Reactive flow in carbonate cores via digital core analysis

摘要

The transport of fluids accompanying chemical reactions in porous rocks pre-sents a complex problem that is central in a wide variety of geochemical processes such as acidizing of petroleum reservoirs, geological storage of CO2 and contaminant transport in groundwater resources. These processes often induce substantial changes in pore structure which, in turn, may significantly change petrophysical and transport properties of the rock. A better understanding of reactive transport at the pore scale can, for example, potentially lower the risk of costly mistakes in the development of hydrocarbon reservoirs and lead to more sound environmental policy for storage of hazardous materialsis. This thesis characterizes the evolution of pore structure and transport properties of real rocks under reactive flow using the emerging technique of X-ray micro-computed tomography (μ-CT).In this thesis, we develop a framework for the characterization of pore scale reactive flow in the presence of microporosity, and apply it to carbonates since these rocks are much more susceptible to alteration than sandstones when exposed to the flow of reactive fluids. We describe the experimental and computational developments that allow imaging of pore-scale structural evolutions in core material. The evolution of pore types and interconnectivity are analysed via µ-CT images for two broad dissolution patterns, namely uniform and wormhole-like patterns. A novel method based on the three-phase segmentation of normalized images, microporosity assignment and 2D histogram of image intensities is developed to formulate voxel-based evolution scenarios during reactive flow. The developed evolution scenarios essentially represent regions of the sample where the void fraction of voxels increases, decreases or remains unchanged. The proposed approach incorporates both the resolvable porosity and the subresolution porosity into the voxel-by-voxel study of the reactive flow-induced microstructural changes.Parallel numerical calculations of morphological and transport properties and experimental measurements allow to use pore-scale physics to understand the cause of anomalies in core-scale behaviour. We discuss the effect of variations in pore connectivity and critical pore-throat diameter on changes in the transport properties of the core. The results provide important insights into the mechanisms governing pore-scale reactive displacement and the evolution of rock properties.