Suction-induced strains in intact rocks

Suction-induced strains in rocks affect subsurface applications that involve immiscible fluids, including CO₂ sequestration, hydrocarbon extraction and the storage of energy fluids. This study explores the underlying processes and parameters that govern the strains intact rocks will experience during wetting and drying. It combines a synthesis of published data with new experimental results. Macroscale water retention measurements together with nuclear magnetic resonance NMR data show that desaturation proceeds through sequential drainage, beginning with the largest interconnected pores, which empty first due to their lower capacity to sustain capillary pressure. Consequently, the suction at air-entry ψ AE is strongly correlated with the characteristic pore size d 85 . Because the rock remains saturated to the verge of air entry, the nominal strain ε ≈ ψ AE /E defined as the ratio between the air entry pressure and the rock stiffness E is an effective indicator of the rock’s susceptibility to suction-induced deformation; this nominal strain can vary from less than 10 −6 in stiff igneous rocks to more than 10 −4 in clay-rich shales. Suction-induced strains can significantly impact fracture transmissivity, which scales with the cube of the fracture aperture. Then, wetting-induced swelling may reduce aperture and enhance self-sealing, while drying can increase aperture and facilitate leakage. During wetting or drying, changes in suction dominate over osmotic effects, although their impact may be partially masked by concurrent processes such as creep and thermoelastic strains. • Suction-induced strains can significantly affect fracture transmissivity in rocks • The nominal strain at air-entry suction is a good proxy for rock susceptibility • Suction at air-entry correlates strongly with the 85th percentile pore size • NMR during drying shows desaturation progressing from large to small pores