Viscosity Reducers for Water-Based Drilling Fluids: A Review of Modified Natural Materials, Industrial Waste Utilization, and Synthetic Polymers

Viscosity reducers are essential additives for water-based drilling fluids (WBDFs), serving to counteract the rheological degradation induced by the high-temperature and high-salinity conditions commonly encountered in deep and ultra-deep well drilling. This paper systematically reviews the research progress in this field, categorizing viscosity reducers into three major systems: modified natural materials, industrial waste utilization, and synthetic polymers. Modified natural material viscosity reducers, derived from renewable materials such as lignin and humic acid via chemical modification, are environmentally friendly products. The preparation of viscosity reducers from industrial wastes realizes both resource recycling and economic benefits. Synthetic polymer viscosity reducers, incorporated with functional monomers such as sulfonic and carboxylic groups, achieve high performance with temperature resistance exceeding 220 °C as well as excellent salt and calcium tolerance via rational molecular design, and represent the current mainstream research direction in the field. This paper provides an in-depth analysis of the action mechanisms of various viscosity reducers, summarizes the performance characteristics, synthesis methods and application status, and identifies challenges in structure–property relationship elucidation, extreme working condition adaptability, and technology transfer efficiency. Finally, future development trends are discussed, with emphasis on precision molecular design, ultimate performance requirements for ultra-deep wells, environmentally sustainable approaches, and the establishment of standardized evaluation protocols. This review aims to provide both theoretical insights and practical guidance to support the efficient development of deep oil and gas resources.