The inferior rheological and filtration properties of formate drilling fluid was the key question to caused unsatisfactory cementing quality. In order to improve the efficiency of annulus drilling fluid displacement and mud cake flushing,the effects of pH value and diluent sulfonated tannin on rheological properties and water loss of formate drilling fluid in a well area of oilfield were studied,the microstructures of formate drilling fluid mud cake formed by before and after adjustment were compared,and the change mechanism of drilling fluid property was analyzed. The results showed that pH value and diluent had a great influence on the rheology and water loss of formate drilling fluid. As the pH value and the addition of diluent increased,the plastic viscosity, dynamic shear force,HTHP filtration and mud cake thickness of formate drilling fluid decreased first and then increased,while the washing efficiency and interfacial bonding strength increased first and then decreased. At 90℃,the suitable pH value of formate drilling fluid on the scene was 11.5,and the best dosage of diluent was 0.3%. The dispersion of clay particles,the performance of drilling fluid,and the compactness of mud cake formation were improved after formate drilling fluid was adjusted by NaOH and diluent. It facilitated the displacement of annulus drilling fluid and the flushing and removal of mud cake on the borehole wall,and improved the cementing quality.
formate drilling fluid