Abstract: The deep shale reservoirs in Longmaxi Formation, Southern Sichuan are subject to severe wall instability during drilling and frequent downhole obstruction, which increases the construction cost and difficulties of later operations. Therefore, an experimental study was conducted on the physical, chemical and mechanical properties of downhole core water, the experimental results were employed to comprehensively analyze the root reasons for dynamic and static sidewall instability in Longmaxi Formation, and the technical countermeasures were developed for sidewall stability. The results showed that the shale in Longmaxi Formation, Southern Sichuan was of high content of brittle minerals, dense lithology, and high mechanical strength; the frequent alternation of small layers was found in the collapsed interval, with well developed bedding fractures at the intersection of small layers, leading to significant anisotropy in rock pore permeability parameters and mechanical parameters, especially the sidewall stability in the inclined interval was greatly affected by the mechanically weak surface effect of the bedding fracture; the downhole differential pressure during the drilling process exacerbated the seepage of the drilling fluid along the bedding fracture, resulting in an increase in the effective stress due to the pressure penetration effect, which reduced the effective support of the drilling fluid to the sidewall and further induced sidewall collapse; for formations developed with bedding fractures, it was necessary to further strengthen the plugging performance of drilling fluid, optimize well trajectory and strengthen the flushing and carrying of downhole cuttings. There is much guiding significance of the study results for safe drilling in deep shale reservoirs.

Key words: shale reservoirs, sidewall stability, bedding fractures, sidewall collapse, Longmaxi Formation