Abstract: The preparation of measures to prevent and control CO₂ contamination requires an accurate determination of the concentration of contaminants and an understanding of the patterns in which the contaminants affect the properties of drilling fluids. Field quantitative method of determining the alkalinity of drilling fluids was improved, based on the principle of color change with acid/base indicators, to have a better accuracy and operability. Analyses of feld sampling and laboratory evaluation were performed to study the effects of CO₂ on the rheology and fltration property of high-density water base drilling fluids. Study was also conducted on the effects of CO₂ on the Zeta-potential of the surfaces of clays used in drilling fluids, particle size distribution of clays and adsorption of mud additives from the point of view of colloid chemistry, and on the mechanisms of the effects. The studies showed that the concentrations of carbonate and bicarbonate ions in high-density water base drilling fluids have critical values; when the concentrations of carbonate and bicarbonate ions exceeds the critical values, the structural viscosity and fltration rate of drilling fluid increase with the increase in the concentrations of the carbonate/bicarbonate ions. Carbonate/bicarbonate ions have altered the extent of the solubilization and the sizes of clay particles, which can be seen from the decrease in the Zeta-potential of the surfaces of clay particles, the decrease in the number of submicron-sized particles and the increase in the number coarse particles. Decrease in the adsorption quantity of colloid protective agents by clay particles results in the coalescence of clay particles and association of the clay particles to form a gel with 3D network structure. These phenomena have revealed the intrinsic mechanisms governing the macro property changes.