Experimental investigation on the failure mechanism of water-bearing sedimentary rocks under true-triaxial stress

Understanding water-induced failure behavior of sedimentary rocks under true-triaxial in situ stress is essential for mitigating damaging formation sliding and casing damage during oil and gas development. However, previous studies on water–rock interactions are limited to uniaxial or conventional triaxial stress conditions, leaving the understanding of failure mechanisms associated with lithological and structural diversity under complex 3D stress states poorly understood. We followed the deformation evolution (stress–strain response) of natural and water-saturated sedimentary rocks through true-triaxial mechanical tests. We elucidated how water dominates the failure through mineral-structural analyses, application of the softening index (SI) and brittle-lubrication index (BI) under true-triaxial stress, and X-ray computed tomography (CT) imaging. This study established a classification framework for water-induced rock deterioration, distinguishing Type I (clay-rich, CMC > 30 %) and Type II (low-clay, CMC < 30 %) evolutionary pathways governed by water-softening and −lubrication effects, respectively. Structural prominences (SAC > 0.4) amplified these effects, as evidenced by increased SI and BI, categorizing rocks into high-risk zones: Type I₁ (CMC > 30 %; SAC > 0.4) with complex fracturing tendencies along strata and Type II₁ (CMC < 30 %; SAC > 0.4) susceptible to bedding-plane shear-slip failure governed by structural discontinuities. These hydro-mechanical synergies significantly increased the risk of casing shear failure. To address this, we propose a geologically informed mitigation strategy: localized buffer layers in high-risk zones (I₁ / II₁) paired with enhanced cementing in stable zones. The findings provide actionable insights for preempting formation slippage and resulting accidents during reservoir development.