Abstract

Systematically discuss the current research progress of deep and ultra deep drilling fluid technology at home and abroad, analyze the key problems that exist, and propose future development directions.In response to the problems of high temperature, high pressure, high stress, fracture development, wellbore instability, and drilling fluid leakage faced in the drilling process of deep and ultra deep complex oil and gas reservoirs, scholars at home and abroad have developed deep and ultra deep anti high temperature and high salinity water-based drilling fluid technology, anti high temperature oil-based/synthetic based drilling fluid technology, reservoir protection drilling fluid technology, and drilling fluid leakage prevention and plugging technology. However, there are still key issues such as insufficient ability to resist high temperature, high pressure, and high stress, easy wellbore instability, and severe drilling fluid leakage.Therefore, the development direction of future deep and ultra deep drilling fluid technology is proposed as follows: ①.High temperature and high salinity water-based drilling fluid technology needs to focus on improving high-temperature stability, rheological properties, enhancing filtration control, and improving compatibility with the formation.②.The high-temperature resistant oil-based/synthetic based drilling fluid technology needs further research and development in low toxicity and easily degradable environmentally friendly additives such as high-temperature stabilizers and flow regulators, as well as related supporting technologies.③.The reservoir protection drilling fluid technology should focus on the research of new high-performance additives and materials, and improve real-time monitoring technology by introducing advanced sensor networks and artificial intelligence algorithms.④.The drilling fluid leakage prevention and plugging technology should pay more attention to the integration and application of intelligent technology, the research and application of high-performance plugging materials, the exploration of diversified plugging technologies and methods, and the improvement of environmental protection and safety production awareness.

0. Introduction

In recent years, China's demand for oil and natural gas consumption has been increasing year by year, and the external dependence on oil and gas supply remains high.In 2022, China's dependence on foreign crude oil reached 71.2%, far above the international oil security warning line, and its dependence on foreign natural gas was 40.2%, seriously affecting China's energy security and urgently requiring the improvement of its oil and gas self-sufficiency capacity.China has enormous potential for deep oil and gas resources, but the degree of exploration is low, making it a realistic field for future oil and gas exploration and development.At present, deep (greater than 4500 m) and ultra deep (greater than 6000 m) drilling and completion technologies still face severe challenges, such as ultra-high temperature and pressure at the bottom of the well, high stress, and developed fractures, which can easily cause serious problems such as reservoir damage, wellbore instability, and malignant wellbore leakage.In the process of deep and ultra deep drilling, drilling fluid technology faces many challenges, including: ① The wellbore of deep and ultra deep drilling is prone to instability, long open hole sections, multiple cracks, easy block falling, collapse, and blockage.The stability of the wellbore is affected by factors such as drilling fluid cooling and subcritical water, and the instability mechanism is complex and the stability technology is limited.② During deep drilling, drilling fluid needs to cope with high temperature, high pressure, and high stress environments. Under conditions of ultra-high temperature, high pressure, and high stress, the degradation of treatment agent materials and structural changes can occur, making it difficult to control the performance of drilling fluids.③ Deep and ultra deep layers are prone to leakage and difficult to seal due to the development of natural fractures and faults. Natural gas in fractured high-pressure gas reservoirs is prone to enter the wellbore with drilling, which may cause blowouts. In addition, the narrow safety density window for leakage makes it difficult to choose a safe drilling fluid density window.This article summarizes the research progress of high-temperature and high salinity water-based drilling fluid technology, oil-based/synthetic based drilling fluid technology, reservoir protection drilling fluid technology, and drilling fluid leak prevention and sealing technology in response to the above-mentioned challenges faced by deep and ultra deep drilling fluid technology. It clarifies the key problems currently existing in deep and ultra deep drilling fluid technology at home and abroad, and proposes the future development direction of deep and ultra deep drilling fluid technology.

1. Research Progress on Deep and Ultra Deep Drilling Fluid Technology

Experts and scholars at home and abroad have developed high-temperature and high-pressure drilling fluid technology, high-temperature oil-based/synthetic based drilling fluid technology, reservoir protection drilling fluid technology, and drilling fluid leakage prevention and sealing technology to address the problems faced in deep and ultra deep complex oil and gas drilling processes, such as high temperature, high pressure, high stress, fracture development, wellbore instability, and drilling fluid leakage. These technologies can effectively solve the problems of wellbore instability and drilling fluid leakage.

1.1 High Temperature and High Salinity Water-based Drilling Fluid Technology

With the expansion of oil and gas exploration into deep and ultra deep layers, traditional drilling fluids are no longer able to meet extreme environmental requirements such as high temperature and high salinity. The high-temperature and high salinity water-based drilling fluid technology aims to improve the high-temperature and salt resistance of drilling fluids, and ensure their stability in deep drilling.

