Drilling Fluid Technology for Well Keshen-15 Penetrating Highly Deep HTHP Gypsum Formations

The well Keshen-15 is a key exploration well drilled in the Keshen 15^# structure of the middle Kelasu tectonic zone, Kuche depression, Tarim. Difficulties that must be faced during drilling include highly deep formations, faults, large hole size, high temperature, high mud density, developed salt and gypsum formations, and narrow mud density window etc. High temperature high density complex organic salt drilling fluids were formulated to have "low water activity, low hydrating potential and low equivalent circulation density (ECD)", in an effort to deal with the difficulties as mentioned above. With these measures, no trouble had ever occurred during drilling, and all of the wireline loggings were successful at the first try. Compared with the well drilled nearby with oil base drilling fluid, the mud density was reduced by 0.05-0.08 g/cm³, indicating that this organic salt drilling fluid had strong inhibitive capacity, low ECD, good rheology, high temperature stability, and resistance to salt, gypsum and drill cutting contamination. Meanwhile, this drilling fluid was environmentally friendly, non-fluorescent, had low corrosion to tubular goods, and can be used as drill-in fluid to save drilling time. The successful drilling of the well Keshen-15 has proved the potential of wide application of the high temperature high density complex organic salt drilling fluid.