonite clay and different functional polymer treating agents [2][3][4]. Bentonite as the most essential component in drilling fluid exhibits incomparable advantages in viscosity and filtration loss control [5,6]. Furthermore, clay particles can plug pores and fractures in the near wellbore area and form a thin and low-permeability filtration cake. However, as the number of deep and ultra-deep wells increases, one of the inevitable problems is that drilling fluid must endure high temperatures [7]. It is worth noting that high bentonite content of water-based drilling fluids at high temperatures would give rise to serious detrimental effects [8][9][10], for example, deterioration of rheological behavior, increase in filtration volume seeping into formation, particularly formation damages caused by dispersive clay particles. Thus, less or no bentonite in drilling fluids capable of providing necessary performance in harsh operating conditions should be contained to The quadripolymer of AM, AMPS, SSS, DMDAAC was synthesized by solution free radical polymerization. The effect of molar ratio of four monomers, dosage of initiator, pH and reaction temperature on optimizing the best synthesis condition was determined (AM: AMPS: SSS: DMDAAC is 10:3:3:2, redox initiator dosage((NH 4 ) 2 S 2 O 8 ) is 0.2 wt%, temperature is 80 • C and reactants system pH is 7) by dosage experiment.
Firstly, a desired amount of mixture monomers was mixed well in a reaction flask and deoxygenated with nitrogen. Secondly, sodium hydroxide was used to adjust the pH value (pH = 7) of the reaction system. The whole reaction process took place in a constant temperature oil bath. Next, a redox initiator ((NH 4 ) 2 S 2 O 8 ) dosage of 0.2 wt% was added into the above solution with a constant stirring speed of 200 r/min. After 5 h of reaction time, the white product was filtered off and extracted with acetone and methanol for three times, and finally dried powders were obtained.

Characterization of Molecular Structure
FT-IR spectrum of the quadripolymer was recorded on a Nicolet 6700 Fourier Transform Infrared Spectrometer. A pellet sample made from a mixture of 1 mg quadripolymer and about 100 mg of potassium bromide (KBr) was prepared under a pressure of 100 psi and tested in the optical range of 400-4000 cm −1 .
1 H NMR spectrum of the quadripolymer was measured by Angilent 400 MHz Nuclear magnetic resonance spectrometer. Mass of 10 mg of sample powder was dissolved into a test tube containing 0.65 mL D 2 O, and then transferred to a sample cavity. 1 H NMR spectrum measurement was carried out with controlled heating and cooling steps.
Thermal stability: The thermal stability of the quadripolymer was tested with HCT-1 Differential Thermobalance analyzer (Beijing Henven Instrument Plant, Beijing, China) in nitrogen gas atmosphere. The heating rate was 10 • C/min, and the temperature was in the range of 30-600 • C.

Sample Preparation
Base formula of bentonite-free drilling fluid (marked as formula1#) was prepared as follows: 1000 mL deionized water, 0.1 wt% NaOH, 0.15 wt% Na 2 CO 3 , desired amount of polymer (the quadripolymer or Drispac), 2 wt% PAC-LV, 2 wt% LOCK-SEAL, 4 wt% super-fine CaCO 3 , 0.5 wt% Na 2 SO 3 were weighted and added sequentially at high speed stirring for 20 min with a GJSS-B12K multi-spindle mixer, and then the solution was stood still for 24 h at room temperature for complete dissolution.

