Drilling Fluid Technology for Complex Deep Cambrian Formationsin Tarim Basin

The complex Cambrian formations in Tarim Basin exhibit complex geological characteristics such as large depth, ultra-high temperature, ultra-high pressure, complex lithology, fluids, and stress. Prominent issues during drilling include the collapse of fractured dolomite formations, instability of wellbore due to dissolution of thin gypsum layers, drilling fluid contamination, and poor stability of high-temperature drilling fluids. Based on theoretical analysis, strategy for preventing the collapse of fractured formations through “reasonable density support, multi-stage plugging while drilling, and static reinforcement with closed mud,” strategy for avoiding sticking in “unsaturated and low-filtration loss” gypsum-containing dolomite formations, and strategy for controlling high-temperature performance of drilling fluids through “polymer and 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 reducers, multi-scale wide-size particle plugging and collapse prevention treatment agents, sulphonation treatment agents, and high-temperature resistant and efficient lubricants were used. Through laboratory experiments, SMDP-2 high-temperature resistant and salt-resistant filtration reducer was selected as the optimal agent, and the formulation of plugging and collapse prevention materials was determined. The optimal dosage of SMJH-1 lubricant was identified and high-temperature resistant, strong plugging, and unsaturated saltwater drilling fluid system capable of withstanding temperatures up to 200 °C was constructed. This drilling fluid has been applied in three wells including Well QSH1, demonstrating good inhibitory properties, plugging and collapse prevention properties, and high-temperature stability, achieving rapid drilling through complex formations. In Well QSH1, the average daily leakage rate of the Lower Qulitalik and Awatagulik formations decreased by 45.6% compared to adjacent 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 Tarim Basin at temperatures below 200 °C, providing technical support for safe and rapid drilling in such formations.