Significant progress has been made in the exploration and development of oil shale in Sinopec’s K-II block. However, frequent downhole blockages, drilling incidents, and wellbore instability have posed challenges for high-inclination horizontal drilling. This study analyzes complex subsurface conditions using field data and core experiments to investigate the physical, chemical, and mechanical properties of the formation. Key factors driving oil shale collapse are identified, and a wellbore stability evaluation method for the Shahejie Formation is developed. Results show that the formation has well-developed fractures, with quartz and clay as its primary components, and that water-based drilling fluids have minimal impact on its mechanical properties. Wellbore stability is significantly affected by inclination, azimuth, and weak-plane fractures. The critical collapse pressure equivalent density ranges from 1.5 to 1.65 g/cm3. Drilling along the maximum horizontal stress improves stability compared to drilling along the minimum stress. Enhancing drilling fluid sealing and adding rigid particles further improve wellbore integrity. These findings provide practical insights for safer oil shale drilling operations.
1. Shi, B., Hu, X., Gao, S., Xu, J., Lin, Y. Visualization sealing simulation test and evaluation of hard brittle shale microfracture. Pet. Drill. Tech., 2014, 42(3), 32–37.
2. Wang, P., Qu, Z., Huang, H. Experimental study of the effect of formation water salinity on creep laws of the hard brittle shale. Pet. Drill. Tech., 2015, 43(5), 63–68.
https://dx.doi.org/10.11911/syztjs.201505011
3. Westergaard, H. M. Plastic state of stress around a deep well. J. Boston Soc. Civ. Eng., 1939, 6(2), 107–114.
Hubbert, M. K., Willis, D. G. Mechanics of hydraulic fracturing. Pet. Trans. AIME, 1957, 210(1), 153–168.
https://doi.org/10.2118/686-G
4. Okland, D., Cook, J. M. Bedding-related borehole instability in high-angle wells. In: SPE/ISRM Rock Mechanics in Petroleum Engineering, July 8–10, 1998, Trondheim, SPE-47285-MS.
https://doi.org/10.2118/47285-MS
5. Gupta, D., Zaman, M. Stability of boreholes in a geologic medium including the effects of anisotropy. Appl. Math. Mech., 1999, 20, 837–866.
https://doi.org/10.1007/BF02452483
6. Chenevert, M. E., Gatlin, C. Mechanical anisotropies of laminated sedimentary rocks. Soc. Pet. Eng. J., 1965, 5(1), 67–77.
https://doi.org/10.2118/890-PA
7. Ong, S. H., Roegiers, J.-C. Influence of anisotropies in borehole stability. Int. J. Rock Mech. Min. Sci. Geomech. Abstr., 1993, 30(7), 1069–1075.
https://doi.org/10.1016/0148-9062(93)90073-M
8. Chen, X., Yang, Q., Qiu, K. B., Feng, J. L. An anisotropic strength criterion for jointed rock masses and its application in wellbore stability analyses. Int. J. Numer. Anal. Methods Geomech., 2008, 32(6), 607–631.
https://doi.org/10.1002/nag.638
9. Jin, Y., Yuan, J., Hou, B., Chen, M., Lu, Y., Li, S., Zou, Z. Analysis of the vertical borehole stability in anisotropic rock formations. J. Pet. Explor. Prod. Technol., 2012, 2(4), 197–207.
http://dx.doi.org/10.1007/s13202-012-0033-y
10. Paslay, P. R., Cheatham, Jr. J. B. Rock stresses induced by flow of fluids into boreholes. Soc. Pet. Eng. J., 1963, 3(1), 85–94.
https://doi.org/10.2118/482-PA
11. Chen, X., Tan, C. P., Detournay, C. A study on wellbore stability in fractured rock masses with impact of mud infiltration. J. Pet. Sci. Eng., 2003, 38(3–4), 145–154.
https://doi.org/10.1016/S0920-4105(03)00028-7
12. McLean, M. R., Addis, M. A. Wellbore stability analysis: a review of current methods of analysis and their field application. In: IADC/SPE Drilling Conference, February 27 – March 2, 1990, Houston, Texas, SPE-19941-MS.
https://doi.org/10.2118/19941-MS
13. Dusseault, M. B., Gray, K. E. Mechanisms of stress-induced wellbore damage. In: SPE Formation Damage Control Symposium, February 26–27, 1992, Lafayette, Louisiana, SPE-23825-MS.
https://doi.org/10.2118/23825-MS
14. Zhu, K., Chen, J., Lu, S. Study on stability mechanism of brittle shale borehole deep in Nanpu-III structure and its application. Drill. Prod. Technol., 2016, 39, 1–4, 100.