Drilling Fluid Performance Measurements
Rheological and filtration properties of bentonite-free drilling fluid were evaluated according to the American Petroleum Institute test program [29], Formulas (1)-(3) were applied to calculate the rheological parameters, including apparent viscosity (AV), plastic viscosity (PV) and yield point (YP), for the fluid system. The readings at 600 rpm and 300 rpm obtained with a ZNN-D6 six-speed rotational viscometer were marked as Φ600 and Φ300, respectively. The initial and final gel strength were also measured and marked as G 1 and G 2 . Before thermal stability evaluation, the bentonite-free drilling fluid was required to be heated at a given temperature for 16 h with a XGRL-4A hot roller oven and then cooled to ambient temperature.
AV (Apparent viscosity) = Φ600/2, mPa·s, PV (Plastic viscosity) = Φ600−Φ300, mPa·s, API filtration loss (FL API ) was determined by a SD-4 API filtration apparatus with a pressure difference of 100 psi at ambient temperature for 30 min. HTHP filtration loss (FL HTHP ) was measured with a GCS71-A HTHP filtration apparatus at 150 • C and 500 psi for 30 min. After the test, the filtrate was collected into the measuring cylinder and the volume was recorded.
HTHP rheology test: After placing the drilling fluid inside the rheometer, the sample was heated up to the desired temperature and meanwhile pre-shear at 100 s −1 was performed before rheological measurement. The pressure was adjusted to 500 psi and the shear stress was measured at the shear rate of 511 s −1 .
Reservoir protection test: The core original kerogen permeability K 0 was measured as follows, Formula (4), wherein, Q, flow rate; µ, Kerosene viscosity; l, Core length; ∆p, Differential pressure before and after flow through the core; A, core, end surface area.
In step 1, the core saturated with simulated groundwater is placed into the core holder and repelled with filtered and dewatered kerosene at 0.4 times the critical flow rate; subsequently, repelled at 0.8 times the critical flow rate until no water flows out and the pressure is stable. According to the above formula, K 0 can be calculated.
In step 2, the core tested for K 0 is quickly loaded into the core holder of JHDS high temperature and high pressure dynamic water loss instrument, and the formulation drilling fluid is injected in the reverse direction at 90 • C and 3.5 MPa differential pressure, and cycled at a shear rate of 300 s −1 for 125 min. The core is loaded into the gripper and repelled at the same flow rate as in step 1, and the contaminated permeability K 1 . is calculated after the pressure and flow rate are stabilized.
In step 3, the end face of the core contaminated by drilling fluid in step 2 is cut off for about 1 cm, and step 2 is repeated to obtain the core permeability K 2 .

Characteristic
The quadripolymer used in this study was milky white dispersion. The scanning electron microscopy (SEM) picture is shown in Figure 1. The microstructure of the quadripolymer had the following characteristic: irregular shapes, different sizes (Figure 1a), and smooth surface (Figure 1b) of the particles were easily recognizable. Among them, the particle size is relatively widely distributed. The largest is around 200 microns and the smallest is just under 10 microns.
at the same flow rate as in step 1, and the contaminated permeability K1. is calculated after the pressure and flow rate are stabilized.
In step 3, the end face of the core contaminated by drilling fluid in step 2 is cut off for about 1 cm, and step 2 is repeated to obtain the core permeabilityK2.

