Reservoir Protection and Filtrate Invasion: Insights from Wireline Formation Testing of Different Mud Systems

Abstract New oil-based mud systems have been widely adopted in the East China Sea. While these materials have demonstrated effectiveness in reservoir protection under laboratory conditions, the variation in invasion depth across different mud systems, influenced by reservoir properties such as porosity, lithology, and fluid type, cannot be accurately replicated in lab settings. The capillary forces and relative permeability between drilling fluids and formation fluids hinder the laboratory-based assessment of mud filtrate cleaning processes. Consequently, quantifying the impact of different mud systems on dynamic production performance remains challenging under laboratory conditions. Wireline formation testing (WFT) stands out as the most crucial and cost-effective technique for characterizing reservoir fluid properties and evaluating mobility in offshore exploration wells. By analyzing data from over 200 sampling stations across more than 50 exploration wells drilled with various fluid systems in the East China Sea over the past decade, this study elucidates the differences in mobility among fluid types—such as gas, oil, water, and water-hydrocarbon coexistence layers—under similar porosity conditions. Furthermore, it examines the effectiveness of oil-based and water-based mud systems in reservoir protection under wellbore conditions. Furthermore, this study compared the differences in the time and volume of formation oil and gas breakthroughs under various drilling fluid systems. Numerical simulations were conducted to replicate the mud filtrate cleaning process and formation fluid breakthroughs at over 150 sampling stations, enabling the determination of drilling fluid invasion depth for each station. Analyzing the differences between oil-based mud (OBM) and water-based mud (WBM) systems is crucial to understand filtrate invasion characteristics and make informed sampling decisions. Our study reveals that OBM has significant advantages over WBM in invasion depth and reservoir damage reduction, in addition to lithology factors, capillary force, and relative permeability, formation fluid type also significantly impact the performance of different oil-based mud systems. This research marks the first industry-wide use of extensive WFT data to analyze the impact of different mud systems on reservoir fluids. Our analysis clarifies the advantages and limitations of various drilling fluid systems under different reservoir properties and fluid types, providing essential guidelines for selecting appropriate drilling fluid types in future block exploration wells.