Effects of CO2 - Rock Interaction on Chemical and Mechanical Behaviour of Shale Rocks

Abstract

An original approach to the problem of chemo-mechanical characterization and creep behavior of shale rocks is presented, which consists of combining techniques like grid nanoindentation, scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS), multispectral image analysis, micro computed tomography (micro-CT) and finite element analysis. X-ray microanalysis of the major elements is performed over the indented region, resulting in high-resolution images of the spatial distribution of different elements over the indentation grid. The individual elemental maps of the image fields are mathematically merged to create a multispectral image proper for segmentation into distinct clustered phases and mapped on the indentation points. Unsupervised clustering analysis is performed on the multispectral image to determine the number of statistically definable mutually exclusive material phases. Digital image analysis was performed on X-ray micro-CT images to realize microstructural evolution and spatial distribution of pores and different material phases. Microstructural modeling was performed to investigate the link between microstructural evolution and creep behavior of shale rocks subjected to chemo-mechanical loading through modeling time-dependent deformation induced by the dissolution-precipitation process. Microstructure models of unreacted and reacted shales are coupled with a finite element model to compute the creep deformation. The method is illustrated through application to different types of shale rocks, demonstrating spatially varying microstructures as a result of exposure to reactive environments. The results show that the method is capable of successfully isolating different material phases at the indentation scale; thus, providing an efficient means to link chemical and microstructural properties to the mechanical response. The application of microstructural and chemo-mechanical characterization methods highlights key chemical reactions, and the resulting mechanical properties changes across the depth of the reacted rock. The results of characterization techniques have the potential to provide instrumented data to inform and validate reactive transport models as well as chemo-mechanical models capable of simulating bulk mechanical response based on local properties.