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Facing the 21st century, in order to improve the recovery rate of old oil fields and efficiently develop low-pressure, low-permeability and heavy oil reservoirs, it is particularly necessary to research and develop advanced and applicable special process drilling technologies.； The drilling and development of high-pressure and high-temperature oil and gas reservoirs, especially high-pressure and high-temperature natural gas reservoirs, urgently needs to break through the technical obstacles of high-pressure and high-temperature drilling. ； The drilling and development of deep oil and gas resources requires further research and development of deep well and ultra-deep well drilling technology. In particular, scientific research and development efforts should be increased on the drilling efficiency of deep exploration wells. 1. High-pressure and high-temperature drilling technology According to the international general concept, when the ground temperature exceeds 150°C, it is called high temperature. When the equivalent density of the formation pressure exceeds 1.8g/m3 or a wellhead device exceeding 70MPa is required, it is called high pressure. Wells that have both are called high-pressure and high-temperature (HPHT) wells. If the bottom hole temperature exceeds 220°C and the bottom hole pressure exceeds 105 MPa, it is called an ultra-high pressure and high temperature well. The temperature of the natural gas layer is higher than that of the oil layer. The geothermal gradient is generally 3~5℃/100m. The ground temperature of a 5000m well may reach 150~250℃. Therefore, most natural gas wells, especially deep natural gas wells, are high pressure and high temperature wells. High-pressure and high-temperature wells, especially high-pressure and high-temperature deep exploration wells, are the most difficult, risky and demanding wells in drilling engineering. For example, a 5000m exploratory well in the central graben of the North Sea costs nearly US$20 million to drill, and the development well costs no less than US$12 million. ； Of the three exploratory wells in the Yacheng 21-1 structure in the South China Sea, the first two wells did not reach the designed target layer, resulting in large economic losses. High-pressure and high-temperature drilling technology is a key technology for the exploration and development of high-pressure and high-temperature oil and gas reservoirs. It is also one of the important symbols representing the development level of drilling technology in the 21st century. 2. Deep well and ultra-deep well drilling technology Deep wells refer to wells with a drilling depth of 4500 to 6000 m ； Ultra-deep wells refer to wells with a drilling depth of more than 6,000m. Deep well and ultra-deep well drilling technology is an indispensable key technology for the exploration and development of deep oil and gas and other resources. Entering the 21st century, deep drilling work in western and eastern my country will be further intensified, and the number of deep and ultra-deep wells that need to be completed will further increase. The areas where deep wells and ultra-deep wells are concentrated in my country include the Tarim Basin, Junggar Basin, Sichuan Basin and Qaidam Basin. Practice has proven that due to the complex geological conditions of deep and ultra-deep wells (such as piedmont structures, high-steep structures, difficult-to-drill formations, multi-pressure systems, unstable rock formations, etc., and some strata also have high-pressure and high-temperature effects), my country's deep and ultra-deep well drilling technology in these areas (or other similar areas) has not yet passed the standard. This is manifested by complex underground wells and frequent accidents, long well construction periods, and high engineering costs, which greatly hinders the pace of exploration and development and increases the direct cost of exploration and development. Compared with the United States, there is still a large gap between my country's equipment and technical level in drilling deep wells and ultra-deep wells (mainly deep exploration wells). The average well construction period and drill bit usage are about twice that of the United States. 3. Special process drilling technology Special process drilling mainly includes directional wells, horizontal wells, cluster wells, extended reach wells, complex structure wells and underbalanced drilling. The research and application of these special process drilling technologies have become relatively mature around the world, and are still being studied and tested in depth to refresh technical indicators. At present, my country has basically mastered the drilling technology of directional wells, horizontal wells and cluster wells. However, the research on complex structure wells, extended reach wells and underbalanced drilling is still relatively weak. Extended reach wells refer to directional wells where the ratio of horizontal displacement to vertical depth is equal to or greater than 2. The main purpose of drilling extended-reach wells is to achieve efficient exploration and development of oil and gas