Xinjiang Oil & Gas

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Current Status and Suggestions for Drilling Technology of CNPC Continental Shale Oil Reservoirs

2024, 20(3): 1-14. DOI: 10.12388/j.issn.1673-2677.2024.03.001

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Shale oil with medium to high maturity is the key field for China's shale oil strategic breakthrough，and strengthening the exploration and development of continental shale oil is an important way to ensure the security of national energy supply. The technical progress of shale oil drilling in Ordos，Junggar and Songliao Basins by China National Petroleum Corporation （CNPC） was introduced in detail，including large platform factory-like well placement，horizontal well casing program optimization，"one-run drilling" in separate well sections，high-performance drilling fluids for horizontal wells，integrated geological steering and other key technologies. By benchmarking against advanced shale oil drilling technologies in North America，the gap and development trend of shale oil drilling in China were analyzed in terms of well placement and wellbore configuration，geological-engineering integration and steering technology，supporting technology for accelerating rates of penetration （ROP），drilling fluid and cementing supporting technology. Development suggestions，such as continuously promoting large platform factory-like operations，optimizing wellbore configurations，integrating ROP acceleration technologies，strengthening research on geological-engineering integration，and accelerating the research and development of new-generation steering tools，were proposed in order to achieve the goals of ROP acceleration，cost reduction，and efficiency improvement in continental shale oil drilling，and moreover，the engineering goals of the shale oil revolution.

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Key Technologies and Practice of Shale Oil Cost-Effective Development in Jimsar Sag，Junggar Basin

QI Hongyan, WU Chengmei, HU Ke, CHEN Yiwei, XU Tianlu, WANG Yongjiao

2024, 20(3): 15-22. DOI: 10.12388/j.issn.1673-2677.2024.03.002

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Xinjiang Jimsar Shale Oil Demonstration Area is the first national-level continental shale oil demonstration project，located in Jimsar Sag，Junggar Basin，China. The target layer is Permian Lucaogou Formation. After ten years of exploration and development，with the obtained abundant production and geological data of the reservoir and deepened geological understanding，the corresponding exploration and development technologies become fairly mature. The fine 3D seismic survey with broadband excitation and well-surface integrated acquisition improves the geological understanding. The fine characterization and classification evaluation techniques of shale oil sweet spots provide a basis for development deployment. The comprehensive mud logging technology of "gold targets" improves the target-layer drilling ratio that is fundamental for delivering high stable production from horizontal wells. The customized fracturing technology of "complex fracture networks" provides an effective means for the efficient development of shale oil. The production capacity of horizontal wells is fully exploited by the flowback and production scheme optimization technique. The marketization has greatly reduced costs and improved both quality and efficiency. The cost-effective development technology system of Jimsar shale oil provides references for the efficient exploration and development of shale oil in China.

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Practice and Understanding of Volume Fracturing Technology for Horizontal Wells in Jimsar Shale Oil Reservoir

XU Jiangwen, WANG Mingxing, WANG Junchao, SUN Haoran, WANG Liang

2024, 20(3): 23-30. DOI: 10.12388/j.issn.1673-2677.2024.03.003

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Addressing Jimsar shale oil reservoir，this study aims to enhance fracture-controlled reserves by conducting theoretical research and technical practices. Focused on four key aspects—fracture network design，construction，proppant effectiveness，and the enhancement of in-situ oil mobility，three generations of technological advancements have been realized. Fundamental theories regarding the effective propagation of hydraulic fracture in thin interlayers and key technologies such as multi-cluster fracturing within a stage and the use of cost-effective materials have been mastered. An integrated horizontal well volume fracturing technology system centered on fracture-matrix matching，precise stimulation，multi-scale proppant support，and CO₂ prepad injection has been developed. The key technical indicators of fracturing have reached the advanced domestic level，supporting the large-scale and efficient development of Jimsar shale oil. In 2023，the oil production of Jimsar shale oil surpassed 60×10⁴ t，with a consecutive annual production increase of 10×10⁴ t in the block for three years. This achievement has provided robust technical support for the establishment of China's first national continental shale oil demonstration zone by 2025 and serves as a reference for optimizing shale oil development technologies in other regions of China.

