Porosity and permeability evolution during calcite dissolution: numerical and experimental exploration of the (sub-)pore scale reactive transport processes

Abstract

Geological storage of CO2 gas is perceived as a potential solution against global warming. Injection of CO2 or acidic solutions into carbonate rocks initiates a series of physical and chemical processes. Optimization of the injection process and later safe storage of the injected gas requires sophisticated modeling tools which can comprehend the fundamentals of the ongoing processes for a range of chemical and physical heterogeneities such as injection flow rate, composition of the injected solution, and rock heterogeneities. Pore scale models are considered a promising tool to understand the fundamentals of reactive transport phenomena due to the fact that the actual physical/chemical processes take place at the pore scale. In this thesis, I have developed a single pore scale model which includes and explicitly defined solid-fluid interface and fully coupled models for simulation of velocity field, transport of solute species, surface, and bulk reactions (i.e., encompasses calcite dissolution system) and an interface of the solid calcite that moves in response to dissolution. This model was used to delineate the control of the imposed reactive -flow conditions on the shape of the pore over the course of the dissolution. We observed that, depending on the dissolution regimes, initially cylindrical pores can attain a spectrum of shapes. This information is important for widely used pore network models, so that the conductance of the pore can be correctly updated as the dissolution progress. The pore-scale model was further used to examine the hydrodynamics contribution of rough calcite surfaces on calcite dissolution rate spectra. This study utilized real surface topographic data to quantify roughness induced reaction rate variability for a range of reaction and flow regimes. In this research, we have proposed new conductance relations for pore network models that incorporate the changing pore shape information as a function of the imposed reactive flow conditions. Finally, we investigated the impact of the composition of brine, mainly Ca2+ and NaCl salt, on the development of the dissolution patterns.