Abstract: The well Guping 28-21 is a horizontal well having the longest horizontal length (2,006 m) in Changqing Oilfield. It has a ratio of horizontal displacement to vertical depth of 1.7. The reservoir is of low porosity, ultra-low permeability, low electric resistance and high shale content. A CQSP-2 compound salts drill-in fluid was formulated to drill the reservoir section. CQSP-2 helped stabilize borehole wall in horizontal section of the well which is 418 m in length, having 20.84% of mudstone and carbonaceous mudstone penetrated along the hole. A gauge hole was obtained, with maximum hole enlargement of 1.61%, minimum of 0.05%, and an average of 0.63%. Friction coefficient in the horizontal drilling was kept at 0.03-0.05, showing good lubricity for the drilling. Hole cleaning performance was satisfactory, with no downhole troubles arising with hole cleaning. Good reservoir protection was achieved as selfflow was found when the well was initially put into production, giving a daily production of 83.6 t, seven times of the average daily production of other wells in Block Heshui.