Abstract

Medium to high maturity shale oil is a key area for China's shale oil strategy breakthrough, and strengthening the exploration and development of terrestrial shale oil is an important way to ensure national energy supply security.This article provides a detailed introduction to the technological progress of shale oil drilling projects by China National Petroleum Corporation (CNPC) in the Ordos, Junggar, and Songliao basins. Including key technologies such as large-scale platform factory well layout, optimization of horizontal well wellbore structure, segmented "one trip drilling", high-performance drilling fluid for horizontal wells, and integrated geological guidance. Compared with advanced shale oil drilling technology in North America, this paper analyzes the gaps and development trends in domestic shale oil drilling in terms of well layout and wellbore structure, geological engineering integration and guidance technology, drilling acceleration supporting technology, drilling fluid and cementing supporting technology, etc.Suggestions have been put forward to continuously promote the industrialization of large-scale platform operations, optimize wellbore structures, integrate supporting acceleration technologies, strengthen geological engineering integration research, and accelerate the research and development of new generation guiding tools, in order to promote the acceleration, cost reduction, and efficiency improvement of onshore shale oil drilling, and lead the achievement of shale oil revolution engineering goals.

The shale revolution in the United States has led to rapid growth in domestic oil and gas production, making it the world's largest oil producing country, significantly reducing its dependence on foreign crude oil, and achieving energy independence.Through the development of "horizontal wells+hydraulic fracturing", shale oil production has achieved rapid growth and has become the largest source of crude oil production in the United States.China's demand for oil and gas is growing rapidly, and its external dependence is constantly increasing. In 2022, the external dependence on crude oil exceeded 73%.The onshore shale oil resources in China reach 283×10⁸ tons, and medium to high maturity shale oil is a key area for China's shale oil strategy breakthrough. Strengthening the exploration and development of onshore shale oil is an important way to ensure national energy supply security.Through continuous exploration and development practices and theoretical and technical breakthroughs, many major breakthroughs and positive developments have been made in the Permian system of the the Junggar Basin, the Triassic system of the Ordos Basin, the Paleogene system of the Bohai Bay Basin, the Jurassic system of the Sichuan Basin, the Cretaceous system of the Songliao Basin, and the Paleogene system of the Subei Basin. China has become the fourth country in the world to achieve a breakthrough in shale oil. In 2023, China's shale oil production will exceed 400×10⁴ tons, reaching a new high and demonstrating a promising industrial development prospect.

The terrestrial shale oil geological resources in China's petroleum mining rights areas are abundant, with a medium to high maturity shale oil geological resource of 201×10⁸ tons, accounting for 71% of the total shale oil geological resources in China. The main distribution is in basins such as Ordos, Songliao, Bohai Bay, and Junggar. In 2023, the annual production of medium to high maturity shale oil in China's petroleum industry is 391.6×10⁴ tons, showing good exploration prospects.

Unlike shale oil in the United States, shale oil in China is mainly deposited in terrestrial deposits, characterized by rapid changes in lithology and lithofacies, unstable distribution, strong heterogeneity, low formation pressure, and high clay mineral content.According to geological conditions and sedimentary characteristics, the sweet spots of continental shale reservoirs in China can be roughly divided into three types: interbedded type, mixed type, and shale type.Among them, the typical representative of interlayer shale oil sweet spot is the 7th member of Yanchang Formation (Chang7th member) in Ordos Basin, the typical representative of mixed shale oil sweet spot is the Lucaogou Formation in Jimusar Sag of the Junggar Basin, and the typical representative of shale oil sweet spot is the Gulongqingshankou Formation in Songliao Basin.

Since 2018, China Petroleum Changqing Oilfield has carried out the construction of a national demonstration project for shale oil in the Longdong region. The National Energy Administration has successively approved the establishment of the Jimsar National Continental Shale Oil Demonstration Zone in Xinjiang and the Gulong National Continental Shale Oil Demonstration Zone in Daqing.This article summarizes the progress of China Petroleum's drilling technology for three sweet spot types of terrestrial shale oil in the Ordos, Junggar, and Songliao basins, and proposes challenges and development suggestions. It has important guiding significance for promoting the shale revolution, accelerating the construction of terrestrial shale oil demonstration areas, and leading the technological progress of unconventional oil and gas engineering in China.

1. Progress of China Petroleum's Onshore Shale Oil Drilling Technology

Since the 13th Five Year Plan, with the advancement of horizontal well technology and the large-scale development of shale oil, the number of horizontal wells has been increasing year by year. As shown in Figure 1, the number of horizontal shale oil wells in China's petroleum industry has grown rapidly since 2019. By 2023, China Petroleum will have completed 349 horizontal shale oil wells, supporting the rapid increase in shale oil production and advances in drilling technology.

