1.2 Key technological progress in shale oil drilling in Jimsar

The shale oil of Lucaogou Formation in Jimusar Sag of the Junggar Basin was discovered in 2011, with a favorable regional resource amount of 11.12×10⁸t. It has gone through three stages of exploration and development pilot test, evaluation and industrial test, technology finalization and large-scale production construction.In March 2020, the National Energy Administration and the Ministry of Natural Resources jointly established the Xinjiang Jimsar National Continental Shale Oil Demonstration Zone in the Jimsar Depression. In 2023, the shale oil production in Jimsar Depression reached 63.5×10⁴ tons, achieving efficient production.

The shale oil in Jimusaer Depression is a thin interbedded mixed shale oil, with the target layer being the Permian Lucaogou Formation, which can be divided into upper and lower sections.The upper dessert is located in the 2nd sand formation of the Lucaogou Formation (Lu 2), mainly composed of feldspar fine sandstone; The lower dessert is located in the 2nd sand formation of the first section of the Lucaogou Formation, mainly composed of cloudy sandstone.There is a large difference in the burial depth of oil reservoirs on the plane, mainly distributed in the depth range of 2800-4200m, making it difficult to optimize the wellbore structure; Vertically developed multiple sets of mudstone layers, with the development of Neogene/Paleogene gypsum mudstone, which can easily contaminate drilling fluids and cause shrinkage; Multiple sets of brown/gray mudstone are developed from Jurassic to Permian, and the wellbore is prone to instability and poor drillability; High quality desserts have a thin layered structure and are difficult to identify, requiring high requirements for horizontal well trajectory control.Through the integration of geological engineering research, continuous optimization of wellbore structure, application of oil-based drilling fluid system to improve wellbore stability, and other technical measures, the main technical system of "double opening wellbore structure+oil-based drilling fluid+segmented drilling with one trip" has been finalized, and the drilling speed and horizontal section extension ability have been continuously improved.In 2023, China Petroleum Xinjiang Oilfield completed 72 shale oil wells in Jimsar, with an average depth of 5720 meters, a horizontal section length of 1817 meters, and a drilling cycle of 30.6 days, as shown in Figure 3.

Deepen the research on geological characteristics and continuously optimize the wellbore structure. From 2017 to 2019, two and three wellbore structures were adopted with a vertical depth of 3000m as the boundary, and successful experiments were conducted on the two wellbore structure in areas deeper than 3000m.Starting from 2020, the standardized promotion of the double opening wellbore structure will be carried out, with the depth of the surface casing adjusted from 500m to 1700m, to seal complex gypsum mudstone layers; Introducing a strong plugging and high lubricating oil-based drilling fluid system to optimize the wellbore structure from three openings to two openings.From 2020 to 2023, 118 horizontal wells achieved a dual wellbore structure with a success rate of 100%. The deepest well depth was 6270 meters, the deepest vertical depth was 4106 meters, and the longest open hole section was 4466 meters.Based on quantitative analysis of wellbore stability and optimization of surface casing size, the surface drill bit/casing size has been optimized to Φ 311.2/244.5mm to achieve further "slimming", speed increase, and cost reduction.At present, 32 surface casing optimization test wells have been completed, achieving preliminary results and laying the foundation for the next step of beneficial development of Jimsar shale oil.

Optimize the drilling fluid system, enhance the performance of the drilling fluid, and ensure safe and efficient drilling. In response to the challenges of stabilizing the wellbore wall in the long open hole section and extending the horizontal section, evaluation and optimization of treatment agents were carried out. In 2019, an 85:15 white oil-based drilling fluid system was developed, achieving a breakthrough in the horizontal section length of 3500 meters and the structure of the secondary wellbore. Since then, oil-based drilling fluids have been widely promoted. In 2021, the oil-water ratio will be further optimized to 80:20.