Characteristic
The quadripolymer used in this study was milky white dispersion. The scanning electron microscopy (SEM) picture is shown in Figure 1. The microstructure of the quadripolymer had the following characteristic: irregular shapes, different sizes (Figure 1a), and smooth surface (Figure 1b) of the particles were easily recognizable. Among them, the particle size is relatively widely distributed. The largest is around 200 microns and the smallest is just under 10 microns. The FT-IR spectrum of the quadripolymer is presented in Figure 2. Moreover, 3444 cm −1 and 3300 cm −1 are assigned to the N-H stretching vibration of AM and AMPS, respectively. The absorbency at 2935 cm −1 results from the characteristic peak of the methyl group. The stretching vibration of C=O at 1699 cm −1 is attributed to AM and AMPS. The absorption peak observed at 1454 cm −1 is due to the C-H bending vibration from DMDAAC unit. Bands recorded at 1185 cm −1 and 1044 cm −1 assigned to S=O stretching vibration in the sulfonic group correspond to SSS unit. The absorption peaks at 766 cm −1 reveal the bending vibration of a benzene ring of the SSS unit.  The FT-IR spectrum of the quadripolymer is presented in Figure 2. Moreover, 3444 cm −1 and 3300 cm −1 are assigned to the N-H stretching vibration of AM and AMPS, respectively. The absorbency at 2935 cm −1 results from the characteristic peak of the methyl group. The stretching vibration of C=O at 1699 cm −1 is attributed to AM and AMPS. The absorption peak observed at 1454 cm −1 is due to the C-H bending vibration from DMDAAC unit. Bands recorded at 1185 cm −1 and 1044 cm −1 assigned to S=O stretching vibration in the sulfonic group correspond to SSS unit. The absorption peaks at 766 cm −1 reveal the bending vibration of a benzene ring of the SSS unit.
The quadripolymer used in this study was milky white dispersion. electron microscopy (SEM) picture is shown in Figure 1. The microstructur ripolymer had the following characteristic: irregular shapes, different size and smooth surface (Figure 1b) of the particles were easily recognizable. the particle size is relatively widely distributed. The largest is around 200 m smallest is just under 10 microns. The FT-IR spectrum of the quadripolymer is presented in Figure 2. M cm −1 and 3300 cm −1 are assigned to the N-H stretching vibration of AM and A tively. The absorbency at 2935 cm −1 results from the characteristic peak group. The stretching vibration of C=O at 1699 cm −1 is attributed to AM an absorption peak observed at 1454 cm −1 is due to the C-H bending v DMDAAC unit. Bands recorded at 1185 cm −1 and 1044 cm −1 assigned to S vibration in the sulfonic group correspond to SSS unit. The absorption pea reveal the bending vibration of a benzene ring of the SSS unit.   The 1 H-NMR spectrum of the quadripolymer is shown in Figure 3. Bands recorded at 7.48 ppm (a 1 ) and 7.24 ppm (a 2 ) indicate the chemical shift in the benzene ring of SSS units. The N-H proton from 5.87 to 5.92 ppm (b) are related to AM. The peaks between 5.51 and 5.63 ppm (c) are due to the N-H proton vibration of AMPS. The characteristic peak at 4.79 ppm belongs to the chemical shift of D 2 O protons. Broad peaks between 3.45 and 3.53 ppm (d) are assigned to N-C proton of DMDAAC. Respectively, the peaks at 2.76 ppm and 1.65 ppm correspond to the chemical shift of -CH 2 and -CH 3 of AMPS (e). The board peak at 2.93 ppm and 1.45 ppm are related to the -CH-linked with the benzene ring and the main chain of SSS, respectively (f). Combined with the FT-IR spectrum, the quadripolymer molecular structure is consistent with the designed one. stals 2022, 12, x FOR PEER REVIEW The 1 H-NMR spectrum of the quadripolymer is shown in Figure 3. Ban at 7.48 ppm (a1) and 7.24 ppm (a2) indicate the chemical shift in the benzene units. The N-H proton from 5.87 to 5.92 ppm (b) are related to AM. The pe 5.51 and 5.63 ppm (c) are due to the N-H proton vibration of AMPS. The c peak at 4.79 ppm belongs to the chemical shift of D2O protons. Broad peaks b and 3.53 ppm (d) are assigned to N-C proton of DMDAAC. Respectively, the ppm and 1.65 ppm correspond to the chemical shift of -CH2 and -CH3 of A board peak at 2.93 ppm and 1.45 ppm are related to the -CH-linked with the b and the main chain of SSS, respectively (f). Combined with the FT-IR spectrum ripolymer molecular structure is consistent with the designed one. From the TG curve (Figure 4), it can be found that the four stages of the t radation process, first stage, the quadripolymer has 8.5% of mass losses be indicating slightly thermal degradation from chain scissionThe second stage °C to 319.5 °C, apparent mass loss of the quadripolymer occurs (27.71%) in t and the fastest mass loss temperature emerges at 300.5 °C in the DSC curv stage is from 319.5 °C to 528.5 °C, the mass loss declines constantly (35.43% of the non-cyclic anhydrides formation acrylic acid repeat unit or the carboxy dant group. Moreover, accordingly, there is no significant fluctuation on the The last stage is from 528.5 °C to 600 °C, 28.36% mass is left. This might b thermal degradation of C=C decomposition in the main chain of AM and S and the breakage of benzene ring in SSS monomers. From the above analysis degradation of the quadripolymer is not obvious before 260 °C and only 2 demonstrates the quadripolymer has strong heat-resistance ability. From the TG curve (Figure 4), it can be found that the four stages of the thermal