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Faults at Various Scales Identification Techniques in Shale Reservoirs

LI Ang, WU Ningyu, ZHANG Liyan, YIN Wen

2024, 20(3): 30-36. DOI: 10.12388/j.issn.1673-2677.2024.03.004

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Qingshankou Formation of the southern Qijia Sag in Songliao Basin develops shale reservoirs，and shale oil is becoming an important replacement field for increasing reserves and production of hydrocarbons in this area. Qingshankou Formation has been cut by different sets of faults during its deposition，and the difficulty in fault identification and fracture prediction has affected the understanding of the main control factors of the reservoir in this area. It is of great significance to accurately identify the distribution characteristics of faults of different levels for shale oil exploration in this area. Seismic attribute analysis is one of the effective means of fault identification and reservoir prediction. Different seismic attributes can describe different geological characteristics，and it is hard to use a single seismic attribute to comprehensively and accurately identify faults. In this paper，the variance cube attribute，3D structural curvature attribute，structural smoothing and 3D edge enhancement technique，ant body attribute as well as other seismic multi-attribute fault identification techniques are comprehensively used to improve the identification accuracy of faults of different levels，and a set of shale formation fault identification technical process is formed which is effective for faults of different levels and scales. The characteristics of fault distribution in the study area are effectively described，and the planar distribution prediction of faults of different levels is realized. This research overcomes the limitations of the single seismic attribute method in fault identification and prediction，and provides a reliable basis for the deployment of oil and gas exploration.

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Calculation of Casing Collapse Strength under Zipper Fracturing in Platform Wells

LI Jun, , ZHANG Xiaojun , LIAN Wei , ZHANG Juncheng , LIU Gonghui

2024, 20(3): 37-45. DOI: 10.12388/j.issn.1673-2677.2024.03.005

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With the extensive applications of new technologies such as zipper fracturing in shale oil and gas development，the casing is subjected to both the multi-stage cyclic internal pressure and the external non-uniform compressive load caused by zipper fracturing operations. Given this，this paper analyzes the variation law of casing collpase strength under multi-stage cyclic load and zipper fracturing of platform wells via the multi-stage cyclic load experiments and numerical simulations. The calculation method of the casing comprehensive collapse strength coefficient under multi-stage fracturing is developed and applied to a casing-deformed well in Xinjiang Oilfield. The results show that the casing collapse strength decreases linearly with the number of load cycles under cyclic loading. Zipper fracturing results in the non-uniform stress distribution around the well，and the casing collapse strength nearly linearly decreases with the increasing stress non-uniformity. Under the combined effects of internal and external loads，the casing collapse strength drops by more than 15%，which increases risks of casing deformation. The calculation method of casing callopase strength in platform wells proposed in this paper provides calculation guidance for the prevention of casing deformation in fracturing practice.

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Research Status of Wellbore Stabilization Drilling Fluid Materials in Complex Shale Formations

HUANG Xianbin, , ZHANG Xuehao, , YUAN Zhenhang, , ZHANG Yang,

2024, 20(3): 46-53. DOI: 10.12388/j.issn.1673-2677.2024.03.006

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In response to the frequent wellbore instability issues encountered during drilling operations in complex shale formations，the drilling fluid technologies aimed at wellbore stabilization have been developed at home and abroad. These include shale hydration inhibition，formation pore plugging，and chemical wall strengthening techniques. Despite these advancements，existing methods have limitations in curbing the surface hydration of shale，and the effect of rapid plugging and wall strengthening agent is considerably reduced under high temperature conditions，making it challenging to address wellbore instability. This paper offers a comprehensive review of the research work on drilling fluid materials for wellbore stability in complex shale formations. By analyzing the current state of research，it elucidates the mechanism of various drilling fluid materials，including shale hydration inhibitors，plugging agents，and chemical wall strengthening agents. It also discusses the strengths and limitations of different types of wellbore stabilization materials. This paper highlights the need for fundamental theoretical research into shale hydration，the development of novel nanomaterials that are stable and effective under high temperature and high salinity conditions，and the establishment of evaluation methods for chemical wall strengthening agents that can simulate downhole conditions. These areas are identified as key focal points and challenges for future research in the field of drilling fluid materials and technologies for wellbore stabilization in complex shale formations. This paper concludes with an outlook on the future development directions for these technologies and materials.