Optimize the standardized segmented drilling speed template and continuously shorten the drilling period. Guided by the concept of "one trip drilling", optimize the drill bit, drilling tools, and drilling parameters in three well sections, finalize the "rotary directional tool+strong attack drill bit", continuously strengthen the drilling parameters, and solidify the acceleration template.Ensure one drilling trip for the first section, one drilling trip for the second section of the vertical well, and 1-2 drilling trips for the inclined section and horizontal section, achieving 3-4 drilling trips for the entire well. In 2023, the proportion of "one trip drilling" in the horizontal section of the Jimsar shale oil horizontal well reached 58.3%, with a maximum single trip footage of 2937 meters.

Develop a domestically produced tough cement slurry system to ensure the quality of well cementing. Starting from 2018, we have been promoting domestically produced economical resilient cement slurry to meet the demand for large-scale volume fracturing. In 2020, we selected an efficient pre fluid system to improve the flushing efficiency of oil-based drilling fluid, significantly enhance the cementing displacement efficiency, and ensure that the cementing quality is qualified or of high quality.

Combining well vibration to construct a three-dimensional geological model, dynamically updating and optimizing the trajectory, and improving the reservoir drilling encounter rate. The key factor determining the productivity of horizontal wells is the "golden target" in the sweet spot, namely the first and second types of sweet spot oil layers.In response to the challenges posed by the "golden target" thickness of only 1.5-2.0 m and the difficulty in controlling well trajectories in Jimsar shale oil, a comprehensive evaluation of geological and engineering sweet spots was conducted based on seismic, logging, and geological information to determine the distribution of sweet spots in shale oil; Comprehensive application of logging curves, rock and mineral analysis data, and inversion data from edge exploration tools for geological guidance drilling while drilling, fine control of horizontal wellbore trajectory.In 2022, the drilling encounter rate of the "golden target" in the Jimsar shale oil of PetroChina Xinjiang Oilfield reached over 85%.

1.3 Progress in Key Technologies for Gulong Shale Oil Drilling

The exploration and development of shale oil in PetroChina Daqing Oilfield has a history of nearly 40 years, which has gone through three stages: discovery and exploration, research and understanding, and experimental breakthroughs.Since 2018, the ancient shale oil Ping-1 well in the deep part of the Gulong Depression in the central depression area of the Songliao Basin has achieved high-yield industrial oil and gas flow by using the shale of the lower part of the Qingyi Formation as the sweet spot target layer, marking an important breakthrough in pure shale oil.In 2021, a predicted reserve of 12.68×10⁸ tons was submitted, and a national demonstration zone for Daqing Gulong continental shale oil has been established. With the deepening of geological understanding, the iterative progress of engineering transformation technology, and the innovation of management mode, the horizontal wells of the Shangyou Formation in the Qingshankou Formation of the Songliao Basin in 2022 have achieved beneficial development of medium to high maturity shale oil.

The sweet spots of Daqing Gulong continental shale oil shale type are mainly distributed in the lower part of the Qing 1 and Qing 2 sections. The lithology is mainly layered and laminated shale, with high brittle mineral content, high clay content, and low carbonate content.The shale of Gulong has extremely developed foliation, with a foliation density of 1000-3000/m and a wide range of pore size distribution (nanometer to micrometer) in the Qingshankou Formation shale.By revealing the mechanism of wellbore instability, forming wellbore reinforcement measures, and exploring the main technology of "one trip drilling", representative technologies such as wellbore stability and oil-based drilling fluid technology, large platform horizontal well drilling technology, and "one trip drilling" main technology have been developed around the key technical issues faced in the early stage of development, such as shale wellbore instability and drilling speed. This has achieved the goal of "drilling well, drilling well, and drilling quickly" in the Gulong shale oil horizontal well.As shown in Figure 4, since the implementation of the strategic breakthrough well "Guye Oil Ping 1 Well", the drilling cycle of the Gulong shale oil horizontal well has achieved a step like leap of 108 d-68 d-28 d-25 d-24 d. By 2023, the average depth of drilled wells will be 4745 m, the horizontal section length will be 2148 m, the drilling cycle will be 23.9 d, and the fastest leap will be 10 d.

