Volumetric response and permeability evolution during carbonation of crushed peridotite under controlled stress-pressure-temperature conditions

Peridotites (olivine-rich rocks) naturally react with CO2-rich 2-rich fluids to eventually form carbonates. Complete conversion involves incorporation of substantial amounts of CO2, which requires prolonged fluid flow. Yet, these reactions also cause a large increase in solid volume (63-84 %), raising questions on how they proceed in nature without this excess solid volume clogging fluid pathways. It has been suggested that reaction-driven fracture, caused by development of crystallization pressure, facilitates continual creation of new pathways, allowing reaction to advance. If indeed so, this could enable injection of industrially captured CO2 into peridotites for permanent sequestration. However, such a fracturing mechanism has not been reproduced experimentally. Here, we report nine reactive flow-through experiments, performed on pre-compacted angstrom heim dunite powder (similar to 88 % olivine) inside a 1D oedometer vessel, to simultaneously measure axial deformation and permeability development. Tests were performed at 150 degrees C and effective axial stresses of 1-15 MPa. After initial flow measurements using deionized water at 10 MPa, during which permeability and deformation remained unchanged, the samples were exposed to inflow of reactive fluid. Samples subjected to CO2-saturated 2-saturated brine/water or NaHCO3-saturated 3-saturated solution showed minor compaction (0-0.38 %), while permeability decreased from 10(-16)-10(-17) to 10-(- 20)-10(- 21) m(2). Microstructural and chemical analyses demonstrate a drastic reduction in porosity of the reaction zone where carbonation occurred. A reference sample exposed to NaHSO4 solution (acidification, but no carbonation) instead showed slightly increased permeability, from 3 x 10(-17) to 8.2 x 10(-17) m(2), associated with 0.05 % compaction strain. Combined, the observations suggest dissolution of olivine at the grain contacts, leading to minor mechanical compaction, followed by precipitation of carbonates inside the remaining pores, clogging transport paths and thus reducing permeability. This indicates volume-increasing precipitation upon olivine carbonation under subsurface conditions clogs transport paths at laboratory timescales, severely limiting reaction rates and thus potential for crystallization pressure development and reaction-driven fracture.