Drilling fluid is the core technology of deep and ultra deep drilling and completion engineering, and is one of the key technologies that determine the success or failure of drilling. This article analyzes the challenges faced by drilling fluids for deep and ultra deep wells, such as wellbore instability, system failure under high temperature and high salt conditions, high friction, and environmental protection. It reviews the latest technological advancements in water-based and oil-based drilling fluids both domestically and internationally. It points out that there is still a gap in the research level of domestic drilling fluids in terms of temperature and salt resistance compared to advanced foreign technologies, and it is urgent to study the failure mechanism of treatment agents, conduct research on anti ultra-high temperature drilling fluid treatment agents and supporting technologies for special formations.

With the increasingly prominent contradiction between the global exploitation of oil and gas resources and the growing energy demand of humanity, the field of oil and gas exploration and development is constantly expanding, gradually moving from shallow to deep and ultra deep layers. According to statistics, the deep oil and gas resources in China can reach 671×10⁸ tons, accounting for over 34% of the total oil and gas resources, and have become a major strategic replacement energy for China's oil and gas industry. According to geological predictions, China has abundant deep oil and gas resources in the tens of thousands of meters, with an estimated oil reserve of 10.17×10⁸t and a natural gas reserve of 1.9×10¹²m³ in the Ordovician Sinian system (up to 12000 meters) of the Tarim Oilfield, and an estimated natural gas reserve of 5.6×10¹²m³ in the Dengying Formation (up to 10000 meters) of the western Sichuan Basin. The deep and ultra deep geological conditions are complex, with widespread occurrence of ultra-high temperature (>200℃) and ultra-high pressure (>140 MPa). The complex formation fluid, lithology, and pressure system have brought huge challenges to drilling. As the "blood" of drilling, drilling fluid will face more severe challenges under the influence of many complex factors. This article analyzes the technical challenges faced by deep and ultra deep well drilling fluids, summarizes the progress in research on deep and ultra deep well drilling fluids both domestically and internationally, and points out the next research directions.

1. Technical Challenges Faced by Deep and Ultra Deep Drilling Dluids

(1) The instability mechanism of deep and ultra deep wellbore is complex. The deep and ultra deep open hole section is long, with natural fractures developed, which are prone to blockages and collapses, leading to blockage, and even jamming and wellbore scrapping; The stability of the wellbore is influenced by multiple factors such as drilling fluid cooling and changes in subcritical water activity (the collapse of the wellbore caused by drilling to 9101 m in KTB well in Germany). The mechanism of wellbore instability is unclear, and the technical means for stabilizing the wellbore are limited.

(2) Drilling fluid faces severe challenges such as high temperature, high salinity, and high pressure. Under ultra-high temperature and high salt conditions, polymer treatment agents are prone to degradation, flocculation, conformational transformation, and dehydration of bentonite; Material failure, drilling fluid rheology, filtration, sealing, stability and other properties are easily out of control; Especially when drilling into high-pressure and ultra-high pressure formations (such as high-pressure salt water layers), it is necessary to significantly increase the density of drilling fluid. Combined with ultra-high temperature and high salt factors, the difficulty of controlling drilling fluid performance increases exponentially; Lack of drilling fluid materials that are resistant to ultra-high temperatures and high salts.

(3) High friction during deep and ultra deep drilling. During drilling, the drilling fluid has a long path (over 9000 meters), high flow resistance, and a fluid pressure loss along the path that may exceed 50 MPa, resulting in an increase in the equivalent circulation density (ECD) of the drilling fluid and a large fluctuation in bottom hole pressure, which can easily lead to low drilling speed and downhole complexity; The adsorption performance of existing lubricants deteriorates under high temperature and high salt conditions, and the effect of reducing friction and drag is extremely unsatisfactory.

(4) The probability of deep and ultra deep leakage is high, and the difficulty of plugging is high. The development of deep and ultra deep natural fractures and faults greatly increases the probability of leakage. The natural gas in fractured high-pressure gas reservoirs in deep formations often enters the wellbore directly with the opening of new formations and even triggers blowout. In addition, the relatively narrow safety density window for leakage further increases the difficulty of selecting a safe drilling fluid density window.

