A quantitative fracability evaluation method and its application to deep shale gas development in Sichuan Basin, China

Abstract

Fracability evaluation is critical for efficiently extracting deep shale gas using hydraulic fracturing to avoid blind drilling and fracking. However, existing fracability indices often fail to systematically consider the mechanical behavior of rocks at high temperatures and high pressures (HTHP), coupled with geostress distributions and heterogeneous reservoir characteristics. This critical omission limits their effectiveness in accurately identifying the optimal fracability sweet spots within deep reservoirs. In this work, a fracability evaluation model was proposed based on the combined weighting method, integrating the improved brittleness index, rock strength, geostresses and natural weakness characteristics. A fracability grading evaluation was carried out to determine the potential fracture characteristics corresponding to shales with different fracability levels. Additionally, the fracability index was used for field validation and applications. Results show that rock brittleness and fracability are not equivalent for deep reservoirs. The fracability index is closely related to the pay zones and actual gas production, with a correlation as high as 84%, implying that the proposed method has practical significance in both experimental and field applications. The above findings can provide theoretical guidance for the selection of fracturing candidates and the optimal design of fracturing in deep resource development.

Keywords

Fracability / Energy-based brittleness / Wufeng-Longmaxi formation / Field application

Acknowledgments

This work was supported by the National Natural Science Foun-dation of China (Nos. U24B2035 and U22A20166), the Postdoctoral Fellowship Program and China Postdoctoral Science Foundation (Nos. BX20250034 and 2024M763503), and the Natural Science Foundation of Hubei Province of China (No. 2024AFD374).

References

[1] Chen SJ, Li P, Qin LL, Zhang JC, Li ZG, Shi M.Novel triangle method for evaluation of fracability in transitional shale:Case study from well ZXY-1 in the southern North China Basin. Energy Explor Exploit 2022; 40(3):1036-56.

[2] Liu YW, Zhang GP, Qiao JM, Tang XH. Micromechanical testing and property upscaling of planetary rocks: A critical review. Int J Min Sci Technol 2024; 34 (9):1217-41.

[3] Xu S, Wen J, Liu KQ, Shi X, Dong T. Brittle minerals, mechanical properties and fracability evaluation of shales. Adv Geo-Energy Res 2024; 14(1):8-11.

[4] Zhao JZ, Ren L, Jiang TX, Hu DF, Wu LZ, Wu JF, et al. Ten years of gas shale fracturing in China: Review and prospect. Nat Gas Ind B 2022; 9(2):158-75.

[5] Li YY, Hu W, Wei SY, Li LH, Zhang ZH, Song SY. Influence of preexisting discontinuities on hydraulic fracture complexity in a naturally fractured reservoir. Eng Geol 2022; 311:106919.

[6] Zou CN, Zhu RK, Chen ZQ, Ogg JG, Wu ST, Dong DZ, et al. Earth Sci Rev Organic-matter-rich shales of China. 2019; 189:51-78.

[7] Zhao GK, Guo YT, Yang CH, Chang X, Guo WH, Yang HZ. Effect of temperature and pressure on mechanical behavior of reservoir shales at different depths: implications for deep shale gas extraction. Rock Mech Rock Eng 2025; 58 (6):6169-94.

[8] Zhou XF, Wei JG, Hadavimoghaddam F, Gayubov A, Ostadhassan M, Chen YH, et al. ‘‘Double sweet spot” identification method via cluster analysis of petrophysical properties from well logg data: A case study of a tight sandstone reservoir. Geoenergy Sci Eng 2023; 227:211821.

[9] Sui LL, Ju Y, Yang YM, Yang Y, Li AS. A quantification method for shale fracability based on analytic hierarchy process. Energy 2016; 115:637-45.

[10] Bai M. Why are brittleness and fracability not equivalent in designing hydraulic fracturing in tight shale gas reservoirs. Petroleum 2016; 2(1):1-19.

[11] Han SC, Cheng YF, Gao Q, Yan CL, Han ZY. Experimental study of the effect of liquid nitrogen pretreatment on shale fracability. J Nat Gas Sci Eng 2018; 60:11-23.

[12] Iyare UC, Blake OO, Ramsook R. Brittleness evaluation of naparima hill mudstones. J Petrol Sci Eng 2021; 196:107737.

[13] Zhang FX, Zhang H, Yuan F, Wang ZM, Chen S, Li C, et al. November 9-12, 2015. Abu Dhabi UAE. Geomechanical mechanism of hydraulic fracturing and fracability evaluation of natural fractured tight sandstone reservoir in keshen gasfield in Tarim Basin. Abu Dhabi International Petroleum Exhibition and Conference. Richardson: Society of Petroleum Engineers, 2015: SPE 177457-MS.

[14] Liu H, Huang YY, Zhang XY, Bie ZJ, Gu JY, He L.Fracability evaluation in deep shale reservoirs based on a fuzzy grey correlation analysis method. 2019 IEEE 18th International Conference on Cognitive Informatics & Cognitive Computing (ICCI*CC). July 23-25, 2019. Milan, Italy. IEEE, 2019: 299-306.

[15] Jarvie DM, Hill RJ, Ruble TE, Pollastro RM. Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment. Bulletin 2007;91(4):475-99.