degradation process, first stage, the quadripolymer has 8.5% of mass losses before 260 • C, indicating slightly thermal degradation from chain scissionThe second stage is from 260 • C to 319.5 • C, apparent mass loss of the quadripolymer occurs (27.71%) in the TG curve and the fastest mass loss temperature emerges at 300.5 • C in the DSC curve. The third stage is from 319.5 • C to 528.5 • C, the mass loss declines constantly (35.43%). the release of the noncyclic anhydrides formation acrylic acid repeat unit or the carboxylic acid pendant group. Moreover, accordingly, there is no significant fluctuation on the DSC curve. The last stage is from 528.5 • C to 600 • C, 28.36% mass is left. This might be due to the thermal degradation of C=C decomposition in the main chain of AM and SSS segments and the breakage of benzene ring in SSS monomers. From the above analysis, the thermal degradation of the quadripolymer is not obvious before 260 • C and only 27.7%, which demonstrates the quadripolymer has strong heat-resistance ability.  The data of quadripolymer molecular weight is follow in Table 2. Based on t lecular structure of the quadripolymer. The weight-average molecular weight is 1 g/mol, and the number-average molecular weight is 684,000 g/mol. Besides, the vi average molecular weight is 1090,000 g/mol calculated with Mark-Houwink-Sa equation (Mη = 802*[η]1.25), among which the η value is 3.236 dL/g measured belohdo viscometer (the flowing time of 0.1 wt% quadripolymer solution). Moreo molecular weight distribution index PDI is 1.6974, indicating that the quadripolym narrow molecular weight distribution. A drilling fluid having good rheological properties (low plastic viscosity valu able gel strength, and high yield point) will provide good cuttings transportati ciency in drilling operation. To investigate the influence of the quadripolymer on t ological behavior of the bentonite-free drilling fluid, various amount (from 0.3 to 0 of two polymers were added to the base formula and the rheological parameters, the apparent viscosity (AV), plastic viscosity (PV), and yield point (YP), were me and compared.
As illustrated in Table 3, the two polymer drilling fluids display a remark crease in rheological parameters (AV, PV, and YP), as the concentration of poly creases in the range from 0.3 to 0.9 wt%, which is mainly because that polymer cont to forming the network structure of fluids and consequently improving the rhe properties. Compared with Drispac, PV values of the quadripolymer containing b ite-free fluid system are relatively lower at the same polymer concentrations on the while YP show relatively higher values. Moreover, the quadripolymer system ex dynamic ratio (YP/PV) of up to 0.80, significantly larger than that of Drispac, at p concentration of 0.9 wt%, indicating that the quadripolymer has good shear th property. Lastly, gel strength of drilling fluid describes the capacity to suspend th ing cuttings when the pump is shut down. It can be observed that the increase in c The data of quadripolymer molecular weight is follow in Table 2. Based on the molecular structure of the quadripolymer. The weight-average molecular weight is 1161,000 g/mol, and the number-average molecular weight is 684,000 g/mol. Besides, the viscosity-average molecular weight is 1090,000 g/mol calculated with Mark-Houwink-Sakurada equation (Mη = 802*[η]1.25), among which the η value is 3.236 dL/g measured by Ubbelohdo viscometer (the flowing time of 0.1 wt% quadripolymer solution). Moreover, the molecular weight distribution index PDI is 1.6974, indicating that the quadripolymer has narrow molecular weight distribution. A drilling fluid having good rheological properties (low plastic viscosity value, suitable gel strength, and high yield point) will provide good cuttings transportation efficiency in drilling operation. To investigate the influence of the quadripolymer on the rheological behavior of the bentonite-free drilling fluid, various amount (from 0.3 to 0.9 wt%) of two polymers were added to the base formula and the rheological parameters, such as the apparent viscosity (AV), plastic viscosity (PV), and yield point (YP), were measured and compared.
As illustrated in Table 3, the two polymer drilling fluids display a remarkable increase in rheological parameters (AV, PV, and YP), as the concentration of polymer increases in the range from 0.3 to 0.9 wt%, which is mainly because that polymer contributes to forming the network structure of fluids and consequently improving the rhelogical properties. Compared with Drispac, PV values of the quadripolymer containing bentonite-free fluid system are relatively lower at the same polymer concentrations on the whole, while YP show relatively higher values. Moreover, the quadripolymer system exhibits a dynamic ratio (YP/PV) of up to 0.80, significantly larger than that of Drispac, at polymer concentration of 0.9 wt%, indicating that the quadripolymer has good shear thinning property. Lastly, gel strength of drilling fluid describes the capacity to suspend the drilling cuttings when the pump is shut down. It can be observed that the increase in concentration of the quadripolymer contributes to a more dramatic increase than Drispac in G 1 and G 2 . The enhanced gel strength is attributed to the interactions among polymer molecules. Therefore, the bentonite-free drilling fluid with the quadripolymer has excellent cuttings suspending and carrying capacity.

