It is one of the important tasks for the national Four-Deep exploration plan to conduct scientific exploration of ultra deep oil and gas at myriametric deep, as well as an important measure to promote the strategic replacement of China's energy and ensure national energy security.The available drilling engineering technology can hardly achieve the geological goal of drilling up to 10,000 meters deep, and there are many bottleneck problems, including lifting of ultra large tonnage drilling tools, expansion of extreme-deep wellbore structure, efficient rock breaking and ROP optimization in ultra deep & ultra-deep layers, multi-factor coupling control of wellbore stability, and downhole tools resistant to high temperature & high pressure. There is an urgent need to make breakthroughs in the key core technologies such as 12,000 meter automated drilling rigs, P110 steel grade expansion pipes, vibration reduction and energy enhancement tools, ultra-high temperature and high-pressure wellbore stability and drilling fluid friction reduction technology, screws resistant to high-temperature, and mechanical inclinometers while drilling. Therefore, CNPC has conducted in-depth research and started two exploration wells over 10,000 meters depth in 2023, in order to further accelerate the research and development process of related technologies and achieve phased achievement. A 10,000-meter ultra deep oil and gas drilling technology system is gradually forming featured by successful drilling, fast drilling and high-quality drilling, helping China achieve the commanding height of oil and gas engineering technology and high-level self-reliance and self-reliance in petroleum technology as soon as possible.

The drilling of deep and ultra-deep wells is conductive to the development of the deep oil and gas exploration.Its main characteristics are as follows: high bottom-hole temperature and great drilling depth,among which the high temperature environment is the main factor limiting large-scale drilling.Considering the influence of high temperature on drilling fluid,this paper analyzes the difficulties faced by ultra-deep drilling fluid from six aspects: rheological control,property change of water molecular,wellbore stability,lost circulation prevention and plugging, drilling friction & pressure reduction and environmental protection property.Moreover,the application and research progress of drilling fluid and treatment agent in deep and ultra-deep wells are reviewed.Finally,the problems existing in the field of drilling fluid research in China are analyzed,and the development trend of drilling fluid technology in deep and ultra-deep wells discussed from the basic theory update,research object adjustment,drilling fluid treatment agent development and other directions.

Marine carbonate rocks in Sichuan Basin are rich in oil and gas resources,but the structural stratigraphic conditions are complex and the wellbore environment is harsh.In recent years,the key technologies for deep and ultra-deep well drilling in Sichuan basin has supported the increase of oil and gas reserves and production,and laid a technical foundation for the exploration and development of ultra-deep oil and gas resources.The research on integrated design technology of geology and engineering is worth carrying out for development of ultra-deep oil and gas resour:including establishment of three-dimensional geomechanical model through the fine depiction of complex geological bodies for guiding the design of wellbore structure and wellbore track,research of wellbore structure expansion based on expansion tube and reaming while drilling,and guiding the design of drill bits and ROP optimization tools through rock drillability profiles.In view of the problems of delayed abnormal monitoring and identification of wellhead overflow and severe vibration of drill strings in deep and ultra-deep wells,it is required to continue to develop complexity prevention and control technologies based on big data analysis.And further research and development is till being performed such as expansion pipes,high-density water-based drilling fluids resistant to 220 ℃,oil-based drilling fluid systems resistant to 260 ℃,cement slurry systems with temperature resistance of more than 240 ℃,high-performance cementing materials and high-temperature plugging materials.At the same time,intelligent technologies such as the Internet of Things,new communications,and big data should be closely combined to realize the operation,analysis and management of the whole process of drilling.

Ultra-deep stratum is key field for increasing reserves and production of Sinopec,which is technically difficult and challenging.In recent years,Sinopec has been continuously tackling difficulties related to the exploration and development demands of ultra-deep stratum in key blocks,the high-temperature and high-pressure environment as well as complex geological conditions,etc.Breakthroughs have been made in key technologies for ultra-deep drilling and completion,forming a set of key technologies with fundamental theories of drilling and completion engineering,automated drilling and completion equipment,high-temperature-resistant drilling and completion fluids,high-temperature-resistant instruments and tools,as well as completion and testing technologies,etc.as the core.The drilling of 120 oil and gas wells with the depth of more than 8,000 m has been successfully implemented,with the drilling capability of ultra-deep well exceeding 9,000 m.This has boosted the construction of large-scale ultra-deep oil and gas fields in Shunbei,Tarim Basin,and the breakthroughs in exploration and development of ultra-deep oil and gas in the marine strata of the Sichuan Basin.Besides,this has provided strong engineering technical support for the development of oil and gas resources and its corresponding economic efficiency.As oil and gas exploration and development continue to advance, engineering geologic risks and new challenges faced during drilling and completion process continue to emerge.Further research which are related to accurate prediction of formation pressure,wellbore trajectory control,high-temperature resistant working fluid system,high efficiency rock-breaking and drilling speed-up tools,geo-engineering integration,etc.need to be carried out to realize the continuous upgrading of key drilling technologies and equipment to support the efficient exploration and development of deep and ultra-deep oil and gas resources.