15. Liang, L., Ding, Y., Liu, X., Xu, L. Seepage–mechanochemistry coupling of wellbore stability in hard-brittle shale. Spec. Oil Gas Reserv., 2016, 23, 140–143, 158.
16. Qian, Z., Wu, J., Li, J., Hong, W., Ma, J., Kuang, X. Application of the oil-based drilling fluid in oil shale formation. Pet. Geol. Oilfield Dev. Daqing, 2016, 35, 170–174.
17. Liu, K., Liu, S., Wen, M. Analyze of borehole wall stability by the action of physical chemistry. Petrochem. Ind. Technol., 2019, 26, 139–140.
18. Chen, Z., Deng, J., Wei, B., Hu, J. Borehole stability analysis of hard brittle shale based on damage theory. Sci. Technol. Eng., 2019, 19(16), 87–94.
19. Zhao, G., Chen, C., Yan, H., Hao, Y. Study on the damage characteristics and damage model of organic rock oil shale under the temperature effect. Arab. J. Geosci., 2021, 14(8), 722.
http://dx.doi.org/10.1007/s12517-021-07046-x
20. Zhao, J., Kang, Z. Permeability of oil shale under in situ conditions: Fushun oil shale (China) experimental case study. Nat. Resour. Res., 2021, 30(12), 753–763.
http://dx.doi.org/10.1007/s11053-020-09717-0
21. Zhang, R., Chen, F., Niu, H., Gao, D., Liu, X. L., Yang, C. X. Study on the collapse pressure of hard and brittle fractured oil shale in ZH sag. Oil Drill. Prod. Technol., 2021, 43(2), 151–159.
22. Ren, S., Wang, L., Yang, D., Kang, Z., Zhang, P. Effect of real-time temperature and shear angle on the mechanical strength and energy evolution of oil shale. Oil Shale, 2022, 39(4), 270–289.
http://dx.doi.org/10.3176/oil.2022.4.03
23. Wang, L., Yang, D., Kang, Z., Zhao, J., Meng, Q. Experimental study on the effects of steam temperature on the pore–fracture evolution of oil shale exposed to the convection heating. J. Anal. Appl. Pyrolysis, 2022, 164, 105533.
https://doi.org/10.1016/j.jaap.2022.105533
24. Feng, Z., Shi, J. Research on the evolution rules and mechanisms of permeability of organic rocks under the action of thermal-mechanical coupling. J. Coal Sci. Eng. (China), 2023, 1–13.
http://dx.doi.org/10.13225/j.cnki.jccs.2023.1296
25. Zhang, X., Yu, L., Ma, Z., Li, Y., Jiang, X. Analysis of anisotropic heat transfer properties of oil shale and simulation of in-situ thermal conduction exploitation. J. Taiyuan Univ. Technol., 2024, 55(2), 260–266.
https://doi.org/10.16355/j.tyut.1007-9432.20230683
26. Cao, J., Yang, D., Kang, Z., Feng, L. Study on shear failure characteristics of oil shale under high-temperature steam treatment. J. Taiyuan Univ. Technol., 2023, 55(2), 252–259.
https://doi.org/10.16355/j.tyut.1007-9432.20230637
27. Chen, M., Jin, Y., Zhang, G. Petroleum Engineering Rock Mechanics. Science Press, Beijing, 2008.
28. Jaeger, J. C. Shear failure of anisotropic rocks. Geol. Mag., 1960, 97(1), 65–72.
https://doi.org/10.1017/S0016756800061100
29. Ma, T., Chen, P., Wang, X., Guo, Z., Liu, Z. Numerical analysis method of pore pressure propagation around the borehole for shale gas reservoirs. Acta Petrol. Sin., 2016, 37(5), 660–671.
https://dx.doi.org/10.7623/syxb201605010
30. Jin, Y., Chen, M. Wellbore Stabilization Mechanics. Science Press, Beijing, 2012.
