A water based drilling fluid loss agent (PAASDA) was synthesized by free radical polymerization method,using acrylamide (AM), acrylic acid (AA), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), sodium p-styrene sulfonate (SSS), and dimethyldiallylammonium chloride (DMDAAC) as synthetic monomers. The structure and thermal stability of PAASDA were characterized using FTIR, ¹H NMR, DTA-TG, SEM and other methods. The suitable synthesis conditions were determined using the single factor method as follows: m (AM): m (AMPS): m (SSS): m (AA): m (DMDAAC)=30:40:8:15:7, with an initiator dosage of 0.6% (w). The results of high-temperature aging experiments show that when the dosage of PAASDA in saline based slurry is 2.0% (w), it can withstand a high temperature of 180℃; At different aging temperatures, the filtration performance is superior to other sulfonated fluid loss agents; In the composite salt water polysulfonate drilling fluid system, the filtration loss after aging is 2.2mL, and the filtration loss at high temperature and high pressure is 8.2mL. Compared with commonly used salt resistant and fluid loss agents, the filtration loss can be reduced by 72.7%, and the filtration loss reduction effect is better.

With the rapid development of modern petroleum industry, oil and gas exploration has gradually entered deeper strata. During the drilling process, drilling fluid can suspend rock cuttings, cool the drill bit, and control formation pressure. At present, both domestically and internationally, the main types of water-based drilling fluid loss agents are modified natural products and artificially synthesized polymer fluid loss  agents. Among them, zwitterionic polymer fluid loss agents will become the main direction for the research and development of artificially synthesized polymer fluid loss agents in the future due to their excellent compatibility, adsorption, and stability.

High temperatures in deep wells can cause cross-linking or degradation of conventional fluid loss agents, making it difficult to control the rheological properties and filtration loss of drilling fluids. High pressure can affect the density of drilling fluid, causing wellbore instability, collapse, and pressure difference jamming. Therefore, there is an urgent need to develop a high-performance drilling fluid loss agent. Huang et al. synthesized a fluid loss agent using acrylamide (AM)/2-acrylamido-2-methylpropanesulfonic acid (AMPS)/dimethyldiallylammonium chloride (DMDAAC)/sodium p-styrene sulfonate (SSS) in a freshwater slurry, which aged at high temperatures up to 240℃. The cationic nature of DMDAAC ensured that the polymer had sufficient adsorption groups at high temperatures. SSS with a rigid benzene ring structure, due to its sulfonic acid groups and large steric hindrance effect, enhanced the inhibitory and thermal stability of the fluid loss agent. Quan Hongping et al. synthesized a fluid loss agent using AM/allyl polyethylene glycol (APEG)/acrylic acid (AA)/SSS. The introduced AA in this fluid loss agent contains carboxyl groups, which can increase the adsorption capacity of hydration groups, improve the polymer's fluid loss performance and stability.

This article adopts the free radical aqueous solution polymerization method, using AA, AMPS, SSS, DMDAAC, AM as raw materials, （NH₄）₂S₂O₈-NaHSO₃ as initiator, to synthesize a water loss agent PAASDA with temperature resistance, salt resistance, and strong inhibitory properties. The suitable conditions for synthesizing PAASDA were determined using the single factor method, and the structure and filtration performance of PAASDA were analyzed using methods such as FTIR, ¹H NMR, DTA-TG, SEM, etc.

1. Experimental Section

1.1 Main Reagents and Instruments

*AM，AA，（NH₄）₂S₂O₈，NaHSO₃，CaCl₂，NaCl，Sodium Formate, Anhydrous Sodium Carbonate, Anhydrous Ethanol: AR, Chengdu Kelong Chemical Reagent Factory;

* AMPS：Industrial products, Sean Chemical Technology Co., Ltd;

* DMDAAC：60% (w) aqueous solution, Aladdin Reagent Co., Ltd;

* SSS: Industrial Products, Shanghai Dibo Chemical Technology Co., Ltd;

*Secondary Bentonite: Xinjiang Zhongfei Xiazijie Bentonite Co., Ltd;

*Sulfonated water loss agent (SMP-3), salt resistant water loss agent (HL-60), viscosity enhancer (PAC-HV), auxiliary viscosity enhancer (CMS), diluent (SMT), cationic emulsified asphalt powder (FT-1), ultrafine calcium carbonate powder (QS-2), barite: Industrial Products, Renqiu High Tech Chemical Co., Ltd.

*WQF520 Fourier Transform Infrared Spectrometer: Beijing Ruili Analytical Instrument Co., Ltd;

*ZNS type room temperature and medium pressure filter loss instrument: Qingdao Hongxiang Petroleum Machinery Manufacturing Co., Ltd;

*Bruker AVANCE III HD 400 nuclear magnetic resonance spectrometer: Brooke Company;

*STA-449F3 thermogravimetric differential synchronous analyzer: NETZSCH, Germany;

*QUANTA-450 environmental scanning electron microscope: FEI Corporation, USA;

*GGS42-2 High Temperature and High Pressure Filter Loss Instrument: Qingdao Tongchun Petroleum Instrument Co., Ltd;

*NDJ-8S digital viscometer: Shanghai Youke Instrument and Meter Co., Ltd.

