ABSTRACT:

Despite the significant hydrocarbon production from unconventional shale resources in the last decade, hydration induced mechanical responses of shales especially at micro-scale level is not yet fully understood. Such mechanical responses especially to hydraulic fracturing fluids control the leak-off rate and near fracture permeability evolution and therefore is crucial for hydrocarbon production. Therefore, we ran a series of systematic experiments by exposing organic matter rich shale samples to ionic solutions (NaCl, KCl, MgCl2 and CaCl2) with different concentrations (0.2, 1 and 2M) at controlled temperature. Images of the samples at their initial and wet states (exposed to ionic solutions for 7 hrs at 25 °C temperature) were taken using environmental electron microscope. The image processing was performed to register the images and extract the evolved features. Digital Image Correlation was further employed to calculate the strain field developed by change in sample structure in order to investigate the likelihood of micro-fracture development. From the results of this study, it was found that a) the sample exposed to 0.2M NaCl solution experienced the most significant micro-structural changes especially clay hydration while the sample exposed to 0.2M CaCl2 solution almost showed no changes, b) the sample exposed to 0.2M KCl solution on the other hand showed no/little swelling but its pore space was increased, c) surprisingly the micro-structural changes were more pronounced at 1.0M concentration of all ionic solutions especially for those with divalent ions, d) Digital Image Correlation analysis confirmed that micro-fracture development is more likely to occur when the sample is exposed to solutions of monovalent ions in particular NaCl.

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