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| Chemical fracture mechanism and experimental study on cracking induced by hydration in illite crystal layers of hard-brittle shale |
| ZHANG Yazhou, JIN Yan*, CHEN Mian, LU Yunhu |
| (State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum (Beijing), Beijing 102249, China) |
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Abstract Microcrack growth induced by water imbibition and hydration in hard-brittle shale has consistently posed a challenge to wellbore stability during drilling. While most current studies rely on modeling assumptions of constant crack width and hydration driving force, as well as semi-quantitative experimental characterizations, they fail to sufficiently reveal the chemical fracture mechanisms underlying hydration cracking in hard-brittle shale. To address these limitations, this work establishes a mathematical model applicable to the nucleation and subcritical crack growth (SCG) of hydration cracking in illite-rich hard-brittle shale, based on the principles of physical equivalence and the framework of linear elastic fracture mechanics (LEFM). The asymptotic solutions of stress fields near the tips of hydration microcracks, under variable crack width and nonlinear hydration stress conditions, were derived, and the long-term stability was also evaluated, clarifying the evolution mode of SCG dominated by intrinsic multi-processes. Additionally, an innovative indoor double torsion test was developed for the first time, wherein water was instantaneously introduced to the tip of a stress-loaded crack to simulate water-rock interactions after wellbore formation. The dynamic behavior of hydration cracking evolution in hard-brittle shale from the Gulong Qingshankou formation was quantitatively characterized. Furthermore, a critical criterion for the transition time from hydration cracking to sudden instability failure was proposed. Results indicate that the double torsion program, which incorporates instantaneous water introduction to the crack tip, is reliable and repeatable. The hydration of illite crystal layers in hard-brittle shale can significantly enhance the rate of SCG by 1 to 2 orders of magnitude, reaching rates of μm/s, and increase the length of SCG to 4 to 13 times that of the original length. Moreover, the model predictions provided by the iterative algorithm align well with experimental results, exhibiting a maximum mean relative error not exceeding 8.2% and achieving rapid convergence under varying initial stress levels, thereby confirming the reliability and robustness of the proposed model. This study clarifies the microscopic origins and progressive evolution behaviors of hydration cracking in illite-rich hard-brittle shale from the perspective of chemical fracture mechanics, highlighting a hierarchical regulation strategy that enhances blockage at the crack mouth, obstructs flow within the crack, and inhibits growth at the crack tip, thereby ensuring overall wellbore stability. The experimental results and theoretical model offer methodological references and data for both indoor expanded tests and on-site engineering practices.
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