1.2 Experimental Methods

1.2.1 Synthesis of PAASDA

At room temperature, add 2.57g of AMPS, 1.29g of AA, and 0.69g of SSS to a beaker, dissolve them completely in 20mL of pure water, and add 30% (w) sodium hydroxide aqueous solution dropwise until the solution pH is 7. Add 1.0g of DMDAAC 60% (w) and 3.43g of AM to pure water in sequence, stirring evenly to completely dissolve all monomers. Add 0.034g of (NH₄) ₂S₂O₈ and 0.017g of NaHSO₃ as initiators, stir well, pass nitrogen for 10 minutes, seal with cling film, and place in a constant temperature water bath at 50℃. After 6 hours of reaction, obtain a transparent gel like polymer. Wash multiple times with anhydrous ethanol, cut and granulate, dry and grind to obtain white powder, which is PAASDA.

1.2.2 Structural Characterization

FTIR spectroscopy analysis of PAASDA was performed using an infrared spectrometer, with a wavenumber range of 500~4000cm^-1, a resolution of 2cm^-1, a scanning frequency of 16, and KBr compression. ¹H NMR characterization of the fluid loss agent PAASDA was performed using a nuclear magnetic resonance spectrometer, and the molecular structure was determined by combining FTIR spectroscopy.

1.2.3 High Temperature Stability Analysis

The stability of the water loss agent PAASDA was investigated using a synchronous comprehensive thermal analyzer in a nitrogen atmosphere with a heating range of 50-450℃ and a heating rate of 10℃/min.

1.2.4 Preparation of Drilling Fluid Base Slurry

Prepare drilling fluid base slurry according to the method specified in Q/SH 0049-2007. Add 2.4g anhydrous sodium carbonate and 40g sodium bentonite to 1000mL of deionized water, stir at high speed for 20 minutes, and cure for 24 hours to obtain saline based slurry. The formula of composite saltwater polysulfonate drilling fluid is shown in Table 1.

1.2.5 Evaluation of Fluid Loss Agent Performance and Rheological Parameters

Test the filtration loss reduction performance according to the method specified in GB/T 16783.1-2014.

Filtration loss experiment: The sample is added to different slurries, and the room temperature and medium pressure filtration loss (FL_API) is measured using a medium pressure filtration loss instrument. The experimental conditions are room temperature and 0.69 MPa; The high temperature and high pressure filtration rate (FL_HTHP) was measured using a high temperature and high pressure filtration meter under experimental conditions of 180℃ and 3.5 MPa.

Aging experiment: Different slurries were subjected to high-temperature aging treatment using a roller heating furnace to investigate the thermal stability of the slurries. The aging temperature was 200 ℃ and the slurry was aged under nitrogen atmosphere for 16 hours.

1.2.6 Microscopic Morphology of Filter Cake

Observe the microstructure of the sample using an environmental scanning electron microscope.

2. Results and Discussion

2.1 Influence of Synthesis Conditions of Fluid Loss Agent PAASDA

Using the single factor method, fresh water slurry was used as the test sample, and the amount of fluid loss agent was 0.6% of the slurry quality. FL_API was used as the evaluation basis to determine the optimal synthesis conditions. 

2.1.1 Monomer Ratio

Three sets of single factor experiments were used to optimize the monomer ratio. Fix the value of m(SSS):m(DMDAAC):m(AA)，examine the value of m(AMPS):m(AM)； Fix the value of m(AMPS):m(AM):m(AA),examine the value of m(SSS):m(DMDAAC)；Fix the value of m(AMPS):m(AM):m(SSS):m(DMDAAC)，examine the dosage of AA.

2.1.1.1 AMPS to AM Ratio

AM acts as the backbone of the main chain and accounts for a large proportion in the monomer ratio; The introduction of AMPS into copolymers will significantly improve their temperature and salt resistance. Fixed m (SSS): m (AA): m (DMDAAC)=8:15:7, the effect of m (AM): m (AMPS) on the filtration loss reduction performance of PAASDA in freshwater slurry is shown in Figure 1. As shown in Figure 1, with the decrease of m (AM): m (AMPS), the filtration loss of PAASDA in freshwater slurry first decreases and then increases, indicating that AMPS has a significant impact on the filtration loss reduction performance of PAASDA. This is due to the presence of sulfonic acid groups on the monomer of AMPS, which has high polymerization activity and can participate in the reaction. Therefore, with the increase of AMPS dosage, the sulfonic acid groups increase, enhancing the hydration ability of the polymer, which is beneficial for improving the mud making performance of drilling fluid and reducing the filtration loss in freshwater slurry. But when the amount of AMPS added exceeds 40% (w), the filtration loss increases, which is caused by excessive polymerization of AMPS. Considering economic costs, the optimal ratio of m (AM): m (AMPS)=3:4 is determined, and the amount of AMPS added is 40% (w).

2.1.1.2 SSS to DMDAAC Ratio

SSS contains sulfonic acid groups and a rigid structure benzene ring, which has good thermal stability; DMDAAC is a quaternary ammonium cationic monomer that can enhance the adsorption capacity of polymers on clay particles. Fixed m (AM): m (AMPS): m (AA)=6:8:3, the effect of m (SSS): m (DMDAAC) on the filtration loss reduction performance of PAASDA in freshwater slurry is shown in Table 2. From Table 2, it can be seen that by changing m (SSS): m (DMDAAC), PAASDA partially exhibits flocculation in freshwater slurry. This is because DMDAAC contains cations, while clay particles generally have negative charges. Under the action of electrostatic force, clay and polymer adsorb each other. When the dosage of DMDAAC is high, the cation content in PAASDA molecules is higher, causing excessive adsorption of clay particles on the polymer, leading to aggregation and flocculation. When the dosage of DMDAAC is 7% (w), the cations and clay particles on the polymer moderately coagulate, and FL_API is the lowest, so the dosage of DMDAAC is determined to be 7% (w).