Interplay of Loading and Adsorption Controls Elastic Deformation of Clastic and Crystalline Rocks

Rock elasticity varies with both mechanical loading and moisture content. Studies to date have only examined each effect independently, although moisture interactions with pore walls are likely coupled to mechanical stress. Here, we present experimental data specifically collected in sandstone and granite under simultaneous control of cyclic loading alongside ambient humidity approaching saturated vapor. Adsorption can account for 40% reduction in Young's modulus, which reduces to (Formula presented.) 10% as uniaxial stress increases from below 1 MPa to below the elastic limit. The observation is explained by a micromechanical model linking grain-scale contact stiffness to pore-scale vapor adsorption, quantitatively capturing coupled stress-induced stiffening and adsorption-induced softening. The coupled behavior is interpreted as adsorption-induced softening becoming inhibited under greater mechanical loads. Our results suggest the coupled effects are strongest at overburden stresses between 3.3 and 10.6 MPa (140–450 m) in sandstone and 6–30.3 MPa (235–1,200 m) in granite.