1.2.2 Key Scientific Issues

1) .Molecular structure design of treatment agents that meet complex working conditions and their physical chemical interactions with reservoirs.

The molecular structure of treatment agents is the key to determining their performance. Different molecular structures of treatment agents have different spatial distribution patterns and interaction mechanisms with formation minerals under complex operating conditions such as high temperature, high pressure, and high salinity, which can easily lead to wellbore instability and reservoir damage during deep and deep water drilling processes. Therefore, it is urgent to develop treatment agents that are resistant to ultra-high temperature, high salinity, and environmentally responsive, reveal their physical and chemical interaction mechanisms with reservoirs, and construct high-performance drilling and completion fluid systems and performance control methods.

2) .Natural gas hydrate inhibition mechanism and integrated regulation method.

Conduct in-depth research on the mechanism of action between deep and shallow natural gas hydrate inhibitors and reservoirs, prevent wellbore instability caused by hydrate blockage in the manifold, and construct a dedicated drilling and completion fluid system for drilling natural gas hydrates; By revealing the complex physical and chemical interaction mechanism between drilling and completion fluids and reservoirs, elucidating the impact mechanism of drilling and completion fluid invasion on mechanical stability, establishing an integrated control method for drilling and completion fluid sealing, leak prevention and plugging, reservoir protection, and wellbore stability in deep water oil and gas formations, and ensuring safe and efficient drilling.

2.Research Progress in Drilling and Completion Fluid Technology for Deep Layer and Deep Water Oil and Gas Drilling

2.1 High Temperature Resistant and Environmentally Friendly Drilling and Completion Fluid System

With the strengthening of environmental management regulations and environmental limitation indicators, the environmental protection of the drilling site is considered an important indicator for assessment in drilling design. The Implementation Measures for the Environmental Protection Management Regulations of the People's Republic of China on Offshore Oil Exploration and Development clearly stipulate that "water-based drilling and completion fluids with an oil content exceeding 10% are prohibited from being discharged into the sea; when using oil-based drilling and completion fluids, when the oil content in the drilling cuttings exceeds 15%, they are prohibited from being discharged into the sea." Although the treatment technology for abandoned drilling and completion fluids has achieved certain results, the fundamental method to solve environmental pollution of drilling and completion fluids is to carry out research on environmentally friendly drilling and completion fluid technology and control pollution from the source. Since the 1990s, environmentally friendly drilling and completion fluid systems have been developed both domestically and internationally. At present, high-temperature resistant and environmentally friendly systems mainly include high-performance water-based systems, polyols, alkyl glycosides, and synthetic groups.

2.1.1 High Performance Water-based Materials

In recent years, in order to reduce costs and environmental pollution, a high-performance and low-cost water-based drilling and completion fluid has been developed. High performance water-based drilling and completion fluids are mainly composed of shale inhibitors, coating agents, dispersants, and fluid loss agents.

In foreign countries, high-performance and environmentally friendly water-based drilling and completion fluid systems such as HydraGlyde and UltraDrill from Schlumberger, PerforMax and Latidrill from Baker Hughes, Shall-Drill from Halliburton, and Evolution from Newpark are representative. These systems all have good rheological properties, lubricity, and excellent shale inhibition properties, which can be applied to deep water drilling and complex shale formation drilling, and have been successfully applied in multiple blocks in the United States. In China, Sinopec Zhongyuan Petroleum Engineering Company has developed a GOF environmentally friendly high-performance water-based drilling and completion fluid system, which can effectively solve technical problems such as wellbore instability, sticking, and slow drilling speed during shale rock drilling. It is safe, environmentally friendly, and cost-effective.

In response to the problems of shale formations in the Longmaxi Formation in Sichuan and Yunnan that are prone to collapse when encountering water, as well as the development of micro cracks and fractures, Kang Yuan et al., Yan Lili et al., and Sun Jinsheng et al. formulated a hydrophobic and strong sealing water-based drilling and completion fluid with modified silica sealing agent as the core. The shale recovery rate of the drilling and completion fluid reached 90.2%, and the penetration depth of 40-60 mesh sand beds was only 1.5 cm, with excellent inhibition, sealing, hydrophobic, and anti pollution capabilities. In addition, the team has also developed a strong inhibition and sealing water-based drilling and completion fluid system. After on-site application, the electric logging was successful once, with a regular wellbore diameter and an average wellbore expansion rate of 5.71%. Jiang Guancheng and others have developed biomimetic wall stabilizers, biomimetic lubricants, and dual hydrophobic high-performance water-based drilling and completion fluids based on the theory of "plugging, inhibition, solidification, dual hydrophobic, and lubrication". Field tests have shown that the drilling and completion fluid system can reduce the complexity of the wellbore, shorten the well construction cycle, and meet the needs of safe, efficient, and environmentally friendly drilling.

