Abstract: Frequent wellbore instability poses a significant challenge during the drilling of large-diameter wellbores in the hard and brittle mudstone of the Dong 3 Member in the Pugu 2 Gas Storage Reservoir. Based on the scale effect theory in rock mechanics, this study conducted comparative mechanical degradation experiments on intact and crushed mudstone samples immersed in drilling fluid. A wellbore collapse prediction model considering the time-dependent seepage damage was subsequently established to reveal the fracture-fluid coupling damage mechanism. The results are as follows: 1) The well-developed micro-fracture network in the mudstone transforms singular hydration diffusion into a severe hydration-seepage coupling damage. 2) The rock degradation exhibits a significant scale effect and nonlinear time-dependent characteristics. During the initial immersion phase (0~4 d), the strength decay rate of the crushed samples is 1.54 times that of the intact ones. 3) A wellbore collapse pressure prediction model considering the time effect of fracture seepage damage was established. The calculation results show that, the critical collapse density of the 9.5 in. well position exceeded the safety warning line on the second day, and compared with the 12-day stable period of the 8.5 in. conventional section, the safe operation window was shortened by more than 80%. Based on this, a collaborative anti-collapse strategy of micron-scale rigid plugging and film-forming agents and a 48 h rapid drilling and completion operation suggestion were proposed, providing a theoretical basis for safe drilling in complex sections of gas storage reservoirs.

Key words: gas storage, scale effect, seepage damage, time-dependent instability, wellbore stability