Advanced Drilling Fluid Technology for Complex Cambrian Formations in the Tarim Basin

The Cambrian complex formations in the Tarim Basin exhibit complex geological characteristics such as ultra-deep, ultra-high temperature, ultra-high pressure, complex lithology, complex fluids, and complex stress. Prominent issues during drilling include collapse of fractured formations in dolomite layers, instability of wellbore due to dissolution of thin gypsum layers, drilling fluid contamination, and difficulty in controlling high-temperature stability of drilling fluid. Based on theoretical analysis, strategies for preventing collapse in fractured formations through "reasonable density support, multi-stage plugging while drilling, and static reinforcement with closed mud," prevention of sticking in "undersaturated, low-filtration loss" gypsum-containing dolomite formations, and control of high-temperature drilling fluid performance through "polymer, sulphonation material synergy against temperature" were proposed. Key treatment agents such as high-temperature resistant plugging and collapse prevention agents, high-temperature resistant and salt-resistant polymer filtration reducer, multi-scale wide-size particle plugging and collapse prevention treatment agent, sulphonation treatment agent, and high-temperature resistant and efficient lubricant were used. Through laboratory experiments, SMDP-2 high-temperature and salt-resistant filtration reducer was selected as the optimal agent, the formulation of plugging and collapse prevention materials was determined, and the optimal dosage of SMJH-1 lubricant was identified. The optimal ratio of SMC, SMP-3, and SPNH was determined to be 2:4:2, constructing a high-temperature-resistant, strong plugging, undersaturated saltwater drilling fluid system capable of withstanding temperatures up to 200 °C. This drilling fluid has been applied in three wells including QSH1, demonstrating good inhibitory properties, plugging and collapse prevention properties, and high-temperature stability, achieving rapid drilling through complex formations. In QSH1 well, the average daily leakage rate of the Lower Qulitalik and Awatagulik formations decreased by 45.6% compared to neighboring wells, with a borehole enlargement rate of 9.52%, and downhole logging and casing were completed smoothly, reducing drilling time by 30.32%. The research indicates that this drilling fluid technology effectively addresses drilling challenges in complex Cambrian formations in the Tarim Basin at temperatures below 200 °C, providing technical support for safe and rapid drilling in such formations.