Synthesis Of High-temperature Resistance Polymer As Filtrate Reducer For Drilling Fluids And Mechanisms Of Filtration Control

With the development of oil industry, drilling fluids had been facing many challenges to keep pace with the drilling technology. One of these challenges was the high temperature drilling, so water-based drilling fluids with improved properties were urgently required. Through a lot of literature search, polymers with improved thermal stability were selected as an additive to improve the properties of drilling fluids. According to theories of molecular structure design, terpolymer of sodium 2-acrylamide-2-methyIpropane-sulfonate (AMPS), N, N-dimethyl acrylamide (DMAM) and N-vinylpyrrolidone (NVP) was synthesized through free-radical polymerization using potassium persulfate (KPS) as initiator and EDTA as chelator. AMPS-DMAM-NVP terpolymer as a filtrate reducer was able to improve the thermal stability of water-based drilling fluids in an environment of high temperature.1. Preparation and characterization of AMPS-DMAM-NVP terpolymerSynthesis conditions of AMPS-DMAM-NVP terpolymer were optimized via orthogonal designs, and the best synthesis conditions were obtained: the chelator EDTA dosage 0.1w.t.% (of total monomers), the monomer ratio AMPS : DMAM : NVP=3 : 6 : 1 (mol), the initiator KPS dosage 0.2w.t.% (of total monomers) and the reaction temperature 75℃. Chemical structure, relative molecular weight and thermal stability of AMPS-DMAM-NVP terpolymer were characterized. The Fourier transform infrared (FTIR) spectroanalysis revealed the presence of three monomers, confirmed the structure of terpolymer and the effectiveness of the reaction. Element analysis showed higher reaction activity of AMPS and DMAM than NVP, and AMPS, DMAM had higher content in terpolymer. The intrinsic viscosity of terpolymer was 2.43dL/g, so the relative molecular weight of terpolymer was obtained from the intrinsic viscosity, which was 77.14×10⁴. Both thermogravimetry analysis (TGA) and the retainable ratio of intrinsic viscosity illuminated the good thermal stability of terpolymer.2. The adsorption of terpolymer onto clay particles and effects of terpolymer on the rheological properties of clay dispersionsIn order to explain the mechanisms of terpolymer to give water-base drilling fluids good thermal stability in high temperature environment, the adsorption properties of terpolymer onto clay particles were studied, and effects of terpolymer on the interaction between clay particles were also studied through rheological experiments. It showed that terpolymer adsorbed onto clay particles in the case of fresh water dispersions, and the adsorption increased with increasing terpolymer concentration until a plateau region presented. The addition of electrolytes conduced to the increase of adsorption, and effects of divalent metallic ions were more significant than that of univalent. Besides, adsorption decreased with increase of temperature.The results of rheological experiments revealed that the flow curves of terpolymer solution at different concentrations could be described by the Power Law model and exhibited pseudo-plastic properties. For terpolymer-clay dispersions, flow curves could be discribed by the Herschel-Bulkely moldel, and the addition of terpolymer increased the apparent viscosity of base clay dispersions. The results of oscillatory measurements illuminated that the viscous modulus of clay dispersions increased with increasing terpolymer concentration, while elastic modulus almost kept constant. The addition of electrolytes had noticeable effects on both viscous and elastic modulus of each of dispersions, and effects of divalent metallic ions was more significant than that of univalent.3. Filtration control properties of terpolymer in high temperature environment and mechanisms of filtration controlEffects of terpolymer on the filtration and thermal stability properties of muds were studied before and after aging experiments at 200℃for 16h. Its mechanisms were explained through the size distribution of clay particles, zeta potential, and so on. For 4w.t.% fresh water mud, terpolymer behaved good filtration control and thermal stability properties. This might attributed to the adsorption of terpolymer onto clay particles, which heightened the zeta potential (absolute value) of clay particles, enhanced the electrostatics stabilities of clay particles. In addition, the adsorption of terpolymer enhanced the steric stabilization, and both of the effects kept the clay particles dispersion and had a good size distribution, which contributed to form a low permeability mud cake; besides, terpolymer could plug the mud cake holes, improved the quality of the mud cake, and increased apparent viscosity of mud. All of those contributed to reduce the filtration loss. For 4w.t.% salt water mud, due to the addition of electrolytes increased the adsorption of terpolymer onto clay particles, it required that terpolymer concentration was enough high. The mechanisms for terpolymer reducing the filtration loss of 4w.t.% salt water mud were that the adsorption of terpolymer enhanced the steric and the electrostatics stabilization of clay particles, which kept the clay particles dispersion and had a good size distribution, in addition, terpolymer could plug the mud cake holes and improved the quality of the mud cake. The high temperature resistance and salt tolerance properties of terpolymer were explained from the aspect of molecular chemical structure.4. Evaluation for high temperature filtration control properties of terpolymercomparing with Driscal DComparing with Driscal D, effects of AMPS-DMAM-NVP terpolymer on the filtration properties of various type muds were studied after aging test at 200℃for 16h. It was found that the filtration control effects of terpolymer on fresh water base mud, 4w.t.% salt water base mud, compound salt water base mud and artificial sea water base mud b were better than that of Driscal D. It accounted for that AMPS-DMAM -NVP terpolymer was a more efficient high-temperature resistance filtrate reducer, had better salt tolerance properties, and withstood Ca²⁺, Mg²⁺ pollution, could give each of four type muds a better thermal stability. While the filtration control effects of terpolymer on artificial sea water base mud a were little lower than that of Driscal D, it could be explained that terpolymer was applied for artificial sea water base mud with low clay content. In addition, the filtration control effects of terpolymer and Driscal D on fresh water base mud, saturated salt water base mud were also studied after aging test at 240℃for 16h. It found that terpolymer still had good high temperature resistance and salt tolerance properties, and behaved a more efficient high-temperature resistance filtrate reducer for fresh water base mud. Terpolymer could withstand saturated salt water on the whole.