Abstract:There are many challenges in the process of geological drilling， among which the problem of leakage is particularly prominent， especially in fractured formations， which occurs more frequently. Therefore， it is of great practical significance to develop a drilling fluid system suitable for loose broken formation to maintain rapid drilling in broken formation. The inert material and formula suitable for plugging in the broken formation were determined by optimization experiments. Meanwhile， Super Absorbent Polymer （SAP） was prepared by aqueous solution polymerization， and the water absorption ratio was more than 100 times under the condition of 0~100℃. The non-permeable drilling fluid loss instrument was used to optimize the particle size and concentration of SAP， and 0.3% of 4~8 mesh SAP was the best choice. Finally，the formula of the drilling fluid system for plugging the broken formation was optimized： water+8%bentonite+0.1%xanthan gum+0.6%HV-CMC+2%walnut shell+1%mica sheet+2%specialty fiber+0.3%SAP of 4~8 mesh. The drilling fluid system can completely plug the sand filling bed of 10~20 mesh， the loss is 0 mL at 0.69MPa pressure， and has good rheological stability in the range of 0~60℃. The apparent viscosity is between 25~48mPa·s， the dynamic shear force is 6~18 Pa， the API filtration loss is 7~11mL， and the lubrication coefficient is 0.311. It shows a good application prospect in plugging loose broken strata.