Salt Resistance
It is shown from Table 4 that all bentonite-free polymer drilling fluids with NaCl have lower rheological parameters than that without salt. Furthermore, as the salt content increases, all the rheological parameters display a remarkable decreasing trend. This is mainly because of charge shielding effect. Na + has strong ability to compress electric double layer of polymer and neutralize negative charge on ionized polymer, which could reduce an electrostatic repulsion force between particles to lower viscosity values, thus destroying network structure between polymer molecules. Furthermore, with a further increase in salt, large amount of Na + will increase the polarity of solution and enhance the hydrophobic associate effect, which can compensate the decrease of viscosity to a certain extent [30]. However, it could be seen clearly that compared with Drispac containing fluids, the quadripolymer shows a less variation in rheological parameters and has higher YP and gel strength values at the same salt concentration of 5 wt% and 10 wt%, respectively, suggesting that the quadripolymer has better salt resistance than Drispac.

Filtration Property
For filtration property, the API filtration (FL API ) experiments were performed under a pressure of 100 psi and room temperature. The effect of salinity and polymers on filtration performance of bentonite-free drilling fluid is presented in Figure 5. quadripolymer Drispac quadripolymer+5wt% NaCl Drispac+5wt% NaCl quadripolymer+10wt% NaCl Drispac+10wt% NaCl Figure 5. Effect of salinity and polymers on filtration property of bentonite-free drilling fluid.
Dramatic decreases in FLAPI of both fresh and salt fluid samples are seen as the dosage of polymer increases from 0 to 0.9 wt%. Firstly, in deionized water, FLAPI of the bentonitefree fluids with 0.9 wt% quadripolymer and Dispac decrease to 3 mL and 3.5 mL, respectively. The two polymers have approximately equal ability to control the filtration loss in fresh water environment. Similarly, in electrolyte solution, the filtration loss variations also present a declining trend with increasing polymer concentration, but the volumes are considerably higher than those in fresh water solution. This is mainly because that the molecular chains of polymer cannot be fully stretched in salt water, and subsequently the repulsive forces between molecular reduce and a high permeable filtration cake is formed. For all that, compared with FLAPI of Drispac containing salt system (18 mL), at 0.9 wt%, FLAPI of bentonite-free drilling fluid with the quadripolymer can be controlled within 12.5 mL, which demonstrates that the quadripolymer has better filtration loss control property in brine drilling fluid.

Thermal Stability
In petroleum industry, it is well known that high temperature would dramatically worsen rheological property and increase filtration volume loss of drilling fluids. Therefore, common used polymers PAM, 80A51, HE150 were compared with the quadripolymer to investigate their high temperature resistance. The initial viscosity was recorded as V1 when polymer solutions were prepared at 0.9 wt%. Then, the polymer solutions were hot rolling aged at different temperatures for 16 h, and the residual viscosity was marked as V2. The viscosity retention rates at different temperatures are shown in Figure 6. Dramatic decreases in FL API of both fresh and salt fluid samples are seen as the dosage of polymer increases from 0 to 0.9 wt%. Firstly, in deionized water, FL API of the bentonite-free fluids with 0.9 wt% quadripolymer and Dispac decrease to 3 mL and 3.5 mL, respectively. The two polymers have approximately equal ability to control the filtration loss in fresh water environment. Similarly, in electrolyte solution, the filtration loss variations also present a declining trend with increasing polymer concentration, but the volumes are considerably higher than those in fresh water solution. This is mainly because that the molecular chains of polymer cannot be fully stretched in salt water, and subsequently the repulsive forces between molecular reduce and a high permeable filtration cake is formed. For all that, compared with FL API of Drispac containing salt system (18 mL), at 0.9 wt%, FL API of bentonite-free drilling fluid with the quadripolymer can be controlled within 12.5 mL, which demonstrates that the quadripolymer has better filtration loss control property in brine drilling fluid.

