Effect of cooling rate on pore structure and mechanical properties of shale wellbore during formation heat exchange driven by drilling fluid circulation

To reveal the thermal shock damage mechanism and influence law of cooling rate on the pore structure and mechanical properties of shale wellbore induced by drilling fluid circulation during formation heat exchange process, a drilling fluid circulation temperature transfer model was established to quantify the cooling rate characteristics under different well depths and circulation times. Then, the shale samples were subjected to variable cooling rate heat treatment in an intelligent high-temperature furnace. The microscopic pore evolution and mechanical property deterioration were analyzed by combining SEM, NMR and triaxial compression tests. The results show that: (1) SEM image processing combined with pore morphological parameters enables the classification of pores and fractures as well as the quantification of porosity; (2) NMR-based T 2 -pore radius conversion and spectral area comparison established a correlation model for pore volume and expansion rate characterization; (3) A higher cooling rate results in a increased pore volume, a greater number of strip-shaped fractures, and accelerated transformation of micro-mesopores to macropores (with a weak thermal expansion effect). Meanwhile, higher porosity triggers more severe deterioration of shale shear strength and elastic modulus, accompanied by faster propagation of micro-mesoscopic fractures; (4) A statistical damage model with the expansion rate as the damage variable is established, and a corresponding shale statistical damage constitutive model is further developed. The predicted values are in good agreement with the experimental results, which can effectively quantify the evolution law of mechanical damage, and provide key theoretical models and calculation method support for the prediction of thermal shock damage of shale wellbore under the drilling fluid circulation condition in deep engineering.