Impacts of Organic Ligands on Forsterite Reactivity in Supercritical CO₂ Fluids

Abstract

Subsurface injection of CO2 for enhanced hydrocarbon recovery, hydraulic fracturing of unconventional reservoirs, and geologic carbon sequestration produces a complex geochemical setting in which CO2-dominated fluids containing dissolved water and organic compounds interact with rocks and minerals. The details of these reactions are relatively unknown and benefit from additional experimentally derived data. In this study, we utilized an in situ X-ray diffraction technique to examine the carbonation reactions of forsterite (Mg2SiO4) during exposure to supercritical CO2 (scCO2) that had been equilibrated with aqueous solutions of acetate, oxalate, malonate, or citrate at 50 °C and 90 bar. The organics affected the relative abundances of the crystalline reaction products, nesquehonite (MgCO3 · 3H2O) and magnesite (MgCO3), likely due to enhanced dehydration of the Mg(2+) cations by the organic ligands. These results also indicate that the scCO2 solvated and transported the organic ligands to the forsterite surface. This phenomenon has profound implications for mineral transformations and mass transfer in the upper crust.

Experimental Study of Organic Ligand Transport in Supercritical CO2 Fluids and Impacts to Silicate Reactivity

“…In this present work, which is a continuation of the Miller et al [25] study, we performed additional experiments in order to accomplish four goals: 1) verify transport of simple organic ligands via solvation by scCO 2 , 2) further investigate the impacts that the organics have on magnesium carbonate precipitation in thin adsorbed water films, 3) attempt to determine the flux of citrate complexes to the forsterite surface in Miller et al [25], and 4) identify evidence of metal-organic partitioning into the water-saturated scCO 2 phase. The in situ spectroscopic experiment was conducted with a calcium acetate solution at similar conditions to EXP 2 in Miller et al [25].…”

“…In the subsurface, the injected scCO 2 produces regions in which the full spectrum of mutual CO 2 -H 2 O solubility can occur [6][7][8][9]. In basalt reservoirs, the CO 2 -rich (water-bearing scCO 2 ) fluids are highly reactive with regard to the minerals present [10], and the scCO 2 may mobilize the organics, as scCO 2 is an effective solvent of organic matter [11], which has been observed in the field and in experiments with geomaterials [12][13][14][15][16][17][18][19][20][21][22][23][24][25]. In our recent investigation, we demonstrated that simple, naturally abundant organic ligands (the protonated forms of acetate, malonate, oxalate, and citrate) were transported through water-saturated scCO 2 and their presence impacted the carbonation of forsterite [25].…”

“…The in situ spectroscopic experiment was conducted with a calcium acetate solution at similar conditions to EXP 2 in Miller et al [25]. Acetate was chosen as it is the most prevalent organic ligand in subsurface waters [26][27][28].…”

“…Acetate was chosen as it is the most prevalent organic ligand in subsurface waters [26][27][28]. The high pressure X-ray diffraction (HXRD) experiments utilized three different citrate-rich solutions, as citrate most affected the forsterite carbonation in Miller et al [25]. Additionally, citrate is a suitable example of a multivalent organic ligand that is often used in forsterite reactivity studies [29][30][31][32][33].…”

“…Additionally, citrate is a suitable example of a multivalent organic ligand that is often used in forsterite reactivity studies [29][30][31][32][33]. Forsterite was chosen as a model silicate mineral in order to compare the results with previous studies [25,[34][35][36][37][38][39].…”