Abstract: The theological property,hole-stability property,and reservoir protection performance of silicate drilling fluids are greatly influenced by pH.The properties of silicate drilling fluids of various pH are analyzed based on conventional methods,as well as the size distribution and surface charges of polysilicic particles are examined on the basis of the dynamic light scattering technology.Research results show that pH exerts the greatest effect on the water loss of silicate drilling fluids.However,the effect of pH on the viscosity of silicate drilling fluids varies a lot.When the silicate concentration is as low as 1.0%,the viscosity changes little with pH.But when the silicate concentration is as high as 5.0%,the viscosity decreases quickly as the pH increases.When the pH increases,the hole-stability property increases initially and then decreases,being the strongest when the pH is 8.0.Since the pH controls the size distribution and surface charges of polysilicic particles,so it determines the flocculation of them and it has effect on the interaction of poly-silicate particles/wellbore clay particles and poly-silicate particles/slurry clay particles.These results are helpful to recognize the properties of silicate drilling fluids in essential,and further to promote its application.