resources through extended-reach extension. Therefore, the key technical indicator for extended-reach well drilling is the length of horizontal displacement. In oil and gas exploration and development projects in oceans, beaches and special areas, obvious economic and social benefits can be obtained by applying extended reach drilling technology. Complex structure wells mainly refer to multi-branch wells. Several branch wellbores can be drilled from the inside of a main wellbore, and each branch well can be re-entered and put into production. If each branch well is a horizontal well, it is called a multi-branch horizontal well. Multilateral wells can be used for both drilling new wells and sidetracking old wells, but are only suitable for oil and gas development purposes. Since multi-branch horizontal wells overcome the shortcomings of "one well and one layer" of conventional horizontal wells, they can achieve "one well with multiple layers" and share a main wellbore and surface oil production facilities. The drilling cost is low, so the single well production is high and the recovery effect is good. Multi-branch wells are compatible with the characteristics of thin and abundant oil layers in my country's continental sedimentary deposits, and are of great significance for stable production of mature oil fields and efficient development of low-pressure, low-permeability and heavy oil reservoirs. The so-called underbalanced drilling is a drilling method that artificially makes the effective flow pressure of the fluid in the well lower than the formation pore pressure. During the drilling process, formation fluids are allowed to enter the well, circulate out of the well, and be controlled at the surface. Underbalanced drilling is conducive to discovering low-pressure reservoirs, avoiding damage to the reservoir, increasing the mechanical penetration rate, reducing drilling costs, and reducing reservoir production stimulation operations. Survey results in the United States show that oil companies are most interested in how underbalanced drilling can effectively reduce or avoid reservoir damage, while technical service companies are more concerned about the high efficiency of underbalanced drilling. Underbalanced drilling is a high-risk drilling operation that can easily lead to well wall instability and blowout accidents. It must have corresponding casing procedures and add a complete set of ground control devices, so the cost is generally relatively high. * * The weight of drilling should be higher. During the "Ninth Five-Year Plan" period, my country has drilled a number of underbalanced wells, but the main equipment used was imported, and the understanding of the mechanism of underbalanced drilling was not deep enough. 4. Three-dimensional controllable and visualized drilling technology The trajectory of the actual drilled wellbore usually changes in the complex three-dimensional formation space and cannot be seen or touched. Especially in special process drilling such as cluster directional wells, horizontal wells, extended reach wells and complex structure wells, how to effectively measure and control the trajectory changes and stability of the actual drilled wellbore, and even achieve the ideal goal of "drilling while watching, drilling at will" is one of the important research topics in the development of oil and gas drilling in the direction of automation and intelligence. After continuous research, in the 1990s, domestic and foreign technologies such as downhole power steering drilling systems and variable diameter stabilizers were mastered. At the same time, foreign countries have further successfully developed rotary steerable drilling systems, such as Baker Hughes's Auto Track RCLS system, Schlumberger's Power Drive SRD system, and Halliburton's Geo-Pilot system, etc. These rotary steerable drilling systems are currently mainly developed to meet the high-tech needs of special process drilling such as extended reach wells. In order to better expose the oil reservoir along the wellbore, simple "geometric targets" must be replaced by complex "geological targets" (such as the interfaces between different rock layers or fluids in the formation), and the precise locations of these "geological targets" are often difficult to predict. Therefore, foreign geosteering technologies while drilling, such as logging while drilling technology and seismic while drilling technology, have been invented to help identify the location of the "geological target", thereby keeping the actual drilling well trajectory within the appropriate track, and reaching the goals of geological exploration and reservoir development faster and better. Using computer visualization technology, we can better understand a large amount of downhole measurement data (including well trajectory parameters, formation characteristic parameters and near-bit mechanical parameters, etc.) and cluster well design data, etc., to achieve "visualization" of the geometric form, geological conditions and mechanical behavior of three-dimensional drilling, and provide information visualization assistance for the optimal control of three-dimensional drilling.