Research On Treatment Technology Of Waste Oil-Based Drilling Fluid Based On CO₂-Triggered Switchable Solvents

The main challenge of the treatment of waste drilling fluid is the high oil content. The most commonly used treatment technologies of waste drilling fluid in engineering are reinjection, landfill. These technologies not only cause a waste of resources, but also not fundamentally eliminate the environmental pollution of oil substances. Currently, the mainly oil recovery treatment methods include thermal distillation, solvent extraction and chemical demulsification separation. However, these methods cannot recycle the solvent or reagent added in the treatment. Although the supercritical extraction process recycle the extraction agent, but the cost of equipment and energy consumption is too high. CO2 based switchable solvent can be used to resolve these contradictions and recycle the solvent without the need of high temperature and high pressure equipment, simple operation, addition of polymer compound. The best-performance switchable solvent reported in the existing literature is N, N-dimethyl ring hexyl amine. Though its performance of oil recycle is good, it is still with some serious problems, such as low flash point, poor security, stimulation odor and high acute toxicity.First, in this paper, we synthetized series of CO2 based switchable solvents by introducing ester base and extending carbon chain based on the structure of N, N-dimethyl ring hexyl amine. The optimized synthesis conditions include (1) N,N-dimethyl ethanolamine and carboxylic acid with Molar ratio 1:1, (2) heating 10h at 130℃, (3) The amount of catalyst is 0.4%.Second, we analyzed 13 basic physical properties of the synthesized switchable solvents, including toxicity, solubility and surface activity and etc., and then evaluated the performances of the solvents. In order to select the best switchable solvent in large scale candidates with multi parameters, a "Reagent selection software" was developed. The reagent named as "C-8" (CH3(CH2)4COO(CH2)2N(CH3)2) was selected as the best switchable solvent for the treatment of waste drilling fluid.Third, the conversion mechanism of the switchable solvent, which is the reversible transformation of the tertiary amine group, is demonstrated by measurement of variance of electrical conductivity, pH value and microstructure. The feasibility of the conversion is verified by thermodynamics and dynamics experiments. In thermodynamics experiment, the interface reaction, ionization, mass transfer and diffusion of the conversion are analyzed. In the experiment of dynamics, study on the reaction kinetics of the conversion shows that activation energy and half-life of hydrophobic to hydrophilic conversion processes are Ea=3339 J·mol-1 and ti/2=5.0min respectively, activation energy and half-life of hydrophilic to hydrophobic conversion processes areEa=7091 J·mol-1 and t1/2=38.5min respectively. The conversion speed of the two direction conversions is studied, and the conversion equation (m=kt) is obtained. The effects of molecular structure, temperature, aeration rate and stirring intensity on conversion speed were analyzed.Fourth, the selected switchable solvent is used to process waste oil based drilling fluids obtained from 5 different oil fields. The oil removal rate is 98%, the oil content in solid residual is 3% which is lower than the second grade discharge standard (GB4914-2008) of oil content of non-water base drilling fluid, average oil content of waste water is 14.5 mg/L which can be directly reused to treatment of waste drilling fluid. The oil recycle rate is higher than 93% and the quality of the recycled oil is good. The recycle rate of switchable solvent about 60% and the structural performance has almost no change after being used several times. Furthermore, we studied the influencing factors of the loss and optimized the experiment condition which increased recycle rate of switchable solvent to 85%. A small scale process equipment for the treatment of the waste oil based drilling fluid using switchable solvent is designed.Finally, the mechanism of treatment of waste oil base drilling fluid using switchable solvent is studied. The variation of plastic viscosity, dynamic shear and flow index and consistency coefficient, viscosity, density, emulsifier concentration the mixture under different switchable solvent content are evaluated. Results shows that when the switchable solvent content increased from 10% to 40%, the plastic viscosity of the mixture decreased 85% and its dynamic shear force decreased from 25 to 0; at low rotation speed, the flow index of the mixed liquor is increased by 17 times and the consistency coefficient is reduced by 98% when the 20% switchable solvent is added; in medium rotation speed and high rotation speed condition, addition of switchable solvent make the mixture liquid change from pseudo-plastic fluid to Newton fluid; at the speed of 3 r/min,10% switchable solvent can make the apparent viscosity of the waste oil base drilling fluid decreased by 86%. The concentration of the solid particles and the polymer was decreased by the addition of the switchable solvent. When the concentration is below the critical value, the "card house" structure of the drilling fluid is destroyed, the dynamic stability and structural strength of the colloidal system is reduced and the viscosity of the mixture liquid is greatly reduced. In addition, since the switchable solvent increases the density difference between oil phase and water phase, it is easy to realize the demulsification after centrifugation separation.