[16] Wang F, Zhang YH, Li X, Han X. Study on a new rock fracability evaluation model of shale gas reservoir. Geofluids 2020; 2020(1):8882899.

[17] Yuan JL, Zhou JL, Liu SJ, Feng YC, Deng JG, Xie QM, et al. An improved fracability-evaluation method for shale reservoirs based on new fracture toughness-prediction models. SPE J 2017; 22(5):1704-13.

[18] Liu XQ, Zhang WD, Qu ZQ, Guo TK, Sun Y, Rabiei M, et al. Feasibility evaluation of hydraulic fracturing in hydrate-bearing sediments based on analytic hierarchy process-entropy method (AHP-EM). J Nat Gas Sci Eng 2020; 81:103434.

[19] Jin XC, Shah SN, Roegiers JC, Zhang B.An integrated petrophysics and geomechanics approach for fracability evaluation in shale reservoirs. SPE J 2015; 20(3):518-26.

[20] Hajiabdolmajid V, Kaiser P. Brittleness of rock and stability assessment in hard rock tunneling. Tunn Undergr Space Technol 2003; 18(1):35-48.

[21] Rickman R, Mullen M, Petre E, Grieser B, Kundert D. A practical use of shale petrophysics for stimulation design optimization:all shale plays are not clones of the Barnett shale. SPE Annual Technical Conference and Exhibition. September 21-24, 2008. Denver, Colorado, USA. Richardson: Society of Petroleum Engineers, 2008: SPE 115258-MS.

[22] Zhao GK, Guo YT, Chang X, Jin PH, Hu YQ. Effects of temperature and increasing amplitude cyclic loading on the mechanical properties and energy characteristics of granite. Bull Eng Geol Environ 2022; 81(4):155.

[23] Zhao GK, Chang X, Guo YT, Yang HZ, Guo WH, Hu YQ. Fatigue of granite subjected to cyclic loading at various temperatures: Experimental insights from deformation and energy conversion. Geomech Geophys Geo Energy Geo Resour 2022; 8(2):64.

[24] Huo ZP, Zhang JC, Li P, Tang X, Yang X, Qiu QL, et al. An improved evaluation method for the brittleness index of shale and its application: A case study from the southern North China basin. J Nat Gas Sci Eng 2018; 59:47-55.

[25] Sun W, Zuo YJ, Wu ZH, Liu H, Zheng LJ, Shui Y, et al. The distribution characteristics of brittle minerals in the Lower Cambrian Niutitang Formation in northern Guizhou. J Nat Gas Sci Eng 2021; 86:103752.

[26] Liu YW, Liu A, Liu SM, Kang Y. Nano-scale mechanical properties of constituent minerals in shales investigated by combined nanoindentation statistical analyses and SEM-EDS-XRD techniques. Int J Rock Mech Min Sci 2022; 159:105187.

[27] Rahimzadeh Kivi I, Ameri M, Molladavoodi H. Shale brittleness evaluation based on energy balance analysis of stress-strain curves. J Petrol Sci Eng 2018; 167:1-19.

[28] Li YW, Long M, Zuo LH, Li W, Zhao WC. Brittleness evaluation of coal based on statistical damage and energy evolution theory. J Petrol Sci Eng 2019; 172:753-63.

[29] Amiri H, Ramezanzadeh A, Chamanzad M, Parhizgar M. Recognizing the best intervals for hydraulic fracturing using a new fracability index. J Petrol Explor Prod Technol 2021; 11(8):3193-201.

[30] Feng G, Kang Y, Wang XC, Hu YQ, Li XH. Investigation on the failure characteristics and fracture classification of shale under Brazilian test conditions. Rock Mech Rock Eng 2020; 53(7):3325-40.

[31] Hu YF, Hu YQ, Zhao GK, Jin PH, Zhao ZR, Li C. Experimental investigation of the relationships among P-wave velocity, tensile strength, and mode-I fracture toughness of granite after high-temperature treatment. Nat Resour Res 2022; 31(2):801-16.

[32] Ma XL, Jiang DD, Sun YH, Li SL. Experimental study on hydraulic fracturing behavior of frozen clayey silt and hydrate-bearing clayey silt. Fuel 2022; 322:124366.

[33] Xu WJ, Zhao JZ, Xu JG. Fracability evaluation method for tight sandstone oil reservoirs. Nat Resour Res 2021; 30(6):4277-95.

[34] Wang XQ, Ge HK, Han P. A new model for fracability evaluation with consideration of natural cracks. J Geophys Eng 2018; 15(4):1492-505.

[35] Ishizaka A, Lusti M. How to derive priorities in AHP: A comparative study. Cent Eur J Oper Res 2006; 14(4):387-400.

[36] Ju Y, Wu GJ, Zhang GL, Liu P. Effects of rock ductility on the fracability of ductile reservoirs: an experimental evaluation. Geoenergy Sci Eng 2023; 227:211864.

[37] Hatheway AW.The complete ISRM suggested methods for rock characterization, testing and monitoring; 1974-2006. Environ Eng Geosci 2009; 15(1):47-8.

[38] Xiong ZH, Wang GM, Cao YC, Liang C, Li MP, Shi XM, et al. Controlling effect of texture on fracability in lacustrine fine-grained sedimentary rocks. Mar Petrol Geol 2019; 101:195-210.