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Types of deep-water drilling fluids at the current stage: The areas where deep-water drilling is most active around the world currently include the Gulf of Mexico, West Africa, and Brazil. Commonly used drilling fluid systems are as follows: (1) Silicate drilling fluids. Silicate drilling fluids have the advantages of being non-toxic, non-fluorescent, and low in cost. Additionally, they stabilize the wellbore through various synergistic effects, which is why they are receiving increasing attention. However, silicate drilling fluids also present many problems: silicate gels and precipitates can clog the pores of oil and gas reservoirs, and this blockage is difficult to remove ； Performance adjustment is difficult, and it is quite sensitive to the pH value of the drilling fluid ； Silicates have poor compatibility with other drilling fluid additives ； High drilling fluid friction, etc. Silicate drilling fluids cause blockages in the pathways through which oil and gas flow; abroad, silicate disruptants have been developed, which can effectively eliminate the damage caused by silicates to oil and gas reservoirs, thus opening up broader prospects for their use. This drilling fluid system is widely used in regions such as the North Sea, Alaska, Newfoundland, and the Gulf of Mexico. (2) Organic salt drilling fluids: Organic salt drilling fluids are also a new type of solid-free water-based drilling fluid system that has been developed in recent years. The organic salts used in drilling fluids are mainly formates (sodium formate, potassium formate, and cesium formate). Organic salt drilling fluids feature good collapse prevention capabilities, protection of oil and gas reservoirs, low corrosivity, environmental friendliness, and recyclability. Formate drilling fluids are more environmentally acceptable than conventional saline systems; they exhibit many superior properties over their halide counterparts without any side effects. Tests on aquatic organisms have shown that both sodium formate and potassium formate can be classified as non-toxic, while cesium formate can be considered practically non-toxic. All formates, once introduced into water and diluted, degrade rapidly and can only exist in water for a short period of time. Biodegradation tests show that the biodegradation rate of all formates exceeds 70% within 28 days, allowing them to be classified as substances that are \"easily biodegradable\". Compared to inorganic salts, formates are more expensive, but formate drilling fluids can be recycled. Currently, formate-based drilling fluid systems are being recognized and valued by the oil industry worldwide as drilling and completion fluids; they have been widely used in Europe and the United States, achieving excellent results ； In China, formate drilling fluids have been subjected to field tests in various offshore oil fields, including the Marine Drilling Company of the Shengli Oil Field and the Drilling Company of the Bohai Oil Field, with remarkable performance. (3) Polyol drilling fluids: Polyol drilling fluids are an environmentally friendly water-based drilling fluid system developed in the late 1980s, using polyols as the main components. They possess the excellent properties of oil-based drilling fluids, yet they do not cause environmental pollution or interfere with geological logging. Both laboratory tests and field applications have shown good technical and economic results. Polyols are a type of non-ionic low-molecular-weight polyether that is soluble in water at room temperature; however, when the temperature rises to a certain level, phase separation occurs, resulting in an emulsion. The droplets in this emulsion can adhere to the well walls or coat the surface of drill cuttings, forming a hydrophobic layer similar to oil, thereby enhancing the drilling fluid’s capabilities in terms of inhibition, collapse prevention, and lubrication. Enright D P et al. conducted a toxicity assessment of the polyol PPG in accordance with the rules set by the U.S. Environmental Protection Agency. The results showed that polyol drilling fluids have very low toxicity. The biodegradability of polyethylene glycol was evaluated using the OECD 306 test procedure, and it was found that the polyol achieved a biodegradation rate of over 70% within 5 days, and was almost completely degraded within 28 days. Due to their excellent comprehensive properties such as collapse prevention, lubrication, and environmental friendliness, polyol-based drilling fluids are widely used in offshore drilling abroad, with good results. In recent years, polyol drilling fluids have also been widely used in the shallow seas of China’s Bohai Sea oil fields, South China Sea oil fields, and Shengli Oil Field. (4) Methylglucoside drilling fluid: The methylglucoside drilling fluid is a new type of water-based drilling fluid system introduced in the 1990s. Since it resembles oil-based drilling fluids in terms of its collapse prevention mechanism and conventional drilling fluid properties, it is also known as an oil-like drilling fluid system. Numerous laboratory studies and production practices have shown that methylglucoside drilling fluids can effectively suppress the hydration and swelling of shales, maintain wellbore stability, and protect oil and gas reservoirs. They also possess good lubricating properties, resistance to contamination, and high-temperature stability. Moreover, they are non-toxic, readily biodegradable, and have minimal impact on the environment. Methyl glucoside can be synthesized directly from glucose, or it can be produced by decomposing starch into glucose at high temperatures and then using a catalyst. It is generally made from starch; it requires simple equipment, is easy to operate, has a wide range of available raw materials, and has low costs. Methylglucoside can adsorb on the surface of formation shales, forming a semipermeable membrane. Thus, by adjusting the water activity of the methylglucoside-based drilling fluid, it is possible to control the movement of the drilling fluid and formation water; even allowing the water in the shales to enter the drilling fluid, which effectively prevents the hydrative swelling of the formation shales and helps maintain wellbore stability. To ensure that the drilling fluid has optimal shale inhibition properties and forms an effective semipermeable membrane, the amount of methyl glucoside should be at least 35%, with an ideal range of 45% to 60%. In offshore drilling where environmental regulations are stringent, methylglucoside drilling fluid systems have been successfully used in highly deviated and horizontal wells abroad. Field trials have been conducted in domestic oilfields such as the Liaohe, Shengli, and Xinjiang fields. (5) Synthetic-based drilling fluids: Synthetic-based drilling fluids are water-in-oil emulsion drilling fluids composed of artificially synthesized or modified organic substances, namely synthetic base fluids, as the continuous phase and brine as the dispersed phase, along with emulsifiers, organic clays, lime, etc. According to performance requirements, loss control agents, rheology modifiers, and barite are added as well. After ester-based drilling fluids were first applied in the North Sea in March 1990 with success, the variety of synthetic-based drilling fluids continued to increase. Initially, these were ester, ether, and polyalphaolefin-based drilling fluids; later, a second generation of synthetic-based drilling fluids was developed after taking into account environmental protection factors and costs. Second-generation synthetic-based drilling fluids are represented by linear α-olefins, internal olefins, and linear paraffin-based drilling fluids. They are characterized by low dynamic viscosity and low drilling fluid costs, but they have poor environmental protection properties. Synthetic-based drilling fluids are non-toxic, biodegradable, and cause no environmental pollution; drilling wastewater, cuttings, and used drilling fluids can be discharged directly into the ocean. In addition, synthetic-based drilling fluids also have advantages such as good lubricating properties, which are beneficial for protecting oil reservoirs and stabilizing the wellbore; they contain no fluorescent substances, so they do not affect logging and well testing data. Although the preparation cost of synthetic-based drilling fluids is much higher than that of water-based and oil-based drilling fluids, it reduces the costs associated with treating waste fluid from oil-based drilling fluids, as well as the downtime and complexity involved when using water-based drilling fluids. Therefore, in most wells, the total drilling cost of using synthetic-based drilling fluids is lower than that of oil-based or water-based drilling fluids. There is still controversy regarding the environmental pollution issues associated with synthetic-based drilling fluids, with the main focus being on their biodegradation under seabed conditions. Therefore, the environmental impact of synthetic-based drilling fluids still requires further study.