(5) It is difficult to achieve both high performance and environmental friendliness of drilling fluid materials. The water-based drilling fluid treatment agents and systems that meet environmental requirements and have good performance are not yet complete, and the problems of wellbore instability and lubrication still need to be solved; Oil-based drilling fluids generate a large amount of oily waste and high treatment costs. Only in front of Kuche Mountain, over 30000 tons of oil and gas are produced annually, while single shale oil and gas wells produce 400-600 tons, and there is no unified emission standard.

2. Current Research Status of Drilling Fluid Technology for Deep and Ultra Deep Wells at Home and Abroad

2.1 Deep and Ultra Deep Well Water-based Drilling Fluid

2.1.1 Deep and ultra deep well water-based drilling fluids at abroad

Foreign high-temperature resistant water-based drilling fluids mainly consist of sulfonated and polymer materials, including lignin sulfonates, high-temperature and salt resistant polymers, low gum and solid-free modified silicates, and other high-temperature resistant water-based drilling fluids. Typical cases of water-based drilling fluid application abroad are shown in Table 1. The deepest vertical depth of the application well is the former Soviet Union SG-3 well, with a depth of 12262 meters and a bottom hole temperature of up to 215℃; The highest temperature of the application well is KTB-HB well in Germany, with a bottom hole temperature of 280℃.

To meet the requirements of improving the average mechanical penetration rate in hard sandstone horizontal wells under high temperature conditions, Halliburton and Shell jointly developed a bifunctional polymer DFSP that can provide low flowability, good high-temperature filtration resistance, and resistance to divalent cations. And a set of high-temperature resistant brine based reservoir drilling fluid has been formed, which has excellent wellbore cleaning effect and can effectively increase the average mechanical drilling speed in hard sandstone horizontal wells by more than 8 times. It has been successfully applied in the oil field of the United Arab Emirates with a bottom hole temperature of 193.3℃.

Baker Hughes synthesized a series of high-temperature resistant polymers COP-1, COP-2, sulfonated polymer Poly-drill, as well as high-temperature stabilizers such as SSMA and Chemral-x, using monomers such as 2-acrylamide-2-methylpropane sulfonic acid (AMPS) and acrylamide (AM), different long-chain alkyl acrylamide, vinyl acetamide (NVA), and vinyl pyrrolidone (NVP). And this type of polymer is used as the core to form a polysulfonate drilling fluid Pyro Drill with related inorganic salts. It has been successfully applied in Well 21-1-3 of Yacheng, South China Sea, with a drilling depth of 4688 meters and a bottom hole temperature of up to 206℃.

Deep and ultra deep layers are often accompanied by high-pressure salt water layers, which pose a risk of pollution from acidic gases such as hydrogen sulfide and carbon dioxide. Therefore, drilling fluids must have good anti pollution capabilities while ensuring high temperature resistance. Schwerk has developed a new type of high-temperature resistant polymer ECF-1868 and formed a high-temperature and high-density water-based drilling fluid system with a temperature resistance of 204℃ and a density of up to 2.30 g/cm³, greatly improving the anti pollution ability and reservoir protection effect of water-based drilling fluid, and maintaining good settling stability even under long-term high-temperature aging conditions. In order to further improve the temperature resistance of water-based drilling fluids, starting from the type of synthetic products, compared to the disadvantage of long chain alkanes in organic materials that are prone to cracking when heated, compounds formed by inorganic elements such as silicon and carbon in inorganic materials have strong temperature resistance advantages. Therefore, the introduction of silicon elements forms an inorganic multi-layer silicon treatment agent SIV. A high-temperature resistant SIV drilling fluid system was formed by combining polymer anti flocculant and ternary copolymer fluid loss additive, with a temperature resistance of 233℃ and good shear dilution performance. The SIV drilling fluid system was applied in the KTB-HB exploration well of the German continental deep drilling project, and the initial results were good. However, after a long period of drilling, the core treatment agent was affected by high temperature, making it difficult to control its rheological properties.

