Abstract

In response to the problems encountered in complex oil and gas drilling processes such as high temperature, high pressure, high salinity, reservoir damage, poor rheological performance, hydrate blockage, and wellbore instability, domestic and foreign scholars have developed high-temperature and environmentally friendly, high-temperature and high salt high-density, and liquid casing drilling and completion fluid technologies. However, as the geological conditions of drilling become increasingly complex, there are still problems such as insufficient high-temperature resistance of drilling and completion fluid materials, poor environmental performance, and serious well leakage and reservoir damage. In order to meet the performance requirements of drilling and completion fluids in complex oil and gas drilling processes, it is necessary to further study the mechanism of drilling and completion fluid treatment agents in the future, establish a safe and efficient multi-functional integrated control method for drilling and completion fluids, construct intelligent drilling and completion fluid theory and technology, and provide technical support for the development of complex oil and gas resources. This article systematically reviews the technical challenges faced by drilling and completion fluids in complex oil and gas drilling, such as deep layers, deep water, and unconventional formations. Combining key scientific issues and research progress, it provides suggestions for the future development direction of complex oil and gas drilling and completion fluid technology in deep layers, deep water, and unconventional formations.

Overview

According to the latest forecast from the US Energy Information Administration, oil and natural gas will still be the top energy consumers in the world before 2050, and for a considerable period of time in the future, oil and gas will continue to be the irreplaceable main energy sources for economic and social development. A large amount of research has been conducted on the exploration of deep oil and gas resources worldwide. 60% of the world's newly added oil and gas reserves come from deep layers, while 83% of China's deep oil and gas reserves need to be explored and developed. Moving towards the "underground Everest" to obtain oil and gas resources is a major strategic task to ensure national energy security. According to predictions from the United States Geological Survey and the International Energy Agency (IEA), the production scale of deepwater oil and gas resources exceeds 10¹¹ barrels, making it an important source of future offshore oil and gas production. The petroleum geological resources of the main sedimentary basins in the South China Sea account for one-third of China's total oil and gas resources, of which 70% are located in the deep water area of 153.7 ×10⁴ km². In the new era, with the rapid development of China's national economy, the demand for oil and natural gas consumption is increasing year by year, and the dependence of oil and gas supply on foreign countries remains high. In 2022, China's external dependence on crude oil reached 71.2%, far above the international oil security warning line, and the external dependence on natural gas reached 40.2%, seriously affecting China's energy security. It is urgent to enhance China's self-sufficiency in oil and gas.

China's exploration and development of oil and gas resources are gradually advancing towards complex oil and gas resources such as deep layers, deep water, and unconventional ones, and complex oil and gas have become an important substitute energy source for China and even the world. However, the safety and efficiency of ultra-deep wells and ultra-deep water conditions still need to be further improved. At the same time, there are also problems such as ultra-high temperature and high pressure at the bottom of the well, large temperature difference, complex and variable formation lithology, strong heterogeneity, narrow safety density window, and serious wellbore instability. At present, China's drilling depth has reached a new level of 9000 meters and advanced towards 10000 meters, and 13000 meters of continental scientific exploration wells have also been put on the agenda. It is reported that PetroChina Southwest Oil and Gas Field has deployed a 10000 meter scientific exploration well in Guangyuan City, Sichuan Province, and the drilling began in July 2023. The successful completion of drilling well Pengshen 6 in the Penglai gas field of PetroChina Southwest Oil and Gas Field has set a new record for the deepest vertical well in Asia, with a maximum depth of 9026 meters, making it known as the "underground Everest".

