Carbonates, such as calcite (CaCO3) and dolomite CaMg(CO3)2, are abundant minerals in the Earth’s crust, common in sedimentary formations. These minerals are susceptible to subcritical chemically-driven fracturing. Understanding chemically-driven fracture is necessary for carbon dioxide (CO2) and nuclear waste storage, resource extraction, and predicting earthquakes. We examined chemical processes controlling subcritical fracture in calcite single crystals, and in carbonate-rich shale caprock, following its reaction with CO2. We developed a novel approach for quantifying the growth rate of subcritical fracture in calcite. We used a Vickers-geometry indenter tip to create identical micron-scale fractures on the calcite (100) surface. These pre-fractured samples were submerged in a series of aqueous fluids and an optical microscope was used to track the fracture growth in-situ. The fracture propagation rate depended on the type of aqueous fluid, and varied from 1.6×10-8 m s-1 to 2.4×10-10 m s-1. The fracture growth rate correlated with the strength of the calcium-ligand complex, and did not depend on the measured dissolution rate of calcite or trends in zeta-potential. We conclude that the growth of subcritical fracture in calcite is limited by chemical complexation reactions at the fracture tip. To examine the coupled chemical-mechanical effects in calcite- and dolomite-rich sedimentary rock we used Mancos Shale. We quantified the changes in mineralogy and micromechanical properties before and after reacting this shale with CO2. The mineralogical changes depended on the pressure of CO2 during shale exposure to the CO2-brine mixtures. Dedolomitization (replacement of dolomite by calcite and gypsum) was observed in the reactors pressurized with 100 psi of CO2, while at the higher pressure (2,500 psi of CO2), the complete dissolution of calcite, partial dissolution of dolomite, and precipitation of magnesite and anhydrite were observed. The mechanical weakening of shale was localized: for dolomite-rich laminae a decrease of up to 50 % in scratch toughness was observed, while the quartz-calcite-rich laminae did not undergo mechanical degradation.

Chemical effects on fracture in calcite single crystals and in carbonate-rich shale

2019 Spring National Meeting

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