2.1.2 Polymeric Alcohols

Polyalcohol drilling and completion fluid is an environmentally friendly water-based drilling and completion fluid system formulated with polyalcohol as the main agent, which can enhance the inhibition, sealing, and lubricity of the drilling and completion fluid. In the 1990s, foreign countries had already developed polyethylene glycol copolymer (COP/PPG) drilling and completion fluid systems. Research has shown that the synergistic effect of polyols and potassium chloride can significantly enhance the inhibitory performance of the system, and has good effects on wellbore instability problems such as block falling and collapse in water sensitive formations. In addition, polyol drilling and completion fluids have been applied in deep-sea marine drilling. Liu Xiaodong et al. developed a polymer alcohol system with a temperature resistance of 200℃ and applied it on site in the Bohai Sea oil field. The system has an EC₅₀ (half effective concentration) greater than 3×10⁵ mg/L, meeting the requirements for biological toxicity emissions in first-class sea areas, effectively solving the problem of high-temperature deep well drilling in environmentally sensitive sea areas.

2.1.3 Alkyl Glycosides

Alkyl glycoside drilling and completion fluid is a water-based drilling and completion fluid with good inhibition and lubricity, stable rheological properties, and environmental friendliness. It is suitable for drilling large displacement wells, high inclination wells, and environmentally sensitive areas such as the ocean. Among them, methyl glucoside drilling and completion fluids were first widely used, such as the WZ-6-9 oilfield in the South China Sea, the Bin 26X1 well in Dagang Oil Field, and the Tick drilling platform in the Gulf of Mexico. In order to improve the inhibitory properties of methyl glucoside drilling and completion fluids, modified alkyl glucoside drilling and completion fluids have been developed. Zhao Hu and others modified alkyl glycosides with cations and developed alkyl glycoside drilling and completion fluids with a temperature resistance of up to 150℃, successfully solving the problem of wellbore instability in shale formations such as the Xiaoheba Formation and Longmaxi Formation. In addition, sulfonation modification of alkyl glycosides can also improve the comprehensive performance of alkyl glycoside drilling and completion fluids.

2.1.4 Synthetic Type

Synthetic drilling and completion fluid is a drilling and completion fluid composed of artificially synthesized organic compounds as the continuous phase, saline water as the dispersed phase, and emulsifiers, fluid loss agents, flow pattern improvers, etc. The environmentally friendly performance and ease of achieving constant rheological properties of synthetic drilling and completion fluids have made them widely used in offshore deepwater drilling. FLAT-PRO synthetic drilling and completion fluid has stable rheological properties, strong rock carrying capacity, and good wellbore cleaning effect. It has been successfully applied in the ultra deep water well Liwan 22-1-1 in the eastern South China Sea. The Zhongjiang 204H well has successfully applied a biomass synthesis based drilling and completion fluid, which has a temperature resistance of 150℃, strong rheological, filtration, lubricity, and inhibition properties, as well as low environmental risks and waste treatment costs. However, there are still certain defects in biomass based drilling and completion fluids. Under long-term high temperature and alkaline conditions, biomass based drilling and completion fluids will undergo hydrolysis, leading to poor performance and increased difficulty in maintenance and treatment.

2.2 High-temperature, High-salt, and High-density Drilling and Completion Fluids

2.2.1 Sulfonation/Polysulfonates

Domestic high-temperature resistant water-based drilling and completion fluids have roughly gone through three stages: calcium treatment, sulfonation, and polysulfonation. Calcium treated drilling and completion fluids suppress clay dispersion through calcium ions, improving the filtration and wall building performance and rheological properties under high temperature conditions. After the 1970s, the three sulfonic acid drilling and completion fluid developed by scholars such as Wang Pingjin significantly improved its temperature resistance. The three sulfonic materials mainly include sulfonated lignite (SMC), sulfonated phenolic aldehyde (SMP), and sulfonated tannin (SMT), with a temperature resistance of up to 180℃. And polysulfonate drilling and completion fluid is a polymer treatment agent with excellent temperature and salt resistance introduced on the basis of sulfonated drilling and completion fluid, which improves the rheological and filtration properties of the system, and enhances the temperature resistance of the system to over 200℃. The foreign DURATHERM system is a water-based polysulfonate drilling and completion fluid with good temperature resistance, with a theoretical maximum temperature resistance of 260℃, good salt resistance, strong inhibition, and good reservoir protection performance. When this system is used in the LD22-1-7 well in the Yingge Basin, the maximum well temperature is 207℃, the maximum density is 2.24 g/cm³, and it can improve drilling speed, improve cuttings removal efficiency, and reduce drilling costs.