Luo Pingya, an academician of the CAE Member, believes that the record of the deepest straight well in Asia has been broken, indicating that the overall level of deep well drilling in China has been further improved, marking that China's drilling equipment and drilling technology are in the leading position in Asia and have generally reached the world's advanced level. The deep water constant rheological synthetic drilling and completion fluid technology developed by China Oilfield Services Company has reached the forefront of international standards in terms of constant rheological temperature range indicators, breaking through the limit temperature of 3℃ and 180℃ respectively, providing a solid technical guarantee for further development of complex oil and gas resources such as deeper water with higher difficulty. At the same time, Academician Sun Jinsheng's team undertook a major project of the National Natural Science Foundation of China in 2019- the mechanism and regulation methods of wellbore working fluid and natural gas hydrate reservoirs, focusing on the problem of well completion with complex structures of natural gas hydrates in the South China Sea, and conducting research on the physical and chemical interaction mechanism between wellbore working fluid and reservoirs; Developing new materials to meet drilling needs, constructing an efficient wellbore working fluid system, and establishing a multifunctional integrated control method for wellbore working fluid have effectively solved technical difficulties such as formation plugging, well leakage, kick, and reservoir damage during the drilling process of natural gas hydrates in the South China Sea. This has laid a solid theoretical and technical foundation for the safe and efficient drilling of natural gas hydrates in the South China Sea. In 2022, Sun Jinsheng's team, relying on China University of Petroleum (East China) and in collaboration with Southern University of Science and Technology and China Petroleum Engineering Technology Research Institute Co., Ltd., jointly applied for the basic science center project "Ultra Deep and Extremely Ultra Deep Oil and Gas Drilling and Production Flow Control", which was approved and funded by the National Natural Science Foundation Committee. This is the first basic science center project in China's oil and gas field. Focusing on the "bottleneck" problems and major technological bottlenecks of anti ultra-high temperature and high salt drilling and production working fluid materials, malignant leakage prevention, flow control software, and intelligent control technology, we will carry out basic research on flow control in ultra-deep and ultra-deep oil and gas drilling and production, innovate the theory and methods of flow control in ultra-deep and extremely ultra-deep oil and gas drilling and production, and lead the international development of ultra-deep and ultra-deep oil and gas science and technology.

1. The Important Role and Key Scientific Issues of Drilling and Completion Fluids in Complex Oil and Gas Drilling

With the increasing global energy demand, the development of complex oil and gas resources such as deep layer, deep water, and unconventional formations has been rapidly developing, becoming a key breakthrough direction for China's oil and gas storage and production increase. During the drilling process, drilling and completion fluids face many challenges, such as insufficient temperature resistance of treatment agents, difficulty in regulating the rheological properties of drilling and completion fluids, poor wellbore cleanliness, easy to cause drilling risks such as wellbore instability and downhole leakage, and difficult to control reservoir damage. These problems have brought many difficulties to drilling work, and at the same time, higher requirements have been put forward for drilling and completion fluid technology.

1.1 The Role of Drilling and Completion Fluids in Complex Oil and Gas Drilling

Drilling and completion fluid is a circulating fluid that meets the needs of drilling engineering, known as the "blood of drilling". It has functions such as carrying rock cuttings, balancing formation pressure, stabilizing wellbore walls, protecting reservoirs, transmitting water power, lubricating and cooling drill bits, etc. At present, deep and ultra deep layers have become the main areas for major oil and gas discoveries in China, and the natural gas exploration and development in the Sichuan Basin is constantly moving towards the deeper part of the Earth. For wells with bottom hole temperatures above 200℃, the high-temperature stability of water-based drilling and completion fluids remains a major issue, especially under high-density and high salt conditions, where the rheological and filtration properties of water-based drilling and completion fluids are difficult to regulate. Oil based drilling and completion fluids have more advantages under ultra-high temperature conditions, with a temperature resistance of up to 240℃ and a density of over 2.6 g/cm³.

There is relatively little research on the protection of deep oil and gas reservoirs. Currently, the main methods are to use solid-free completion fluids and micro manganese weighted completion fluids to reduce reservoir damage caused by solid phases, or to use oil-based/synthetic based drilling and completion fluids to reduce water sensitivity effects. However, there is still a drawback of high cost.  Deepwater drilling is facing technical challenges such as wellbore instability and natural gas hydrate generation, and China has made significant progress in the technology of deepwater oil and gas drilling and completion fluids. In terms of deep-water water-based drilling and completion fluids, the constant rheological temperature range of high-performance water-based drilling and completion fluids can reach 150℃. Modified additives based on natural polymer materials have also been developed, ensuring that the drilling and completion fluid system has good performance while also possessing good environmental performance. In terms of deepwater oil-based/synthetic based drilling and completion fluids, the independently developed deepwater synthetic based drilling and completion fluid technology has reached an international leading level.

