Study and Application of "Zero Potential" Water Base Drilling Fluid

The "zero potential" theory was used to optimize the formulation of drilling fluid used in deep well and ultra-deep well drilling in Tarim Basin to overcome shale swelling and pipe sticking in top formations and difficulties in mud property maintenance in lower deep formations. Using optimized cationic polymers, a "zero potential" water base drilling fluid was formulated. This drilling fluid had high concentration of cations, and clay particles of high zeta potential, almost equal to or slightly higher than the zeta potential of the "in-situ" drilled cuttings, meaning that the potential difference between the formation and the drilling fluid is "zero", hence greatly mitigating the possibility of borehole wall instability caused by frequent migration of electric charges between formation and drilling fluid. This drilling fluid also had good rheology and filtration property, and was resistant to contaminations from the introduction of salt, calcium and clays, especially at high temperatures in deep and ultra-deep wells. The "zero potential" drilling fluid had been used in 20 wells in Tarim Basin. Compared with conventional water base drilling fluids, total time spent in dealing with drilling fluid-related downhole troubles was reduced by 10.2%, ROP increased by 14.9%, and drilling time reduced by 9.8%. The successful application of the "zero potential" drilling fluid in Tarim Basin provides a means of fast, safe and efficient drilling deep wells, and enriches and further develops the idea of "zero potential" drilling fluid.