Abstract
Subsurface mineralization of CO2 by injection into (hydro-)fractured peridotites has been proposed as a carbon sequestration method. It is envisaged that the expansion in solid volume associated with the mineralization reaction leads to a build-up of stress, resulting in the opening of further fractures. We performed CO2-mineralization experiments on simulated fractures in peridotite materials under confined, hydrothermal conditions, to directly measure the induced stresses. Only one of these experiments resulted in the development of a stress, which was less than 5% of the theoretical maximum. We also performed one method control test in which we measured stress development during the hydration of MgO. Based on microstructural observations, as well as XRD and TGA measurements, we infer that, due to pore clogging and grain boundary healing at growing mineral interfaces, the transport of CO2, water and solutes into these sites inhibited reaction-related stress development. When grain boundary healing was impeded by the precipitation of silica, a small stress did develop. This implies that when applied to in-situ CO2-storage, the mineralization reaction will be limited by transport through clogged fractures, and proceed at a rate that is likely too slow for the process to accommodate the volumes of CO2 expected for sequestration.
Highlights
Mineralization by direct reaction of CO2 with a suitable mineral phase is an emerging strategy for carbon sequestration that has received significant attention (e.g., [1,2,3,4,5,6,7,8,9,10,11])
Estimates from our thermodynamic model show that the solid volume increase occurring during the carbonation of olivine has the potential to result in a significant force of crystallization, provided that sufficient reaction progress and fluid supersaturation are achieved, while impinging grain contacts remain accessible to diffusive transport [67,69,70])
The discrepancy between the observations and a model of the relevant reactions may have arisen from the reliance of the thermodynamic model on the assumption that all reactions would proceed until the free energy change of the reaction driving it is negated by a stress-induced change in the chemical potentials of the solid phases—i.e., the model neglects potential mechanisms that would slow reaction progress, such as fracture or pore clogging that would render the transport of reactants more sluggish, or exceedingly slow reaction kinetics, for example due to changes in fluid composition, that could arrest the development of a reaction induced stress altogether
Summary
Mineralization by direct reaction of CO2 with a suitable mineral phase is an emerging strategy for carbon sequestration that has received significant attention (e.g., [1,2,3,4,5,6,7,8,9,10,11]). Several technologies that utilize mineral carbonation are under consideration as strategies to reduce the input of CO2 to the atmosphere Some of these approaches involve mining and processing olivine and serpentine for reaction with CO2 [13], whereas other routes propose sequestering carbon dioxide within the subsurface by injecting CO2 and water into ultramafic rock bodies that are rich in Minerals 2017, 7, 190; doi:10.3390/min7100190 www.mdpi.com/journal/minerals. To evaluate the fluid pressures and stresses that will lead to the fracture of peridotite during mineral carbonation, data relating to failure criteria for magnesium silicate minerals under in-situ conditions are required Information of this sort, such as the uniaxial tensile strength (T0 ) of ultramafic and mafic rocks under (upper-)crustal temperature conditions is sparse in the open literature. A small percentage of serpentinization (10%–15%) has been shown to significantly weaken peridotite rock [28], and the tensile strength of serpentinite or serpentinized peridotite is likely to be significantly lower, i.e., 2–50 MPa (see Table 1)
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