Thermal Stability
In petroleum industry, it is well known that high temperature would dramatically worsen rheological property and increase filtration volume loss of drilling fluids. Therefore, common used polymers PAM, 80A51, HE150 were compared with the quadripolymer to investigate their high temperature resistance. The initial viscosity was recorded as V 1 when polymer solutions were prepared at 0.9 wt%. Then, the polymer solutions were hot rolling aged at different temperatures for 16 h, and the residual viscosity was marked as V 2 . The viscosity retention rates R v at different temperatures are shown in Figure 6. Aging temperature(C) quadripolymer 80A51 PAM HE150 Figure 6. Viscosity retention rate of polymers at different aging temperature.
As illustrated in Figure 6, as the aging temperature increases, the viscosity retention rates of different polymers generally show downward trends, but the change degree varies. Among them, the viscosity of the two polymers, PAM and 80A51, has been basically lost after aging at 140 °C. In contrast, HE150 and the quadripolymer have higher viscosity retention rates at the same temperature, especially for the quadripolymer, the viscosity retention rate is as high as 40% even when the aging temperature reaches to 180 °C, and nearly 10% at 200 °C. Therefore, compared with other three polymers commonly used in drilling fluids, the quadripolymer has better temperature resistance.
In order to further study the effect of the quadripolymer on the thermal stability of the bentonite-free drilling fluid, the rheological and filtration properties were evaluated before and after hot rolling aging at various temperatures (25 °C, 100 °C, 120 °C, 140 °C, 160 °C, 180 °C, 200 °C), and the results are shown in Table 5. Note: bentnoite-free drilling fluid formulation: deionized water + 0.1 wt% NaOH + 0.15 wt% Na2CO3 + 0.9 wt% quadripolymer + 2 wt% PAC-LV + 2 wt% LOCKSEAL + 4 wt% CaCO3 + 0.5 wt% Na2SO3. As illustrated in Figure 6, as the aging temperature increases, the viscosity retention rates of different polymers generally show downward trends, but the change degree varies. Among them, the viscosity of the two polymers, PAM and 80A51, has been basically lost after aging at 140 • C. In contrast, HE150 and the quadripolymer have higher viscosity retention rates at the same temperature, especially for the quadripolymer, the viscosity retention rate is as high as 40% even when the aging temperature reaches to 180 • C, and nearly 10% at 200 • C. Therefore, compared with other three polymers commonly used in drilling fluids, the quadripolymer has better temperature resistance.
As shown in Table 5, it can be clearly observed that in the aging temperature range of 100-180 • C, all parameters variation are within the acceptable range, showing a good thermal stability. PV value of the quadripolymer bentonite-free drilling fluid still remains at 20 mPa·s after aging at 180 • C. Meanwhile, FL API and FL HTHP are controlled within 8.6 mL and 24.8 mL, respectively. However, when the aging temperature rises to 200 • C, the rheological parameters of the bentonite-free drilling fluid with the quadripolymer show an obvious decreasing trend, and the amount of filtration loss markedly increases, indicating that the comprehensive properties of drilling fluid are out of control. Based on the above analysis, it can be demonstrated that the bentonite-free drilling fluid prepared by the quadripolymer possesses excellent ability to maintain viscosity and filtration loss at the temperature of 180 • C.