2.2.2 Oil based Type

In recent years, with the continuous deepening of oil and gas resource exploration and development at home and abroad, the number of deep and ultra deep wells has gradually increased, and the upper limit of wellbore temperature has also been constantly refreshed (for example, the depth of Well Gulong 1 in the Songliao Basin in Daqing exceeds 6300 meters, and the wellbore temperature reaches 260 ℃). Oil based drilling and completion fluids have the advantages of good inhibition, maintaining wellbore stability, protecting reservoirs, good lubrication, and improving drilling speed, gradually becoming drilling deep and ultra deep wells The inevitable choice for high difficulty wells and horizontal wells. In the 1960s, foreign countries attached great importance to the development and application of oil-based drilling and completion fluids. With amine free emulsifier as the core, MI Swaco Company has formed an ultra-high temperature and high pressure oil-based drilling and completion fluid system, UHTHP, in combination with high-performance temperature resistant organic soil, modified tannin mixed high temperature fluid loss agents, etc. The indoor evaluation results show that the system can maintain excellent rheology, lotion stability and filtration at 300℃, and has been successfully applied in Thailand. An oil-based drilling and completion fluid with a density of 2.2 g/cm3 developed by Halliburton Company, which can withstand high temperatures of 204℃, has been successfully applied in multiple domestic and foreign oil fields such as the North Sea oilfield. In China, Wang Jianhua and others synthesized nanometer polymer plugging agent for oil-based drilling and completion fluid by lotion polymerization. The performance evaluation results show that the high-temperature and high-pressure filtration loss is reduced by 34%, and the plugging rate is increased from 90% to 100%. The plugging agent can form a sealing layer on the surface of micro fractures, which is expected to solve the problem of shale wall stability. Qin Yong et al. synthesized a high-temperature resistant main emulsifier HT-MUL and auxiliary emulsifier HT-WET for oil-based drilling and completion fluids using Tall oil fatty acids and maleic anhydride as the main raw materials. Based on this, a set of high-temperature resistant oil-based drilling and completion fluids was constructed and successfully tested in shale gas development wells on the Wei 204H5 platform, which was widely promoted and applied.

2.2.3 Polymer Type

The technology of high-temperature resistant polymer drilling and completion fluids in foreign countries started early and developed rapidly. The research and application of polymers began as early as the 1960s. The cesium formate solid-free system developed by Shell has a temperature resistance of 220℃ and a density of 2.37 g/cm³. It has good rheological and lubricating properties and can effectively avoid the sedimentation problem of barite under high-density conditions, but it is expensive. The high-temperature resistant polymer system Envirotherm NT constructed by MI-Swaco company has a temperature resistance of 232℃, a density of 2.2 g/cm³, strong pollution resistance, temperature and salt resistance, and minimal damage to the formation.

At present, typical high-temperature resistant polymer systems that have been successfully applied domestically and internationally include: non dispersed low solid phase polymer systems, biomimetic polymer systems, double hydrophobic high-performance polymer systems, cesium formate solid-free systems, high-temperature resistant polymer systems, etc. Shengli Oilfield first adopted non dispersed low solid polymer technology in 1973, which increased drilling speed by more than 20%. Wang Yan et al. synthesized a five component copolymer fluid loss agent, and the experimental results showed that the synthesized five component copolymer has a temperature resistance of 180℃, salt resistance to saturation, and calcium resistance of 1.25%. It has good performance in polymer drilling and completion fluids. Xuan et al., Jiang et al., and Ni et al. developed a series of biomimetic drilling and completion fluid materials, and innovatively established the theory and technology of biomimetic polymer drilling and completion fluid. When drilling in the horizontal section of Su 53 block in Sulige, Changqing, the average drilling speed was increased by 27%, and the comprehensive cost of drilling and completion fluid was reduced by 26.4%, resulting in significant results. In addition, Jiang et al. have further developed a leading technology for polymer drilling and completion fluids with high efficiency and ultra dual hydrophobic properties, based on their research and development of ultra dual hydrophobic agents, achieving rapid development in polymer drilling and completion fluid technology.