Study On Dispersion Technology And Dispersion Mechanism Of Nano - CaCO3 For Water - Based Drilling Fluids

In drilling process, it is important to prevent water penetrating into the oil and gas reservoir for protecting reservoir. The general drilling fluid could not form dense mud cake to prevent water invasion in shale, but nanometer materials could enter pores of shale to block pore and form dense mud cake because of their small size, thus reduce the amount of drilling fluid filtration. However, the nanoparticle’s surface energy is large, meanwhile, the nanoparticles are easy to attract each other forming precipitation, which would affect their application in drilling fluid. Therefore, the study of dispersing technology and mechanism of nanometer materials in the drilling fluid has important practical value and theoretical significance.According to the basic physical and chemical properties, water sedimentation performance and the drilling fluid’s filtration performance,3#nanometer CaCO3was preferably selected as the research object, then its original morphology and aggregation behavior pattern was analyzed by scanning electron microscopy (SEM). Nanometer CaCO3was dispersed with polyethylene glycol (PEG), carboxylate, hexadecyl trimethyl ammonium bromide(CTAB), sodium dodecyl sulfate(SDS), KH-550and polyacrylamide(PAM), and the dispersion effect was evaluated by the sedimentation stability method and spectrophotometric method. The result showed that the carboxylate was preferably selected as dispersant to optimize disperse nanometer CaCO3. The dispersion optimal condition was determined through the absorbance ratio r24and particle size distribution. Ultrasonic processing in ultrasonic cleaners in9min, dispersion environment of pH value of9and4%carboxylate (relative to nanometer CaCO3) could achieve the best dispersion stability performance and its sedimentation rate was lowest. The suspension dispersive effect was evaluated by scanning electron microscope, laser particle size instrument and zeta potential analyzer, the result showed that nanometer CaCO3particle size distribution was concentrated in357nm.To further explore the mechanism of dispersed modified nanometer CaCO3by carboxylate, the adsorption behavior of carboxylate on the surface of nanometer CaCO3and the zeta potential test and the surface wettability of nanometer CaCO3were measured,and then the relationship between adsorption dispersant, pH and zeta potential was obtained. We found that the dispersion process was complex and neither simple steric hindrance nor electrostatic repulsion could explain the dispersed modified principle.The maximum adsorption amount of carboxylate on the surface of nanometer CaCO3reached to5%and the maximum zeta potential value was-47.7mV. The excessive amount of carboxylate could affect the effect of dispersion because of the electric double layer compression and long hydrophobic chains intertwine of carboxylate; the adsorption of carboxylate on the surface of nanometer CaCO3decreased with the increase of pH, meanwhile it was beneficial to the degree of ionization and the increases of absolute value of ZETA potential. Finally, the dispersion mechanism w summed up by dispersing stability formula VT=VA+VR+VS+VST. Under the combination effect of steric hindrance of adsorption layer and electrostatic repulsion of ZETA potential, the dispersion stability state of nanometer CaCO3was obtained.Finally, the application results of nanometer CaC03in drilling fluid was evaluated through the measurements of filtration performance,lubrication performance,rheological properties, shale recovery rate and linear expansion rate. The research results showed that the application effect of nano CaCO3in drilling fluid was good, and the API filtration of drilling fluid is6mL, it could withstand temperatures up to140℃after24h hot rolling by high-temperature furnace. The ability of reducing fluid filtration is outstanding, and hydration swelling tests results showed that nano drilling fluid could effectively inhibit hydration expansion of shale, expansion rate was only15%, shale recovery rate was higher and reached88.6%, meanwhile, it showed excellent lubrication performance, lubrication coefficient decrease rate reached84%.