While under the condition of high temperature and high pressure,the safe drilling in thin safe density window is the technical difficulty that can not be solved well around the world.

Because carbonate rock in Carboniferous system which was main reservoirs develop many cracks that caused not only mud loss but also well kick,and had narrow safety density window of drilling fluid,first lost circulation then well blowout is easily to happened,so drilling fluid rheological property became as the important factor of affecting pressure balance in well.

Based on the comprehensive analysis of logging data,geological information and laboratory tests results,this paper studied the pore pressure,in-situ stresses and rock strength of Liuhua 11-1 oil field,analyzed the borehole stability during the drilling of shale and limestone payzones for extended-reach wells,and calculated the safe mud density window for the extended-reach wells.