The Effect of Surface Forces on Subcritical Crack Growth and Macroscopic Failure of Carbonates in Aqueous Solutions 

Objectives

The main objective of this research is to develop a mechanistic theory for the subcritical cracking of mineral crystals in aqueous environments, and utilize it to inform the chemomechanical modeling of deformation, delayed failure, and band formation of brittle rocks in subsurface conditions. 

Quantitative understanding of subcritical crack growth (SCG) in brittle rocks is necessary for two reasons: 1) SCG controls important natural processes such as delayed earth rupture, weathering of surface rocks, and relaxation of internal stresses in rock systems; 2) A chemical-mediated slow-growing crack in individual grains can, through a chain of events, undermine critical deep subsurface operations by causing induced seismicity, surface subsidence, and reduced reservoir permeability. Modeling the full SCG curve is challenged by its highly nonlinear multi-stage characteristics and the complex, unresolved controlling mechanisms during each stage. Difficulties also exist when extrapolating the knowledge of SCG in mineral crystals to interpret large-scale phenomena at geological time and length scales. Understanding the impact of SCG in the chemomechanical responses of bulk rocks at intermediate scale is a necessary step in this regard.

To fill these gaps, we formulate the following overarching hypothesis: the full SCG curve can be predicted by focusing on two key surficial attributions, namely the alternation of surface forces due to the intrusion of sorptive species and the transport of these species in nanoconfined spaces along the crack. This will lead to the development of a novel Surface Force based Fracture Theory (SFFT) for fracture simulations. We further hypothesize that the chemically mediated time-dependent compaction, failure, and localization of porous rocks can be understood in the same vein by resolving the surface energetics and chemistry during new surface area creation (i.e., damaging). This hypothesis will be tested through injecting interfacial dynamics to our Surface Poromechanics framework. 

This study will focus on calcite crystal and carbonate rocks to synergizes with the planned experimental activities at LBNL. In addition to deepening the fundamental understanding of calcite chemomechanics at multiple scales, this research will advance our knowledge and modeling capability of other geological materials under similar subsurface conditions. For example, the proposed theory can be extended to describe the chemical weakening of siliciclastic rocks which are frequently considered as a stable host formation for long-term CO2 storage.