Treatment of CO₃^2- and HCO₃^- Contamination in Water Base Drilling Fluid Used in Drilling the 3rd Interval of the Well HT2

During drilling of the φ311.1 mm section between 4513 m and 5785 m of the well HT2, invasion of anions from the Donghe sandstones (main pay zone of the Hetianhe gas field) and the non-productive Ordovician limestone and dolomite caused CO₃^2- and HCO₃^- contamination to the drilling fluid used for 140 d. Calcium concentration in the drilling fluid were always zero throughout the section. The drilling fluid was required to have high viscosity (funnel viscosity = 120-150 sec) high gel strength (10 sec/10 min gel strengths =(4-5) Pa/(15-25) Pa) to clean the hole because of three times of pipe sticking in broken formation sections, and this high viscosity/high gel strength caused the drilling fluid to have difficulties degassing and pumping. The rheology of the drilling fluid were then becoming difficult to control, finally the drilling fluid turned to a jelly state at high temperatures which no technical articles can be used for reference. In the early stage, quicklime, calcium chloride, organic salts and nanophase emulsion were introduced into the drilling fluid for property maintenance, and in the later stage, the properties of the drilling fluid were maintained with high concentration sulfonated polymer solution and alkaline liquor, without the addition of thinners in the drilling fluid. The properties of the drilling fluid were thus maintained stable through the whole section, ensuring safe drilling and realizing the geological goals expected.