The instability of wellbore is the primary challenge in the early stage of horizontal well drilling construction. Through the study of the instability mechanism of Gulong shale wellbore, the instability mechanism of "mechanical equilibrium for oil-based and chemical stability for water-based" is elucidated, and it is clarified that the interlayer short-range repulsion generated by illite hydration is the biggest cause of shale instability.Adopting the strategy of "mechanical balance+enhanced plugging technology", based on experimental data of seismic, logging, and rock mechanics parameters, a pressure prediction model for the Gulong shale formation was established, forming collapse and leakage pressure profiles, establishing safe density windows for drilling fluids in different well areas, and forming a high-performance oil-based drilling fluid system with good rheological properties, stable performance, and outstanding plugging and anti collapse effects.By optimizing the particle size distribution of the sealing material and coordinating multiple sealing materials, the sealing of nano and micro pores and seams is strengthened, greatly reducing the probability of wellbore instability and ensuring "success".

In response to the difficulties of collision prevention in factory construction of large platforms and geological guidance of thin oil layers, we adhere to the concept of integrated geological engineering and carry out key technical breakthroughs such as optimizing the well network of large platforms and fine geological guidance.Innovatively proposing a "large platform, multi-layer system, and three-dimensional" well layout method, tackling the optimization design technology of smooth wellbore trajectory and reducing friction and torsion, optimizing the "large three-dimensional" design track into a "double two-dimensional" track, completing the offset distance of the second well section, and constructing the two-dimensional horizontal well normally with the third well section, effectively reducing the slope of the third well construction (more than 20%), forming track design templates under different offset distances, achieving friction reduction and drag reduction, and reducing the difficulty of anti-collision construction.We have developed a horizontal well geological guidance analysis system with independent intellectual property rights, forming a shale oil horizontal well geological guidance technology system. By approaching the target depth layer by layer, automatic prediction is carried out to guide the design of the drilling trajectory, achieve precise trajectory control, and achieve a high-quality reservoir drilling encounter rate of 90%, achieving "good drilling".

Supporting the main technology of "one trip drilling", the acceleration effect is significant. Optimize the well area, reasonably determine the depth of the surface casing, optimize the wellbore structure from three to two wells, and shorten the drilling cycle by more than 7 days on average per well.By selecting short crown high attack PDC drill bits, long-life high torque screws, wellbore cleaning monitoring, and highly reliable drilling instruments, solidifying drilling tool combinations, upgrading equipment matching, strengthening drilling parameters, continuously iterating and upgrading learning curves, the acceleration effect is significant.In 2023, the success rate of "one trip drilling" for Gulong shale oil reached 78.18%, with a record drilling cycle of 12.49 days for the third well and 9.98 days for the second well, achieving "fast drilling".

2. Benchmarking analysis of shale oil drilling technology at home and abroad

Through continuous research and practice, China Petroleum's onshore shale oil drilling technology has made significant progress, breaking through a number of key core technologies, and some indicators of horizontal wells are comparable to those in North America.However, due to various factors such as complex geological conditions and harsh surface environments, there is still a gap in overall drilling efficiency compared to North America. Key technologies and equipment such as water-based drilling fluids and factory operations need to be further benchmarked against international advanced levels to find gaps.At present, we should deepen technological innovation and engineering demonstration, further tap into the potential for improving quality and efficiency, and provide technical support for the scale and benefit development of terrestrial shale oil.

2.1 Overall Technical Indicators Benchmarking

2.1.1 Benchmarking of overall drilling indicators

North American shale oil is commonly developed in multiple layers, using large well clusters, factory operations, two/three wellbore structures, and casing cementing and completion.The fast drilling technology represented by the "one trip drilling" of horizontal wells has promoted the large-scale application of long horizontal sections of horizontal wells, continuously shortened the drilling cycle, and continuously reduced the drilling cost.As of the end of 2022, the inclined section and horizontal section have generally reached over 3000 meters, with the longest length of "one drilling" reaching 6340 meters.The average drilling cycle of shale horizontal wells with a depth of 4000-5000 meters has been reduced to about 10 days, and the mechanical drilling speed has reached over 25 m/h. The cost of building horizontal wells has been reduced to 00/m.