Phase change microcapsule for drilling fluid cooling and its preparation method and application

A preparation method of a phase change microcapsule for drilling fluid cooling comprises the following steps:

• (1) drying, heating and melting, cooling and grounding core materials of NaNO3 and KNO3 to obtain a phase change material mixture;
• and the mass ratio of NaNO3 to KNO3 in the phase change material mixture is 3:1-5;
• (2) adding emulsifier into a linseed oil, then heating and stirring to obtain an oil phase;
• the emulsifier is sorbitan oleate, and the concentration of the emulsifier in the oil phase is 1-10 mg/mL;
• (3) adding the phase change material mixture into a suspension of cellulose nanofibers (CNFs), then stirring followed by adding ammonium hydroxide to obtain a water phase;
• the concentration of the suspension of CNFs is 0.006-0.01 g/mL;
• the ratio of the mass of phase change material mixture to the volume of the suspension of CNFs is 0.5-1.5 g:
• 1 mL;
• the concentration of ammonium hydroxide is 25-28 wt %;
• the ratio of the volume of the ammonium hydroxide to the mass of the phase change material mixture is 0.5-1.5 mL:15 g;
• the suspension of CNFs is prepared according to the following method:
• dispersing an eucalyptus powder into deionized water, and adding 2,2,6,6-tetramethylpiperidine oxide (TEMPO), sodium bromide and sodium hypochlorite solution to yield a mixture, stirring the mixture at room temperature for 4 h;
• then adding 0.1 mol/L of hydrochloric acid solution and 10-30% of sodium hydroxide solution by weight to adjust pH to 10-10.5 to yield a mixed solution 1;
• adding three times of the mixed solution 1 of ethanol by volume to yield a mixed solution 2;
• wherein the mass ratio of the eucalyptus powder to the volume of deionized water is 1 g:100 mL, the mass ratio of TEMPO to the eucalyptus powder is 0.01-0.02:1, the mass ratio of sodium bromide to the eucalyptus powder is 0.05-0.15:1, the mass ratio of sodium hypochlorite solution to the eucalyptus powder is 6-7:1, and the mass fraction of sodium hypochlorite solution is 5-6%;
• then centrifuging the mixed solution 2 and collecting a sediment, dispersing the sediment to deionized water, and performing an ultrasonic treatment to obtain the suspension of CNFs;
• the concentration of the suspension of CNFs is 0.006-0.01 g/mL;
• the power of the ultrasonic treatment is 800 W;
• the ultrasonic treatment time is 30-60 min;
• and
• (4) mixing the water phase and the oil phase, heating and stirring to form a water-in-oil (W/O) emulsion;
• then adding tetraethyl silicate into the W/O emulsion for a reaction at 65-75° C. for 10-15 h to yield a reaction product, centrifugating, washing and drying the reaction product to obtain the phase change microcapsule;
• wherein the mass ratio of the tetraethyl silicate and the phase change material mixture in the water phase is 3-5:10;
• the volume ratio of water in the water phase and the linseed oil in the oil phase is 1:2-4;
• wherein the phase change microcapsule has a phase change temperature at 212-236° C., a latent heat between 186-196 J/g, and an encapsulation efficiency between 73-76%.