Coupling of geochemical reactions with hydrological and mechanical processes in nano-porous carbonate rocks can lead to complex behaviors involving the change of pore topology (e.g., precipitation, dissolution, compaction) and mineralogy. Fluid-rock interactions also change hydrological, mechanical, and geophysical properties (e.g., permeability, rock strength, elastic, acoustic velocity) across spatial and temporal scales. Here we analyzed a Liège chalk sample that have been flooded with MgCl2 in a triaxial cell for 516 days, which was compared to unflooded sample. To identify the impact of chemo-mechanical coupling on the change of mineralogy, pore-structure, and mechanical properties, various multiscale imaging techniques were applied and nano-indentation testing was performed. Multiscale imaging includes dual focused ion beam-scanning electron microscopy (FIB-SEM), micro computed tomography (micro-CT), 2D and 3D energy dispersive spectroscopy (EDS), backscattered electron microcopy (BSE), and MAPS mineralogy. First, we obtained 2D mineralogical mapping and pore structures over a large sample area (~ 0.4cm x 2cm) using EDS and BSE, respectively, to determine the locations of FIB-SEM analysis. The mineralogical mapping reveals the interface of reaction front and the textural fabrics revealed by FIB-SEM images (i.e., 1000 image stacks) at 10 nm resolution and elemental mapping by 3D FIB-EDS at 10 nm resolution show alteration patterns due to dissolution, precipitation, and compaction compared to unaltered sample with clear grain boundary. FIB-SEM-EDS images of the altered sample at multiple locations show distinct pore features including discrete micro-cracks in the chemically unaltered zone, sharp precipitation boundaries in the chemically altered zone, and smooth and relatively large pore space in the boundary zone of chemical reaction. Nano-indentation testing at multiple scales is performed to evaluate the impact of the change of pore structure and mineralogy on the mechanical properties. Lattice Boltzmann simulations and nano-indentation simulations are used to evaluate the change of permeability and mechanical properties at several different scales. Overall, these results clearly demonstrate that it is very critical to characterize the change of multiscale pore structure associated with chemical reactions and mechanical deformation. Multiscale and multiphysics analysis of coupled processes in nanoporous media with broad compositional range and physical and chemical heterogeneity will enhance a fundamental understanding of poromechanical and flow responses of the materials.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
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