Abstract: Solid-free organic salt drilling fluids used for drilling reservoirs in the Weizhou 12-1 Oilfield have rheological problems that resulted in a failure to perform borehole cleaning in long open horizontal intervals.Therefore, a solid-free organic salt drilling fluid was formulated by optimizing the additive quantity and particle size distribution.Performance evaluation showed that optimized solid-free organic salt drilling fluids had a high low shearing rate viscosity,and may provide satisfactory for cutting,carrying and suspension capacities.In addition,these drilling fluids perform well in protecting reservoirs,with permeability recovery rate of cores contaminated by these fluids more than 90%.The drilling fluids also have high inhibition capacities with linear expansion rate of mud shale as low as 10%,with rolling cutting recovery rate up to 96.48%.Field application results indicated that no pipe sticking has been observed during tripping operations in horizontal intervals in the Weizhou 12-1 Oilfield when using the new fluid.More specifically,by using optimized solid-free organic salt drilling fluids,skin factors were reduced from over 20.0 to -3.0,oil production was 2 times of that expected.It was concluded that optimized solid-free organic salt drilling fluid could meet the requirement of drilling horizontal intervals in the Weizhou 12-1 Oilfield,and effectively improve well cleaning and reservoir protection in horizontal intervals.