Abstract: In Changbei Block,formate drilling fluid was used to avoid shale instability in long horizontal open hole sections.Sodium formate was selected as weighting agent and inhibitor.Through experimental study on base mud viscocity and shear force,and damage on cores under simulated formation conditions,the inhibition and weighting performance of sodium formate were evaluated.On this basis,a sodium formate drilling fluid was developed for horizontal wells in Changbei.Its field application in two wells shows that this fluid has the characteristics of strong inhibition,small filtration,good rheology and less solid content,and it is effective for anti-collapse.Multiple horizontal shale sections were penetrated efficiently with the sodium formate drilling fluid,indicating that it can stabilize borehole effectively and prevent downhole complexities.Therefore,it can ensure safe drilling in long open horizontal sections in Changbei Block.