Abstract: According to the experiments and applications, it was proved that the Fuzzy-ball drilling fluid can effectively solve the leakoff, block and collapses induced by quickly changed lithology. MBS26-11 and MBS24-5 were two directional production wells in Qinshui Basin. The drilled layers were many and complex. And the lithology was alternated frequently. And in the second interval of MBS26-11, the clay-free fuzzy ball drilling fluid was used. The density covers from 0.95~1.05 g/cm³. The viscosity was from 35 to 38 seconds. The average ROP was 10.79 m/h which was 9.2% more than adjacent well. In the first interval of MBS24-5, by applying bentonite drilling fluid, the leakoff was severe and the loss rate was 6.95m³/h. Then, by using fuzzy-ball drilling fluid, the loss rate was controlled within 1.68m³/h. In the second interval, the clay-free fuzzy ball drilling fluid was also applied. The density of drilling fluid was 0.93 g/cm³ to 0.98 g/cm³ with the viscosity of above 38 seconds, the plastic viscosity of 6 to 14mPa·s, a yield value of 3.00 to 6.64YP/Pa and a ratio of dynamic to plastic viscosity of 0.43~1.66Pa/mPa·s. The average ROP was 5.6 m/h.