A (bio)chemo-hydro-mechanical framework for enhanced cracking in geomaterials

Abstract

Macroscopic crack propagating into a stressed geomaterial and cavity expansion due to internal fluid pressurization are frequently encountered in many modern-day geo-energy and geo-environmental engineering problems including the emerging geo-technologies in unconventional shale gas exploitation, enhanced geothermal systems, carbon capture utilisation and storage. Often, acidic substances are incorporated in the injection fluid as additives, posing a chemically aggressive environment to the surrounding geomaterial, which diffuses into the matrix and affects the rock’s mechanical properties (strength, stiffness, etc.) as well as hydraulic properties (porosity, permeability, etc.) via, for instance, mineral mass removal. In this project we propose a reactive chemo-hydro-mechanical model to study the complex interplay between the time-dependent coupled processes of damage evolution, mass removal, rock strength degradation and an alteration of the hydraulic field due to progressive chemical erosion occurring in a degrading elasto-viscoplastic carbonate rock. An extension of Perzyna’s overstress model into the chemical domain is proposed, based on the concept of reactive chemo-plasticity with the yield limit dependent on the progress of mineral mass removal through chemical dissolution. Mineral mass dissolution (here calcite), via a rate equation, is described as a function of acid intensity and a variable specific surface area of the solid-fluid interface per unit volume. The latter is in turn assumed to be a function of the irreversible deviatoric strain. Therefore, a two-way coupling between the chemical reaction and the mechanical deformation is formed. Our constitutive formulation captures a combination of the effect of mineral dissolution on Young’s modulus, damage-enhancement on chemical softening, and a chemical “ductilization” effect observed in the laboratory post-yield behaviour of carbonate rocks. Meanwhile, the evolution of the irreversible damage is coupled to the hydraulic field via the additional porosity generated by internal chemical erosion, affecting local acid delivery. In the final set of governing equations, multiple physio-chemical processes occurring within the deforming material at their characteristic scales, including micro-cracking, chemical reactions, reactive-diffusion processes, delivery of acid, and the yielding of solid matrix, are fully coupled. Based on the developed reactive chemo-hydro-mechanical model, this study contributes to a better understanding of how a stressed geomaterial behaves under a chemically reactive environment by an integration of numerical and experimental investigations. Main findings include (1) Chemical dissolution (together with diffusive transport) plays a crucial role throughout the cracking process in terms of the redistribution of stress and deformation field around the crack tip and the nonlinear propagation of the macroscopic crack itself. (2) Subcritical cracking can be chemically driven. The more susceptible the rock is to microcracking under stress, the more accelerated a hydro-fracture propagates. (3) Radial distribution of transport properties and mass removal is highly nonlinear near the cavity wall upon substantial acid-assisted pressurized treatment. (4) A transition in the transport field from diffusion-dominant to diffusion-advection regime can be enabled by progressive chemical erosion. (5) Key coefficients in the proposed framework are preliminarily calibrated with bio-cemented specimens and a “chemical ductilization” effect is captured indicated by a change in failure mode in the post-peak regime.