Chemo-hydro-mechanics in a reactive rock under cylindrical fluid pressurization

Abstract

Cylindrical cavity expansion in a geomaterial due to internal fluid pressurization occurs in many geo-energy and geo-environmental engineering problems involving fluid injection into the subsurface. Often, acidic substances are incorporated in the fluid injection posing a chemically aggressive environment at the cavity wall which diffuses into the matrix affecting the rock’s mechanical as well as hydraulic properties via mineral mass removal. Here we propose a coupled chemo-hydro-mechanical model for such scenarios to study the complex interplay between the process of chemical mass removal, damage evolution, rock strength degradation and an alteration of the hydraulic field due to chemical erosion. A homogeneous fully saturated carbonate rock is chosen for qualitative and quantitative assessments as a representative of chemically reactive geomaterials. Plane strain conditions are assumed and all the fields involved in the reaction-transport-deformation process zone are axisymmetric. The rock behaviour is chemically controlled in both the elastic and plastic domain adopting a chemo-elasticity concept and a yield limit depending on the accumulated mass removal, i.e. the total amount of mineral dissolution under high Damköhler number conditions. The rate of chemical dissolution at a continuum scale is described as a function of a variable specific surface area of the solid–fluid interface per unit volume affected by the irreversible micro-cracking process, as well as the local acidity. The irreversible damage evolution is coupled to the hydraulic field via the porosity generation by chemical mass removal, affecting the acid delivery. The presented framework is the first rock cavity expansion model to our knowledge that underpins a spontaneous transition from the diffusion-dominant to a diffusion–advection regime enabled by damage-enhanced chemical mass removal.