1.2 Technical Difficulties and Key Scientific Issues Faced in Complex Oil and Gas Drilling

1.2.1 Technical Problems

1).Deep oil and gas drilling encounters high-temperature and high-pressure formations, lacking drilling and completion fluid materials and systems that are resistant to ultra-high temperature and high salinity.

Deep oil and gas drilling mainly faces the following challenges: ultra-high temperature, high pressure, and high stress environments at the bottom of the well; The lithology and geological structure of drilling formations are complex and unpredictable; The fluid and physicochemical properties of the formation/reservoir are variable; Reservoir pores are denser and more heterogeneous; Poor environmental performance; Severe leakage in fractured formations, incomplete materials and systems for leak prevention and plugging; Lack of ultra-high temperature resistant materials makes it difficult to form a stable drilling and completion fluid system.

This leads to a high degree of blindness in the design of drilling and completion fluids, resulting in frequent and severe drilling risks such as lost circulation, wellbore collapse, sticking, rock carrying, and blowout. Reservoir damage is severe, and even catastrophic accidents such as well damage and human death can occur. In response to these challenges, domestic and foreign scholars have developed high-temperature resistant, high salt, and high-density drilling and completion fluids, environmentally friendly drilling and completion fluids, temporary plugging type protection oil and gas reservoir drilling and completion fluids, and leak prevention and plugging technologies, and have achieved good application results.

2) .Deep water oil and gas drilling has weak pressure bearing capacity and prominent wellbore stability issues

Deep water formations are relatively complex, and in addition to facing the same problems as deep oil and gas drilling, there are also the following challenges: The low temperature of the mud line and the high temperature at the bottom of the well, known as the "ice fire dual day", result in alternating high and low temperatures, making it difficult to regulate the rheological properties of the drilling and completion fluid; Drilling into hydrate layers and shallow gas may cause gas hydrate blockage in the manifold; Deep water areas are prone to collapse, expansion, and dispersion due to low overlying pressure, which can easily cause wellbore instability; Drilling risks include poor wellbore cleanliness, settling of barite, narrow safety density window, and susceptibility to leakage. In response to the above challenges, domestic and foreign scholars have studied the factors affecting the performance of drilling and completion fluid treatment agents under high and low temperature conditions, marine natural gas hydrate related treatment agents and drilling and completion fluid systems, deep-water constant rheological drilling and completion fluid and reservoir protection drilling and completion fluid systems, all of which have achieved certain results.

3) .Severe reservoir damage during deep water drilling requires the formation of reservoir protection technology while drilling.

During deep layer and deep water drilling, varying degrees of damage to oil and gas reservoirs can occur, leading to a decrease in production and even "shooting" of oil and gas reservoirs. Researchers have established shielding temporary plugging, fine temporary plugging, physical and chemical film temporary plugging, and biomimetic temporary plugging to protect oil and gas reservoir drilling and completion fluid technologies, gradually improving the effectiveness of protecting oil and gas reservoirs. However, with the increasing exploration and development efforts for unconventional, complex, ultra deep layer, and ultra deep water oil and gas reservoirs, existing protection technologies are difficult to meet the requirements. At the same time, international cutting-edge anti-collapse and plugging technologies are difficult to suppress surface hydration, seal nano micron pores and fractures, and cannot improve the strength and density of reservoir wellbore rocks by cementing rock particles, leading to increased reservoir damage; The complex oil and gas reservoirs mainly composed of horizontal wells and most reservoir rocks resembling "grindstones" have significant frictional damage. Currently, lubrication and drag reduction technologies at home and abroad are difficult to form a smooth surface on the surface of drilling tools and wellbore walls, resulting in high frictional resistance and long drilling operation time, exacerbating reservoir damage.