Harnessing mineral carbonation reactions to seal fractured shales and sequester carbon

Abstract:

Shale oil and gas are being developed widely in the United States despite the potential for long-term climate impacts driven by burning these new hydrocarbon resources and by fugitive emissions from fractured formations. Here the carbonation of calcium-based silicates is studied as a method to re-seal fractured shale formations and to store significant amounts of CO₂ after hydrocarbon extraction. Ex situ mineral carbonation has been studied extensively for trapping CO₂ from power plants but the application of these reactions directly within shale matrix under in situ conditions to seal shales and sequester carbon has not been studied. The reaction requires the solid calcium-based silicates being present within the shale fracture matrix and flooded with high-pressure CO₂. The pressure and temperature within most shale formations would enable this carbonation reaction to precipitate solid calcium carbonate, which would clog fractures. Silicates could be injected in the same way that proppants are injected into shale gas wells. Wollastonite was tested here but other silicate minerals such as olivine could also be used in much the same way.

To prove this concept, batch experiments were carried out under reservoir conditions representative of the Marcellus Shale in the presence of ground shale particles (39-177µm) and CaSiO₃ powder. X-ray diffraction (XRD) patterns revealed the conversion of CaSiO₃ into CaCO₃ after 24 hours. Quantitative XRD analysis was used to determine that the conversion ratio of CaSiO₃ was ~55% at 3100 Psi and 75^oC. The reaction was sensitive to both temperature and pressure with ~58% conversion at an increased temperature of 95^oC and only ~50% at lower pressure (2200psi). The morphology observed by Scanning Electron Microscopy (SEM) reveals that the shale particle surfaces are covered with precipitated calcite crystals ranging in size from 1 to 5 µm. Using energy-dispersive X-ray spectroscopy (EDS), the locations of residual CaSiO₃and crystallized CaCO₃ were determined. The cavities of Ca and Si on the corresponding elemental distribution map indicated the areas where the dissolution of CaSiO₃ happens. In a fractured rock, the layer of precipitated CaCO₃ would be likely to block the flow pathways and decrease the conductivity of the fractured formation.