CHEMICAL AND PHYSICAL EFFECTS OF PORE FLUIDS ON POROUS ROCKS

Abstract

Fluid pressure and chemistry exert dual mechanical and chemical controls on rock deformation, strength and failure mode. Classical poroelastic theory treats pore fluid pressure (Pf) as a scalar quantity that counteracts the total applied stress according to the effective stress law. However, the traditional rock deformation frameworks do not account for the complicated chemical nature of the pore fluid itself nor the physical complexities of real rock microstructure. This dissertation integrates triaxial and hydrostatic loading tests along with detailed microstructural analyses to discern the chemical and physical roles of Pf on rock deformation at various stages of loading in moderate (~15 % initial porosity) and high (~25 % initial porosity) porosity sandstones. In the highly porous sandstone, increasing Pf had contrasting effects on the drained volumetric response when subjected to hydrostatic versus triaxial loading. Hydrostatic loading promoted stiffening at high Pf while triaxial loading experiments showed enhanced compaction (softening) in experiments deformed at high Pf. In the moderately porous sandstone, conventional triaxial experiments revealed enhanced subcritical crack growth at high Pf likely promoted slow faulting through a combination of chemical and physical effects only at slow deformation rates. Furthermore, samples saturated with ethanol and water were deformed to discern chemical from mechanical effects in the same rock. Ethanol was found to suppress stress corrosion cracking and promote fast failure accompanied by thin planar faults in thin section. Water was found to promote stress corrosion and slower failure and was accompanied by more anastomose faulting. The totality of the results suggest that simple effective stress is often insufficient to accurately model rock deformation and failure. A complete understanding of the dominant micromechanical mechanisms and fluid-rock interactions is necessary to predict how a rock deforms.