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Non-Uniform Perforation to Balance Multi-Cluster Fractures Propagation and Parameter Optimization

SHENG Mao , DENG Chao , LI Jie , GU Mingzhe , WANG Tianyu , TIAN Shouceng,

2024, 20(3): 54-63. DOI: 10.12388/j.issn.1673-2677.2024.03.007

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Multi-cluster perforation staged fracturing of horizontal well has become one of the key technologies for completion and stimulation in unconventional oil and gas reservoirs. However，the fractures of the central perforation clusters in each fracturing stage are significant affected by stress interference from the fractures of heel and toe clusters，leading to substantial propagation resistance of the central cluster fractures. This is a major cause of the unbalanced propagation of multi-cluster fractures. This study optimized the design of non-uniform perforation distribution pattern between clusters to regulate the perforation parameters，balance the fluid distribution among clusters，mitigate the stress interference between fractures，and promote the balanced propagation of fractures. Therefore，a multi-stage，multi-cluster fracture propagation model that accounts for stress superposition between stages/clusters was established. The model was used to compare and analyze the fracture propagation patterns and mechanisms of spindle-shaped，sloped and uniform perforating patterns. The difference in fracture length and height propagation morphology was utilized to assess the equilibrium of fracture propagation. The perforation distribution pattern was optimized，and an orthogonal test was designed to refine the parameters of non-uniform perforation distribution pattern. The findings indicate that under typical shale oil reservoir conditions，the spindle-shaped perforation pattern achieves the best fracture propagation equilibrium，followed by uniform pattern and then the sloped pattern. The mechanism is that，with the spindle-shaped perforation pattern，the clusters at both ends have a perforation friction 1.4~16.7 times greater than that of the central clusters，reducing the stress interference from the heel and toe fractures on the central-cluster fractures. This results in an increased fluid inflow distribution in the central clusters，enhancing the fracture propagation equilibrium by 17.2 % compared to the uniform pattern. Conversely，the sloped perforation pattern，with over 35% of the holes concentrated at the toe cluster and accounting for 49.3 % of the fluid inflow，exerts a significant squeezing effect on the central cluster fractures，which is counterproductive to achieving a balanced fracture propagation. The optimization of the spindle-shaped perforation parameters reveals that an fracture propagation equilibrium is achieved with a total of 49 holes，a perforation diameter of 10 mm，and an end-cluster perforation proportion of 24.5%. The research results are expected to offer an effective approach for the design of non-uniform perforation in multi-cluster fracturing for unconventional oil and gas reservoirs.

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Research on CO2 Matrix Penetration Distance in Tight Reservoirs

MOU Jianye , YAN Xiaolun , ZHANG Shicheng , SHI Lei , LI Dong , MA Xinfang

2024, 20(3): 64-71. DOI: 10.12388/j.issn.1673-2677.2024.03.008

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During depletion recovery of tight oil，the reservoir pressure and production rate decline rapidly. Energy supplement is a necessary measure to enhance oil recovery. CO₂ is a common medium used for energy replenishment （CO₂ huff and puff，energy recharging by CO₂ prepad）. The performance of CO₂-assisted enhanced oil recovery （EOR） is highly dependent on the penetration distance. Therefore，the penetration distance of CO₂ through the matrix of tight reservoirs was investigated via laboratory experiments and numerical simulation. Mass transfer of CO₂ involves both convection and diffusion. Firstly，CO₂ diffusion experiments were performed using a high-temperature high-pressure reactor and the cores of a tight formation. Based on the experimental results and Fick's diffusion model，the diffusion coefficient of CO₂ was obtained. Next，CMG was used to build a model of CO₂ convection and diffusion，and extensive numerical simulations were performed to analyze permeability，considering the effect of diffusion and flooding duration on CO₂ penetration distance. The results show that the diffusion coefficient is dependent on permeability. The diffusion coefficient ranges from 10^-10 to 10^-8 m²/s at the permeability level of the target formation. Diffusion and convention both affect the CO₂ penetration distance. Yet，the effects of convection are dominant with higher permeability，while those of diffusion are stronger with lower permeability. Permeability is a decisive factor of the matrix penetration of CO₂. The CO₂ penetration in tight reservoir matrix is rather low and reaches less than 10 m after 60 days.