2.1.2 Deep and ultra deep well water-based drilling fluids at China

The domestic high-temperature resistant water-based drilling fluid is mainly composed of polysulfonate drilling fluid system, with salt water drilling fluid having a maximum temperature resistance of 220℃ and fresh water drilling fluid having a maximum temperature resistance of 240℃. The typical cases of domestic water-based drilling fluid application are shown in Table 2, which lists the typical record wells and landmark achievements of water-based drilling fluid application in China in recent years.

Sun Jinsheng first established the theory of water-based drilling fluid film formation, and successfully developed a new chemical film forming material that can automatically form a film on the wellbore wall by introducing strong hydrophobic groups, effectively preventing water phase from entering the formation and causing damage to the formation rocks. Further addressing the issue of unstable performance of drilling fluid in salt gypsum layers, a series of high-temperature and salt resistant treatment agents have been developed, forming a water-based drilling fluid technology with a temperature resistance of 220℃ and a salt resistance of>15%. Successfully conducted on-site tests on more than 20 wells, including Qiye 6-36-1 well and Huabei Xinghua 11 well. Among them, Jilin Changshen 39 well has a depth of 5885 meters and a bottom hole temperature of up to 200℃.

In recent years, it has been discovered that the dehydration of high-temperature clay is a key factor leading to the instability of water-based drilling fluid systems under high temperature conditions. A special polymer high-temperature protective agent GBH with strong adhesive protection function has been specifically developed, accompanied by corresponding series of treatment agents, which greatly improves the temperature resistance of drilling fluid and forms a freshwater based drilling fluid system with a temperature resistance of 240℃. Although there have been breakthroughs in the research and development of high-temperature resistant water-based drilling fluids in China, there are few water-based drilling fluids that can truly withstand ultra-high temperatures on site, and they still rely on sulfonated materials and the introduction of foreign technologies. In the later stage of the Songke 2 well, the bottom hole temperature reached 241℃, and the drilling could only be completed smoothly with the help of high-temperature resistant potassium polysulfonate and high-temperature resistant polysulfonate drilling fluids, as well as the introduction of foreign products Driscal D and Soltex.

The organic salt drilling fluid has strong temperature and salt resistance, and has been successfully applied in the Jiantan well 1. The bottom hole temperature is as high as 235℃, but it also has the problem of rheological deterioration when drilling is completed. Chuanqing Drilling Company has selected a high-temperature resistant and efficient diluent to address the problem of deterioration of drilling fluid rheology caused by carbon dioxide gas invasion in deep wells. It is matched with a composite salt fluid loss agent, a high-efficiency sealing agent, and a lubricant to form an acid resistant gas pollution drilling fluid. By adopting appropriate treatment processes, the pollution of carbon dioxide and acidic gases in deep wells in the Penglai gas region of Southwest China has been effectively alleviated, achieving resistance to carbon/bicarbonate pollution of over 10000 mg/L, maintaining the overall stability of drilling fluid performance, and effectively reducing the risk of safety accidents caused by acidic gas pollution underground.

In response to increasingly severe environmental protection issues, high-performance water-based drilling fluids and environmentally friendly and efficient water-based drilling fluids have become research hotspots in recent years. Sixiqiang synthesized a near oil based drilling fluid NAPG using natural products such as alkyl glycosides (APG), polyethers (PE), and organic amines, and used it as a continuous phase to optimize treatment agents such as viscosity enhancers, fluid loss agents, and sealing agents to form an alkyl glycoside near oil based drilling fluid with characteristics of temperature resistance of 180℃, environmental protection, low cost, and high performance. The system has currently been applied in 21 high difficulty wells on site, with the highest application temperature being 177 ℃ of Shunbei 53-2H well and the deepest completed drilling depth being 9093 m of Shunbei 11X well.