For deepwater oil and gas well drilling with the issue of highly mineralized formation brines, it is required that the drilling fluid properties are not affected by the high salinity formation fluid and the low-temperature environment. A novel drilling fluid mainly consists of fuzzy-ball fluid, which has an excellent tolerance to salinity and low temperature, has the potential to be applied for deepwater drilling to achieve safe and efficient drilling tasks.

Experimental studies and field tests have been carried out to characterize the rheological properties of fuzzy-ball drilling fluid under different conditions of brine salinity and test temperature. In the experimental test, fuzzy-ball fluids with different densities are prepared with brines of varying salinity mineralized by Ca2+ and Mg2+. The rheological curves are plotted from different apparent viscosities and shear stresses under different shear rates measured by a rheometer under 10 ∼ 80°C and a six-speed viscometer at 0°C.

The results positively showed that the yield stress of fuzzy-ball fluid is stable at a relatively high value with the variation of fluid salinity and temperature. The apparent viscosity slightly increases with the decrease of salinity and temperature. It can be claimed that the rheological properties of fuzzy-ball drilling fluid are stable within the range of salinity and temperature investigated experimentally.

Field tests have been conducted during several workover jobs, the rheological properties of fuzzy-ball drilling fluid were stable at temperatures and salinities on site. Thus, it can be proved that the novel fuzzy-ball drilling fluid could be applied in deepwater drilling under the high salinity formation fluid and low-temperature environment.

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