1.1.1 High temperature resistant sulfonated/polysulfide water-based drilling fluid

The development of high-temperature resistant water-based drilling fluids in China has roughly gone through three stages: calcium treatment, sulfonation, and polysulfide.Calcium treated drilling fluid suppresses clay dispersion through calcium ions, improving filtration and rheological properties under high temperature conditions; Sulfonated drilling fluid is a drilling fluid formulated with sulfonated lignite, sulfonated phenolic resin, and other treatment agents. It can withstand temperatures up to 180℃, but has weak salt and calcium resistance. It is mainly suitable for deep freshwater drilling; Polysulfonic drilling fluid is based on sulfonated drilling fluid, which introduces temperature and salt resistant polymer treatment agents to improve the rheological and filtration properties of the system, and increase its temperature resistance to 200℃ or above.In the research of polysulfide drilling fluid, the main focus is on its performance optimization and adaptability improvement. By adjusting the proportion of polymers with different relative molecular weights and the content of bentonite, its rheological properties can be improved; By increasing the dosage of high molecular weight polymer coating agents or adding appropriate amounts of inorganic salts, their inhibitory properties can be enhanced.In addition, the types of polysulfide drilling fluid treatment agents can be adjusted according to the characteristics of different well sections and formations in deep layers to achieve the best drilling results.At present, China's high-temperature resistant water-based drilling fluid is mainly based on the polysulfide drilling fluid system, with saltwater having a maximum temperature resistance of 220℃ and freshwater having a maximum temperature resistance of 240℃.The Songke 2 well in the Songliao Basin has a depth of 7018 meters and a bottom hole temperature of 241℃. With the help of high-temperature resistant potassium polysulfide and high-temperature resistant polysulfide drilling fluid systems, the drilling was successfully completed, setting a new record for drilling under the highest temperature conditions in China at that time (241℃); The Tarim Luntan 1 well has a depth of 8882 meters and a bottom hole temperature of 175℃. It was successfully drilled using a potassium polysulfide drilling fluid system, setting a record for the first deep well on land in Asia at that time.Table 1 shows typical application cases of high-temperature resistant polysulfide water-based drilling fluids in deep and ultra deep layers in China at present.

Foreign high-temperature sulfonated water-based drilling fluids are mainly lignin sulfonate salt water-based drilling fluids. In the high-temperature deep well drilling applications in Louisiana and Lake Pontchartrain in the United States, the highest temperature resistance can reach 260℃.At the same time, Baker Hughes Company synthesized a series of high-temperature sulfonated high molecular weight polymers and high-temperature stabilizers using monomers such as 2-acrylamido-2-methylpropanesulfonic acid, acrylamide, and alkyl acrylamide with different long chains. They also prepared a polysulfide drilling fluid system by adding inorganic salts and other materials, which has been well applied in the Yancheng 21-1-3 well in the South China Sea.

1.1.2 High temperature polymer water-based drilling fluid

The research on high-temperature resistant polymer drilling fluid technology started early and developed rapidly abroad. The development and application of polymer drilling fluids began as early as the 1960s.At present, significant progress has been made in the research of high-temperature polymer based water-based drilling fluids abroad, continuously developing new treatment agents to improve the performance stability of drilling fluids in harsh environments such as high temperature and high salt, providing important technical support for deep and ultra deep well drilling.M-I SWACO company has developed a polymer drilling fluid system with a temperature resistance of 232℃ and a density of 2.20 g/cm³. It is temperature resistant, salt resistant, and pollution resistant, with minimal damage to the formation.Halliburton and Shell have jointly developed a high-temperature polymer water-based drilling fluid technology, which has excellent wellbore cleaning effect and can increase the average mechanical drilling speed of hard sandstone horizontal wells by more than 8 times. It has been successfully applied in the Hail oilfield in the United Arab Emirates with a bottom hole temperature of 193.3℃.M-I SWACO has developed a polymer water-based drilling fluid system with a temperature resistance of 204℃ and a maximum density of 2.30g/cm³, greatly improving the anti pollution ability and reservoir protection effect of water-based drilling fluid. It still maintains good settling stability under long-term high-temperature aging conditions.Wandji et al. designed a new type of high-density polymer nanocomposite material, which uses ethylene functionalized nano silica (VMS) with thermal thickening effect and amphiphilic properties as a nano filler to form a thermally stable polymer nanocomposite bentonite hybrid system. The system has a viscosity retention rate of over 60% and low viscosity attenuation amplitude at 220℃ and NaCl mass fraction of 20%, and has good application effect in high salt and high temperature drilling.

At present, new progress is being made in the research of high-temperature polymer based water-based drilling fluids in China. Efforts are being made to improve the stability of drilling fluids in harsh environments such as high temperatures by developing new additives, improving fillers, and applying modification technologies.The typical high-temperature resistant polymer drilling fluid systems successfully applied in China include non dispersible low solid polymer systems, biomimetic polymer systems, dual hydrophobic high-performance polymer systems, and high-temperature resistant high-density polymer saturated saltwater systems.Xuanyang et al. developed a series of biomimetic drilling fluid materials and established the theory and technology of biomimetic polymer drilling fluid, which was used for drilling the horizontal section of Su 53 block in Changqing Sulige gas field. The average drilling speed increased by 27%, and the comprehensive cost of drilling fluid decreased by 26.4%, with significant effects.In addition, Huang Xianbin et al. designed and developed three polymer based water-based drilling fluid high-temperature treatment agents, and constructed an ultra-high temperature high-density polymer saturated saltwater drilling fluid system with excellent sealing and lubrication properties.