HDCDF (High Density Cationic Drilling Fluid)system is developed to deal with downhole problems caused by abnormal formation pressure, high pressure brines and salt-gypsum-contained formation etc. in the foothill belt drilling. HDCDF system uses the high molecular weight cationic polymer as an encapsulating and flocculating agent, the organic quaternary compound as a shale inhibitor, the cationic polymer and amphoteric polymer as fluid loss reducers, the low molecular weight amphoteric polymer as a deflocculant and barite or hematite as a weighting agent. Results of lab studies and applications in foothill belt ultra-deep well drilling show that HDCDF system, whose density is higher than 2.20g/cm3 in lab and up to 2.44g/cm3 in the YK-1 well drilling, has a lot of advantages, such as excellent inhibition, preferable rheology, low fluid loss, high temperature stability, minimal formation damage and so on. HDCDF system is fit for geology, drilling and other engineering requirements in troublesome ultra-deep well YK−1 drilling.

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