1.1 Key Technological Progress in Longdong Shale Oil Drilling

China Petroleum Changqing Oilfield has achieved a major breakthrough in shale oil exploration and development in the Longdong area by using sandstone interlayers in the shale formation of the Chang-7 Member in the Ordos Basin as a sweet spot, and has explored the Qingcheng shale oil field with a scale of 10×10⁸t.In 2021, the shale oil production of Qingcheng Oilfield reached 131.6×10⁴ tons, taking the lead in building China's first million ton integrated shale oil development demonstration zone. In 2023, the shale oil production reached 207×10⁴ tons, maintaining a growth rate of 30×10⁴ tons for five consecutive years.

The shale oil reservoir in the Chang-7 section of the Ordos Basin is composed of dark mudstone interbedded with fine sandstone, which has the characteristics of small-scale development in the plane and multiple sets of development in the vertical direction. It is located in a loess plateau with crisscrossing gullies, and the drilling well site area is limited, resulting in high environmental pressure.Through continuous research and field practice, the three-dimensional well layout technology for large well clusters and the three-dimensional horizontal well drilling technology for small well fields and large well clusters have been developed.

By tackling the drilling technology of horizontal wells in large well clusters, the number of completed wells on the platform continues to increase.Based on the vertical stacking of multiple small layers and the characteristics of surface gullies, the principle of "one-time deployment of well network and reasonable optimization of well spacing" has been adopted to optimize the well layout and construction technology, forming unconventional reservoir well network optimization technology as well as large well clusters, multi-layer systems, and three-dimensional well layout modes.Based on the idea of minimizing frictional torque, a trajectory design method of "spatial arc+segmented design" was proposed, forming a large offset three-dimensional horizontal wellbore profile optimization technology, and optimizing a 7-segment profile. By finely controlling the three-dimensional wellbore profile and trajectory with large offset, optimizing the horizontal section length of 1500-2000m, reasonable well spacing of 400-500m, and increasing the maximum offset from 1030m to 1266m, the number of platform wells continues to increase, achieving maximum utilization of single platform reserves.The HuaH100 platform has drilled 31 wells, setting a record for the most horizontal wells on a single platform in China. The average horizontal section length is 2008 meters, and the average drilling cycle is 17.12 days, as shown in Figure 2.

Tackle the drilling and completion technology of ultra long horizontal wells, and continuously extend the length of the horizontal section.Using sodium formate and potassium formate as key additives in drilling fluid, the solid content of the drilling fluid is low and the inhibition is strong. The rock debris recovery rate reaches 90.4%, forming a strong inhibition and anti collapse drilling fluid system.Solved the problem of wellbore collapse in long open hole sections, reduced drilling friction torque, significantly improved horizontal extension capacity, and supported the drilling of ultra long horizontal wells.In response to the problem of fault leakage zones in the horizontal section of some horizontal wells, the technology of plugging while drilling and the use of water blocking type curable fiber plugging working fluid and matching plugging technology are adopted to effectively seal and solidify the leakage layer.Through technological breakthroughs such as high lubrication water-based drilling fluid, fault plugging technology, and ultra long horizontal downhole casing, the length of the horizontal section has been increasing year by year, reaching a maximum of 5060m.

Key tools and drilling parameters have been strengthened, and the drilling cycle has been continuously shortened. Continuously selecting high wear-resistant PDC drill bits, high-power long-life screws, hydraulic oscillators and other key acceleration tools, optimizing drilling tool combinations in sections, and forming a shale oil "one trip drilling" technology with high wear-resistant PDC drill bits and equivalent life downhole tools as the core.Continuously supporting high-performance drilling and completion equipment such as electric drilling rigs, top drives, and high-power drilling pumps, by strengthening drilling parameters, aggressively drilling with "high drilling pressure, high displacement, and high speed", establishing learning curves and speed improvement technology templates.Through scale application, the drilling cycle of the 1500m horizontal section has been shortened from 29.1d in the initial stage to 17.8d, and the proportion of "one trip drilling" in the horizontal section has increased from 35.1% to 54.9%. The drilling speed increase effect is significant.

Developing high-strength and toughness cement slurry system, the quality of horizontal well cementing has been improving year by year.By continuously selecting reinforced and toughening materials and optimizing the formulation of the cement slurry system, a high-strength and tough cement slurry system has been formed. Compared with conventional water loss reducing cement slurry and tough cement slurry, this system reduces the elastic modulus of cement stone by 30% and increases the compressive strength by 40%.Select key tools such as high pressure resistant anti overturning floating clamps, integral elastic stabilizers, and floating couplings to form a multi floating casing and secondary floating technology. The integrated application of ultra long horizontal section safety cementing technology has increased the qualification rate of cementing quality from 75% to 81.1%.