HTHP Rheology
In order to determine the rheological characteristics of the synthetic quadripolymer under high temperature and high pressure environment, the HTHP rheology test of four common high temperature resistant polymers were performed. A constant shear rate (511 s −1 ) was applied at different temperatures. Moreover, the curves of viscosity changing with temperature are presented in Figure 7.
Crystals 2022, 12, x FOR PEER REVIEW 11 of 14 As shown in Table 5, it can be clearly observed that in the aging temperature range of 100-180 °C, all parameters variation are within the acceptable range, showing a good thermal stability. PV value of the quadripolymer bentonite-free drilling fluid still remains at 20 mPa·s after aging at 180 °C. Meanwhile, FLAPI and FLHTHP are controlled within 8.6 mL and 24.8 mL, respectively. However, when the aging temperature rises to 200 °C, the rheological parameters of the bentonite-free drilling fluid with the quadripolymer show an obvious decreasing trend, and the amount of filtration loss markedly increases, indicating that the comprehensive properties of drilling fluid are out of control. Based on the above analysis, it can be demonstrated that the bentonite-free drilling fluid prepared by the quadripolymer possesses excellent ability to maintain viscosity and filtration loss at the temperature of 180 °C.

HTHP Rheology
In order to determine the rheological characteristics of the synthetic quadripolymer under high temperature and high pressure environment, the HTHP rheology test of four common high temperature resistant polymers were performed. A constant shear rate (511 s −1 ) was applied at different temperatures. Moreover, the curves of viscosity changing with temperature are presented in Figure 7. As shown in Figure 6, heating causes the rapid drop in the viscosity of polymer solutions, especially for PAM, 80A51 and HE150. Among the four polymers, the viscosity reduction rates of the three polymers mentioned above are much higher than that of quadripolymer. When temperature rises to 180 °C, the viscosities of the PAM and 80A51 solution are close to 0. In contrast, the quadripolymer has higher viscosity values at each temperature point than those of the other three polymers in the temperature range of 200 °C, As shown in Figure 6, heating causes the rapid drop in the viscosity of polymer solutions, especially for PAM, 80A51 and HE150. Among the four polymers, the viscosity reduction rates of the three polymers mentioned above are much higher than that of quadripolymer. When temperature rises to 180 • C, the viscosities of the PAM and 80A51 solution are close to 0. In contrast, the quadripolymer has higher viscosity values at each temperature point than those of the other three polymers in the temperature range of 200 • C, which further suggests that the quadripolymer has good temperature resistance at high temperatures.

Reservoir Protection Performance
ZDY50-180 core flow tester and oil reservoir cores taken from Well TK10 at the depth of 5298.83-5299.50 m, were used for reservoir protection evaluation of the bentonite-free drilling fluid with the new quadripolymer.
The degree of reservoir damage caused by drilling fluid can be characterized by the change of permeability before and after core contamination by drilling fluid, i.e., the lower permeability of core contamination means the higher degree of reservoir damage. It is clear that the permeability of both cores (sample a and sample b) after contamination is restored to more than 85% (Table 6). Moreover, the permeability recovery rate of two cores are obviously improved and reaches up as high as 95.7% and 94.2%, respectively, after cutting off 1 cm of contaminated core section, which indicates that the bentonite-free drilling fluid has a good protective effect on reservoir. Note: K 1 is the initial permeability of cores; K 2 is the permeability of contaminated cores.

Conclusions
A novel hydrophilic polymer was prepared with AM, AMPS, DMDAAC, and SSS monomers via a method of free radical polymerization in aqueous solution. The new polymer was characterized by 1 H NMR and FT-IR analysis. Results indicated the quadripolymer molecular structure was consistent with the designed one. Moreover, the rheological and filtration properties of the bentonite-free drilling fluid containing the quadripolymer and Drispac were examined in deionized and salt water at various temperatures. As a result, the rheological parameters regularly increased with increasing dosage of polymers. Simultaneously, the viscosity of the salt water fluid system was explicit smaller than that of the fresh water system. Additionally, when the adding amount was 0.9 wt%, it was confirmed that FL API of two polymers drilling fluid systems decreased to 3 mL and 3.5 mL, respectively, which are in acceptable range. The thermal stability test of bentonite-free drilling fluid showed that the quadripolymer drilling fluid could maintain a certain viscosity value, and FL API and FL HTHP were also controlled within 8.6 mL and 24.8 mL, respectively. In HTHP rheology test, the synthetic polymer showed superior resistance to high temperature than commonly used polymers. As for its damage to the reservoir, the permeability recovery of cores reached more than 95.72%. In general, the synthesized polymer has good properties of high temperature resistance and salt tolerance in controlling rheology and filtration loss of the bentonite-free drilling fluid system.