In recent years,more than 30% of newly discovered offshore oil and gas structures have been found in deep layers,making deep oil and gas an important resources for future energy supply growth.China National Offshore Oil Corporation (CNOOC) has achieved technological breakthroughs in the Bohai region,which serves as the main field for deep oil and gas reservoir.A series of key technologies have been developed,including accurate prediction and monitoring of formation pressure,quantitative evaluation of fractured leakage,multi-field coupled wellbore stability analysis,ROP optimization in deep hard rock drilling and protection of fractured reservoirs,enabling efficient exploration and development of oil and gas resources in the Bohai region at depths ranging from 5,000 meters to 5,500 meters,thus meeting the requirements of deep oil and gas drilling.Based on these key technologies,a series of deep oil and gas fields such as BZ19-6 condensate gas field,BZ13-2 deep oil field,and BZ26-6 100-million-tonne buried hill oil field have been successfully drilled and discovered in the Bohai Sea.The drilling time decreased from the initial 140 days to less than 40 days with a record-setting operation time of only 31 days for a 5,100-meter well,building a strong foundation for further exploration and development of deep oil and gas reserves in the Bohai Sea.

Currently,the development of ultra-deep/extra-deep layers oil & gas resources in Tarim Oilfield is crucial for meeting Chinese economic development needs and ensuring energy security.After decades of exploration and practice,Tarim Oilfield has innovatively developed a series of ultra deep drilling technologies to ensure significant breakthroughs in ultra deep oil & gas exploration and creating multiple records,including well structure design methodology of ultra-deep complex wells,ROP optimization in ultra-deep and hard-to-drill formations,drilling technology of ultra-deep highly deviated and horizontal wells,drilling & completion fluids of ultra-deep complex wells,ultra-deep complex formation cementing technology,HTHP well integrity,and well control technology of ultra-deep complex wells et al.With the continuous increase in the number of ultra-deep wells,the more complex engineering geological conditions and the more harsh downhole environment,the ultra-deep drilling process is bound to face new engineering problems and challenges,which seriously restricts the safe and efficient development of deep oil & gas resources.Therefore,the key problems and development directions of drilling technology from ultra-deep to extra-deep in the Tarim Oilfield are pointed out by summarizing the development process of drilling technology in Tarim Oilfield and combining the new exploration and development situation and oil and gas development requirements,which may provide reference and guidance for safety and efficient drilling of ultra deep-complex oil & gas wells.

As the exploration and development move towards ultra-deep layers, the well control risks faced by engineering operations are becoming increasingly prominent.With the increase of the risk of kick and loss and the difficulty of well killing in the drilling process,how to improve the timeliness of detection and the success rate of well killing is an on-site problem that needs to be solved urgently.This article investigates the research progress of kick and loss detection equipment and well killing technology in ultra-deep well drilling process, elaborates on the research and development status of kick and loss detection and well control equipment, including precise monitoring technology for large displacement outlet flow rate, automatic fill-up during tripping and kick and loss detection technology, continuous monitoring technology for annular liquid level, and digital monitoring technology for circulating tank volume. It also discusses the research progress of well killing theory and technology in ultra-deep wells. It is discussed about the development trend of AI drilling kick and loss detection system, downhole early kick and loss monitoring technology, automatic and intelligent well kill technology, etc. The downhole-surface kick and loss detection equipment for the whole drilling process has been formed, and the early warning software and the complex well condition kill simulation system based on multi-source data fusion developed, which further improves the timeliness and accuracy of detection, improves the success rate of complex well killing, and provides technical support for the safety well control of ultra-deep well.

The deep formation will be one of the main development directions of China's oil and gas exploration and development with the further development of oil and gas exploration and development,which has important strategic significance for ensuring national energy security.Investigating the development status of deep formation exploration at home and abroad,this paper analyzes the development of key drilling technologies in deep formation,such as casing program design,efficient rock breaking,ROP optimization,HTHP measure and control tools,HT drilling and completion fluid,wellbore pressure control,complex downhole conditions and problems prevention,HT cementing and drilling equipment.It summarizes the key technologies to be improved in the future deep formation drilling,such as drilling of the upper large-size wellbore,safe drilling with multi-pressure systems,ultra-high pressure wellbore working fluid and ultra-high temperature and ultra-high pressure measurement while drilling.On the basis of the coordinated development of geological engineering,the research ideas for large size wellbore ROP optimization,safe drilling with multi-pressure system,ultra-high temperature resistant wellbore working fluid,ultra-high temperature and ultra-high pressure measurement-while-drilling and downhole tools,completion and oil testing technology and equipment development are proposed,which is expected to provide reference for the development of drilling and completion technology for deep and ultra-deep wells in China.