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ABSTRACT: Wellbore cooldown via injecting and circulating cold fluid is a practical method to lower temperatures in the borehole and prevent damage to equipment associated with high temperature geothermal reservoirs in Enhanced Geothermal Systems. Extensive cooling often induces significant thermo-poro-elastic stresses, altering the near-wellbore stress distribution. A finite element model is developed in ABAQUS to investigate the magnitude of the thermally induced stresses during both the cooling and warmup phases. Our simulation results reveal a notable tensile near-wellbore stress in both directions for potential hydraulic fracturing initiation (longitudinal and transverse) due to cooldown. Such stress alteration is largely dependent on the duration of cooldown. Furthermore, the thermally induced stresses diminish with the increased waiting (warmup) time before hydraulic fracturing stimulation. Our findings highlight the pivotal role of borehole cooldown conditions in shaping the near-wellbore stress distribution. 1 INTRODUCTION To increase the survivability of tools during drilling, logging, and well stimulation in the high-temperature reservoirs of Enhanced Geothermal Systems (EGS), extensive injection and circulation of cold fluid prior to the operations in geothermal wells are often deployed to lower the temperature inside the borehole (Sinha and Joshi, 2011; Brown et al., 2012; Lu et al., 2023a, 2024). The cold fluid circulation effectively cools down both the borehole and the nearby rock matrix. Significant thermo-poro-elastic stresses are induced in regions adjacent to the well as a result of cooling (Perkins and Gonzalez, 1984; Li et al., 1998; Ghassemi and Zhang, 2004), which inevitably exert strong alteration to the near-wellbore stresse distributions (Wang and Papamichos, 1994; Ekbote and Abousleiman, 2006; Lu et al., 2023a). The near-wellbore stress field is considered a crucial factor in determining the breakdown pressure of hydraulic fracturing stimulation (Hubbert and Willis, 1957; Haimson and Fairhurst, 1967; Detournay and Carbonell, 1997; Bunger and Lu, 2015; Lu et al., 2017). In addition, different stress distribution along preferential directions, namely the longitudinal direction parallel to the wellbore axis and the transverse direction perpendicular to the wellbore, coupled with hydraulic fracturing (HF) mechanics (Detournay, 2016) can potentially affect the fracture initiation geometry (Lecampion et al., 2013; Benouadah et al., 2021). In the case of mini-frac (mini-HF) testing, where small scale hydraulic fractures are created for in-situ stress measurements (Cornet and Valette, 1984; De Bree and Walters, 1989; Haimson and Cornet, 2003), the near-field stresses induced by wellbore cooldown can have a substantial impact on the pressure evolution (Stephens and Voight, 1982; Lu et al., 2023b) and the orientation of the induced fracture (Lu et al., 2023a, 2024) through the coupling of the thermal effect with hydraulic fracturing mechanics. Hence, it is imperative to account for the thermally induced stresses in the three-dimensional (3D) configuration in any effort to predict HF breakdown and fracturing geometry.
Lu et al. (Sun,) studied this question.