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Performance and Displacement Effect Evaluation of Multifunctional Fracturing-Enhanced Oil Recovery Materials

GUO Jixiang, ZHANG Xiaojun, CHU Yanjie, ZHAO Kun, PENG Zhongying

2024, 20(3): 72-82. DOI: 10.12388/j.issn.1673-2677.2024.03.009

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Poor pore-permeability，high shale oil viscosity，strong reservoir-wellbore flow risk，low primary recovery and single unsuitable slickwater fracturing fluids are the problems facing the development of Jimusar shale oil. SDY-1+XC-4，a multifunctional slickwater fracturing fluid system，was developed based on SDY-1，a multifunctional fracturing-enhanced oil recovery (EOR) material for viscosity reduction，oil washing，and imbibition. The performances of temperature and shear resistant，viscosity reduction，oil washing and imbibition were evaluated through laboratory experiments. Its applicability under reservoir temperature and salinity was analyzed. And the slickwater system has been applied in Jimsar. Results showed that SDY-1 has high compatibility with XC-4. SDY-1+XC-4 exhibits good temperature and shear resistance at reservoir temperature. At 30 ℃，the viscosity reduction rate of SDY-1+XC-4 is 93.68%，the oil washing efficiency is 66.7%，and the imbibition recovery rate is 33.24%. SDY-1+XC-4 works well under the reservoir temperature and salinity. The field testing results showed that the test well presents cumulative oil increase of 2 031.6 t，compared with the reference well. The development and application of the multifunctional fracturing-EOR materials can provide a solid foundation and technical support for the efficient development of Jimsar shale oil.

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NMR-Based Experiments of Fracturing Fluid Assisted CO₂ Huff-n-Puff for Enhancing Shale Oil Recovery

XIAO Wenlian, CHEN Shengen, YI Yonggang, CHEN Haoyu, REN Jitian

2024, 20(3): 83-90. DOI: 10.12388/j.issn.1673-2677.2024.03.010

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To develop a huff-n-puff development mode of shale oil facilitating continuous reservoir energy supplement，the integrated simulation experiment of CO₂ huff-n-puff assisted by fracturing fluids for Jimsar shale oil reservoir was completed using low frequency nuclear magnetic resonance （NMR） core analyzer. The characteristics of pore throat fluid production and recovery rate variation of the core during the composite development of multi-medium cyclic injection and flooding were investigated to evaluate the enhanced oil recovery （EOR） performance of the fracturing fluid-assisted CO₂ huff-n-puff. Finally，the novel development mode of huff-n-puff energy supplement was proposed for shale oil. The experimental results showed that in the case of conventional CO₂ huff-n-puff，the recovery rate of core samples decreases rapidly from cycle to cycle，and only the oil in medium and large pores is produced. The oil production is mainly contributed by the first two cycles，and the ultimate recovery factor is 30%~40%. For the CO₂ huff-n-puff assisted by fracturing fluids，in which the CO₂ injection end was used for production，the produced oil is also mostly limited to medium and large pores. The oil production is mainly attributed to the first three cycles，and the ultimate recovery factor is 30%~40%. In terms of the CO₂ huff-n-puff assisted by fracturing fluids，with the fracturing fluid injection end used for production，the oil in all pores is produced，and the ultimate recovery factor is 70%~80%. This mode leaves evenly distributed remaining oil and the highest development performance. The findings of this research provide a theoretical basis for the field practice of shale oil EOR based on huff-n-puff reservoir energy supplement.

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Quantitative Evaluation of Frac Hits Based on Shut-in Pressure Drop Curve：Taking Shale Oil Platform Wells in Jimsar as an Example

WANG Fei, DONG Zhuo, XU Tianlu, CHEN Lu, WANG Zhengkai, LI Zhanjie

2024, 20(3): 91-98. DOI: 10.12388/j.issn.1673-2677.2024.03.011

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