During the drilling process in complex sections of the formation

due to the leakage of the drilling fluid

the borehole instability occurs easily. In view of this situation

we put forward the film formation and plugging project of the drilling fluid. In the indoor experiment

the microstructure of the mud cake was analyzed by scanning electron microscopy（SEM）. The hydration resistance of the plugging film was evaluated by the water permeation tests. The improved sand bed experiment had simulated the plugging performance of the drilling fluid on the formations with different permeability. Then

we analysed the positive pressure capacity and flowback effect of the plugging film by the self-designed unconsolidated sand core flow experiments. The experimental results showed that the film-formation drilling fluid was resistant to the highest temperature of 150 ℃

and the mud cake formed by API leak-off was denser. In the same period

the water permeability of the mud cake remained unchanged and the plugging film had certain hydration resistance. When the dosage was between 2 % ~ 3 %

the sand beds with different mesh were totally blocked

without the leak-off of the drilling fluid

and the positive pressure value was high after the plugging film formed

which could reach 16 MPa. In the reverse driving

the breakthrough pressure was between 0.5 ~ 1 MPa with good flowback effect.