Study and Field Test of Drilling Fluid with Constant Rheology at High Temperature in West Yueman Block, Tarim Oilfield

Safe density window of drilling fluids in the deep formations of the West Yueman Block in Tarim Oilfield is narrow, and as a result, downhole complications such as lost circulation, well collapse, drill string sticking, and salt water contamination are frequently encountered. Some drawbacks exist in available drilling fluids including thickening at high temperature and a weak resistance to pollution by CO₃^2–/ HCO₃^– and poor quality soil. In this paper, a drilling fluid with constant rheology at high temperature was studied, with a high-temperature-resistant polymer fluid loss additive (APS220) and a new high-temperature stabilizer (HTS220) as the base. The basic formula was determined on the basis of the performance analysis of the main agents. Laboratory tests were performed to evaluate the constant rheology at high temperature and the resistance to CO₃^2– / HCO₃^– and sodium bentonite pollution of the drilling fluid. The results showed that, the drilling fluid had a plastic viscosity ratio of 1.3, dynamic shear ratio of 1.5, initial shear ratio of 1.7 and final shear ratio of 1.2 at 100 °C and 180 °C. As temperatures rose, it presented a variation range of rheological parameters significantly lower than that of other commonly used drilling fluids, with the resistance to pollution by 2% CO₃^2– / HCO₃^– and 10% sodium bentonite. In the field test of two wells in the West Yueman Block, the drilling fluid demonstrated stable rheological properties, with smooth drilled borehole and considerable downhole complication reduction.