Abstract: With high risks in drilling operations,the Jin-58 Well Block in the Hangjinqi Block frequently experiences severe lost circulation and borehole instability in drilling.To solve these problems,innovative drilling fluids have been developed for the Block.Through an in-depth analysis of root causes and key technical challenges related to lost circulation and sidewall collapsing,technologies with outstanding performances in pressure-bearing,lost circulation prevention,plugging and sidewall stabilizing were developed by selecting optimal treating agents in drilling fluids and by optimizing formula and properties of drilling fluids to ensure the safety of drilling operations.The newly-developed technology was successfully implemented in three horizontal wells(Well JPH 351,Well JPH393 and Well JPH404) of Jin-58 Well Block.Lost circulation and sidewall collapses were greatly decreased or avoided.For an example,the horizontal Well JPH351 penetrated the Jiujiagou Formation which is highly susceptible to lost circulation.The time required for plugging operations was over 40% shorter than those required in operations with conventional drilling fluids.In addition,to reduce the frequency of lost circulation,the time required to process these conditions is reduced dramatically.Sidewalls in the Shiqianfeng Formation and the Shihezi Formation displayed outstanding stability with average diameter expansion of 4.73% only.In the horizontal interval,11 intervals of mudstone with total length of 513.00 m,or 35.92% of total length,were penetrated.The 3 200.00-3 292.00 m mudstone interval maintained excellent sidewall stability 52 days after penetration.Research results showed the newly developed drilling fluids and matching technologies could effectively solve problems related to lost circulation and sidewall instability.These technologies have bright prospects in field applications at larger scales.