Permeability of Serpentinized Harzburgite and its Implications on Mineral Carbon Storage

Storage of anthropogenic CO2 as permanent carbonate minerals has emerged as a promising method for control of carbon dioxide. One approach consists of injecting low-pH, CO2-rich water in ultramafic rock that serves as a buffering medium. Accordingly, the rock dissolution will increase the solution pH such that carbonate minerals can precipitate. The permeability of the rock matrix, however, plays an important role in the buffering (dissolution) process and controls chemical reaction rates. Using X-ray diffraction spectroscopy (XRD), we analyzed the composition of the (i) vein filling material in a serpentinized harzburgite, and (ii) matrix adjacent to the vein. The XRD analysis showed that hydrated carbonates mineralized in the vein as well as the intact matrix adjacent to the surface. Permeability of the serpentinized rock matrix adjacent to the vein surface was then measured using the pulse decay approach. It is shown that a permeability on the order of 10-21 m2 and rock matrix with less than 25% serpentine minerals allowed for fixing CO2 as thermodynamically stable, hydrated carbonate minerals in the matrix. 1. INTRODUCTION Harzburgites of the upper mantle sequence in most ophiolites are partially to fully serpentinized (O'Hanley 1996; Mevel 2003; Kelemen and Matter 2008; Boudier et al. 2010). The hydration and carbonation of facies that have undergone less serpentinization and hence are rich in olivine and pyroxene can be described by coupled dissolution-precipitation reactions: (equation) (equation) Most accessible parts in ultramafic formations of the upper mantle in ophiolites, however, are pervasively serpentinized and hence contain considerable amounts of serpentine group minerals (e.g. lizardite, chrysotile, antigorite). The hydration and carbonation of serpentinized harzburgites include the dissolution of serpentine minerals (Luce 1969): (equation) followed by carbonation reactions where the liberated magnesium ions will bind with carbonate ions to form magnesite: (equation) The permeability of the rock matrix, however, plays an important role in the dissolution and buffering processes, controls chemical reaction rates and the extent of carbonation and hydration processes. Using X-ray diffraction spectroscopy (XRD), we analyzed the composition of the (i) vein filling material in a serpentinized harzburgite, and (ii) matrix adjacent to the fracture surface. The XRD analysis showed that hydrated carbonates mineralized in the vein as well as the intact matrix adjacent to the vein surface. Permeability of the serpentinized rock matrix adjacent to the vein surface was then measured using the pulse decay approach. It is shown that a permeability of the order of 10-21 m2 and rock matrix with less than 25% serpentine minerals allowed for fixing CO2 as thermodynamically stable, hydrated carbonate minerals in the matrix.