1. IntroductionAs global energy demand continues to grow and conventional oil and gas resources become increasingly depleted, oil and gas exploration and development are progressively expanding into deep, ultra-deep, and complex geological formations. Deep and ultra-deep well drilling has become an inevitable trend in the petroleum industry, which brings unprecedented technical challenges. As the “blood” of drilling engineering, drilling fluid directly affects the safety, efficiency, and cost of drilling operations. During deep well drilling, the downhole temperature increases significantly with well depth, often exceeding 150 °C and potentially reaching above 240 °C [1]. Simultaneously, salt-gypsum layers and high-salinity formations are more and more frequently encountered than ever before. High-temperature and high-salinity environments pose serious threats to the rheological properties of water-based drilling fluids. High temperature intensifies clay particle hydration expansion and polymer degradation, while high-concentration electrolytes compress the electrical double layer of clay particle surfaces, weakening the electrostatic repulsion between particles [2]. These effects collectively cause sharp increases in drilling fluid viscosity and gel strength, which in turn, result in troublesome problems under the downhole environment.
To effectively control drilling fluid rheological properties under harsh conditions, viscosity reducers are widely used as key additives. Their core function is to disrupt or inhibit the formation of spatial network structures among clay particles, thereby reducing viscosity and gel strength of drilling fluids and restoring or maintaining good fluidity. Traditional viscosity reducers, such as ferrochrome lignosulfonate (FCLS), exhibit good temperature and salt resistance; however, the chromium ions (especially highly toxic hexavalent chromium) generated during their production and application pose serious environmental and health hazards and have been restricted or banned by environmental regulations in many countries [2,3]. Under the global consensus of green, low-carbon, and sustainable development, the development of efficient, environmentally friendly viscosity reducers capable of adapting to extreme conditions (high temperature, high salinity, and high density) has become an urgent research topic in drilling fluid chemistry.
In recent years, researchers worldwide have conducted extensive work on water-based drilling fluid viscosity reducers and formed diverse technical approaches and product systems. Historically, inorganic and organic phosphates represent one of the earliest classes of viscosity reducers. These include sodium-based organic phosphates such as ethylenediamine tetramethylene phosphonic acid (EDTMPS), which, however, exhibits poor salt tolerance, as well as inorganic phosphates like sodium phosphate, sodium tripolyphosphate, and sodium hexametaphosphate [4,5]. However, the broader family of phosphate-based viscosity reducers generally suffers from low temperature resistance, typically not exceeding 80 °C for most conventional types. Due to this inherent limitation, they are not suitable for the high-temperature conditions encountered in deep and ultra-deep wells, which are the primary focus of this review. Therefore, these early phosphate-based additives are not covered in the review.
According to the material sources and synthesis strategies, the viscosity reducers that are the focus of this review can be divided into three main categories: modified natural materials, industrial waste utilization, and synthetic polymers, as shown in
Figure 1. Among these, modified natural materials will be introduced first, followed by industrial waste-derived products, with synthetic polymers receiving the most detailed attention as the current mainstream research direction. These studies have not only promoted the development of high-performance viscosity reducer products but also deepened understanding of the microscopic viscosity reduction mechanism. This paper aims to comprehensively and systematically review the research progress on the development and application of water-based drilling fluid viscosity reducers. By elaborating on the molecular design principles, synthesis methods, performance characteristics, and mechanisms of various viscosity reducers, this review summarizes the current technological achievements and existing deficiencies and discusses future development trends and key research directions, so as to provide valuable references for subsequent innovative research and engineering applications in this field.
Figure 1.Main categories of water-based drilling fluid viscosity reducers: modified natural materials, industrial waste utilization, and synthetic polymers.
